For achieving cultural control, the agricultural practices can be categorised under various groups:
Practice # 1. Tillage:
Characteristics of soil such as texture, chemical composition, moisture, temperature and soil fauna directly influence the survival of soil-infesting insects. These characteristics also influence the quality of food which determines the abundance of a given pest. Thus, with the proper stirring and management of the soil, many insects can be controlled, provided the details of the life history and the behaviour of insects are known.
The depth of cultivation, its frequency and timing will depend on the insect species concerned. Although the value of deep tillage in insect control cannot be considered all embracing, a deep, thorough, and frequent cultivation of the fields is useful in controlling, for instance, the root-infesting aphids of maize and apple.
The soil-infesting insects, in general, can be killed easily by exposing them to weather. The insects are particularly helpless in the pupal stage, and can be easily killed, an example being the Oriental fruit moth, Cydia molesta (Busck). Packing the soil hard or caking is useful in controlling cutworms.
Packing the soil with a roller also tends to raise the water level. The subterranean insects rise to the soil surface, where they are picked up by their natural enemies. Light earthing at the early stage of sugarcane crop during May-June is helpful in checking the shoot borer.
It is a common sight in India to see a number of birds, such as the king-crow, the myna, the starling, etc., following the plough and picking up a number of insects, when exposed. If ploughing is done early in the spring, a number of hibernating insects which would have emerged later are exposed.
Some of the common hibernating species found in the soil are the cotton semilooper, Tarache notabilis (Walker); the lucerne caterpillar, Spodoptera exigua (Hubner); Bihar hairy caterpillar, Spilarctia obliqua (Walker); cutworms, Agrotis spp., etc.
Most of the surface grasshoppers and field crickets lay eggs in the summer or during the autumn, in the upper 7-10 cm of soil. When a field is cultivated, the eggs in the soil are brought to the surface where they are either desiccated, or picked up by birds and other predators.
A majority of the remaining eggs do not hatch in the spring because their emergence holes are blocked. Similarly, the larvae of the pink bollworm, Pectinophora gossypiella (Saunders) and the spotted bollworms, Earias spp., of cotton found in the soil in damaged bolls or cotton seed, if buried deep by ploughing, cannot emerge as moths in the spring.
The type of implement used for ploughing or cultivating the soil is also important. The mouldboard ploughs are, in general, more useful in burying the hibernating insects and at the same time bringing eggs and grubs of other species to the surface.
A disc plough treatment caused upto 73 per cent mortality of overwintering larvae of Dectes texanus Le Conte, a pest of sunflower as compared to 40 per cent mortality recorded using a sweep plough. With frequent hoeing and inter-culture, the soil-dwelling insects, such as the grubs of the ber beetle, Adoretiis spp.; the cotton grey weevil, Myllocercus undecimpustulatus Faust, are disturbed and killed.
Practice # 2. Clean Seed:
A number of insect pests are carried over from one crop to the next through seeds, cuttings or other infested plant parts. Certified seeds, free from pests and diseases, should be used for raising a new crop. Wheat galls should be separated by dipping the wheat seed in 5-10 per cent salt solution.
The hibernating larvae of the pink bollworm are found in the double seeds of cotton and can be killed easily by fumigating the seed in ginneries. Methyl bromide, phosphine and carbon bisulphide are the effective fumigants. If unfumigated seed is sown, the pest is carried over to the new crop.
In order to protect new orchards from infestation by the San Jose scale, Quadmspidiotus perniciosus (Comstock), it is advisable to fumigate nursery plants before dispatching them. Citrus plants should be similarly treated, so that the red citrus scale, Aonidiella aurantii (Maskell), is not allowed to spread.
Practice # 3. Seed Rate:
Adoption of appropriate seed rate ensures proper stand, spacing and crop canopy that helps in adoption of proper spray technology and checks the unwanted growth of crop. The traditional practice of using a high seed rate helps to maintain optimum plant stand and reduce insect damage in cereals.
Use of high seed rate is recommended in those crops where removal of infested plants is helpful in minimising the incidence of insect pests, viz. maize borer in maize and shoot fly in sorghum and other crops. The farmers in north India use 25-50 per cent less seed rate in cotton to avoid thinning. Low seed rate along with untimely rain and high temperature during sowing, drought conditions, termite attack and incidence of root rot, further reduce the plant stand.
The plant stand estimated at the time of harvest is 12-30 per cent less than the recommended plant population. The use of appropriate seed rate in cotton leaf curl virus affected areas helps to maintain proper crop stand even after rogueing of the diseased plants at an early stage. Besides this, too low or high seed rate adversely affects spraying operation.
Practice # 4. Irrigation:
When water is applied to the fields by flooding, a large number of insects present in the soil are drowned. Some of the others are driven out and exposed to their natural enemies. After lucerne is harvested, many lucerne caterpillars remain in the fields. A newly harvested field should be irrigated and the pest is killed by drowning. The cotton bollworms can be shaken off by dragging a rope over the plants and throwing them into standing water.
Sugarcane and wheat crops can be saved from the attack of white-ants by irrigating them. The potato tuber moth, Phthorimaea operculella (Zeller), is effectively suppressed by frequent overhead irrigation of potatoes. The stalk borer of sugarcane, Chilo auricilius Dudgeon, is more serious in flooded areas and, therefore, water should be controlled.
Flooding of fields has been recommended for reducing the attack of cutworms, armyworms, termites, whitegrubs, etc. On the other hand, draining the rice fields for 3-4 days during infestation controls brown planthopper and whorl maggot. Early termination of irrigation (by end of September) to hirsutum cotton in Punjab helps in reducing diapausing larval population of pink bollworm.
Sap sucking insects like aphids, jassids and whitefly are especially sensitive to changing water levels in their host plants. Maximum fecundity of mustard aphid reared on sarson (Brassica compestris Linnaeus cv. yellow sarson) and raya [B. juncea (Linnaeus) Czern.] host plants were recorded when the water level was maintained continuously above the field capacity.
The concentration of acidic amino acids and amides increased significantly under lower soil water regimes. Nutritional imbalance created by this higher concentration coupled with increasing burden on the excretory mechanisms of the aphid was mainly responsible for reduction in the fecundity of the aphid under such conditions. Due to this reason, economic threshold of the pest in the field was reached only at the highest soil water regime.
Indian mustard irrigated once harboured significantly higher aphid population than the unirrigated one. However, increase in number of irrigations from one to two did not result in a further increase in aphid incidence. In case of B. campestris cv. Chinese cabbage, the incidence of mustard aphid increased considerably with the increase in number of irrigations from 3 to 6.
The buildup of aphid in relation to irrigation levels may, thus, vary in different Brassica spp. Waterlogging enhances the multiplication of a number of borers including stalk borer, internode borer, plassey borer and whitefly on sugarcane crop. Increase in soil moisture level due to frequent irrigation also helped in buildup of Pyrilla population on this crop, while black bug damage is maximum in unirrigated fields.
Practice # 5. Fertilizers:
Healthy and vigorous plants are able to resist the attack of a given pest better and for a longer period than the sickly, undernourished plants. Plant growth can be stimulated with proper manuring. According to the requirements of a particular soil, NPK should be applied in the right proportions.
It has been observed that by the application of nitrogenous manures and increased irrigation, the incidence of whitefly of cotton is reduced significantly. However, in another locality, where the whitefly attack is not so serious, the excessive application of nitrogen might increase the attack of cotton jassid.
It is desirable to stimulate the growth of sugarcane during April-May with the application of ammonium sulphate, particularly after the shoots attacked by the top-borer have been cut mechanically. The application of manure at this stage also induces tillering.
Beneficial effect on the growth of potato crop by the application of Bordeaux mixture is well known. The crop so stimulated, becomes less susceptible to the attack of jassid, blight and other fungal diseases. Wheat grown in well-fertilized fields has a good stand and is comparatively free from the attack of molya disease, caused by a nematode.
High levels of nitrogenous fertilizers increase incidence of yellow stem borer, rice leaf folder and gall midge on rice; leaf folder, whitefly and bollworms on cotton; and internode borer, stalk borer and pyrilla on sugarcane. On the other hand, application of potash and sometimes phosphorous, either singly or in combination results in lower incidence of many insect pests, viz. Empoasca kerri Pruthi and Spdoptera litura (Fabricius) on cowpea; brown planthopper and whitebacked planthopper on rice; aphid and thrips on chillies, and Hypera variabilis Herbst and Aphis craccivora Koch on lucerne.
Practice # 6. Clean Culture:
Clean cultivation as a method for minimising the incidence of insect pests was advocated more than a century ago. Destroying or removing crop residues from fields is one of the basic ways to eliminate pest overwintering sites and reduce the spread of infestation. This may be achieved by ploughing directly or shredding and chopping, burning residues or raking and scooping them into piles for burning.
Removal of cotton sticks by 1st August every year was enforced under the Cotton Pests Act, 1911 which helped in reducing the incidence of pink bollworm in erstwhile Madras state. The incidence of boll weevil, Anthonomus grandis Boheman on cotton; European corn borer, Ostrinia nubilalis (Hubner) and South-western corn borer, Diatraea grandiosella Dyar in corn and Hessian fly, Mayetiola destructor (Say) in wheat in USA was reduced by sanitation measures.
Collecting and burning of stubble and chaffy panicles reduces the carryover of spotted stem borer, Chilo partellus (Swinhoe) and midge, Stenodiplosis sorghicola (Coquillett) in sorghum. Stalks from the previous season should be fed to cattle or burnt before the onset of monsoon rains to reduce the carryover of C. partellus.
Piling and burning of trash at dusk in the field attracts the adults of white grubs, Holotrichia consanguinea (Blanchard), and the red hairy caterpillar, Amsacta moorei (Butler), and kills them. This helps to reduce the oviposition and damage by these insects. Sanitation measures have helped to reduce the incidence of planthoppers, leafhoppers, leaf folder, rice hispa, gall midge and stem borer on rice crop.
Removal and destruction of rice stubble has been found highly effective in minimising overwintering populations of many species of stem borers. It has been estimated that stubble left in a field on one hectare with 6.8 per cent population of immature stages is a potential source of emergence of 1,20,000 moths.
Burning of rice stubble has been reported to reduce the stem borer infestation on the subsequent crop by 67 per cent. Removal of damaged plant parts and uprooting of infested plants including those showing deadhearts at thinning time have been recommended to lower maize borer, shoot fly, armyworm, pyrilla, aphids, cutworms and termite incidence in maize crop.
Detrashing of dry leaves from August onwards reduces the attack of pyrilla and scale insect on sugarcane crop. Detrashing at fifth, seventh and ninth month’s checks internode borer damage and detrashing in October and November results in the reduction of stalk borer damage.
Destruction of water shoots protects the crop from the ravages of stalk borer and internode borer. Burning of trash after harvest of the crop up to specific period before the pest migrates to nearby crop kills the carryover population of pyrilla, black bug and scale insect to some extent.
Avoiding ratooning of pigeonpea during off-season helps in reducing the carryover of pod fly and eriophyiid mite, Aceria cajani Channabasavanna, an important vector of pigeonpea sterility mosaic virus. Selective pruning of pigeonpea can be useful in reducing the incidence of leaf folder and spotted caterpillar.
Cotton sticks followed by cotton seeds are the major sources for the carryover of pink bollworm in the Punjab.
Following measures have been recommended to reduce the carryover population of pest:
i. After the last picking, sheep and goats should be allowed to graze on unopened left over green bolls and shed material in the field.
ii. The left over bolls, wherever feasible, should be removed manually before cutting the sticks. These bolls may be kept on roofs of houses, in thin layer for opening to extract good quality seed cotton and the remaining unopened and small bolls should be burnt.
iii. During summer months, on an average, 50.7 per cent (31.6-71.4%) of the larvae in cotton sticks died due to high atmospheric temperature which may go as high as 49°C in some years. The mortality of larvae in cotton stacks put in the open was higher (53.5%) than those kept under tree shade (32%). Similarly, the mortality of larvae in stacks stored in bundles in upright position was higher (53.6%) than in sticks stored untied horizontally (37.7%). Therefore, cotton sticks with left-over bolls should be stacked in the open away from tree shade in the villages, preferably after tying them in bundles and storing in upright position.
iv. The ginning of seed cotton and extraction of oil from cotton seeds should be accomplished by end of April and May, respectively, so that diapausing larvae in the seed cotton and cotton seeds are killed during these processes well before the onset of fruiting bodies in the ensueing cotton crop.
Undesirable plants in gardens give protection and provide food to the newly emerged nymphs of the mango mealybug, Drosicha mangiferae (Green), in January and February. For about one month, they feed on a number of weeds and then ascend the mango trees when fresh leaves appear.
The eradication of baru grass, Sorghum halepense, is very helpful in controlling sugarcane mites. The moths of the red hairy caterpillar, Amsacta moorei (Butler), lay eggs on the weed lahni, Heliotropium eichwaldi, and on emergence, and the caterpillars also feed on it.
Melon, guava, peach, tomato fruits, etc. which harbour maggots of fruit flies and tissue-borers should not be allowed to remain on the farm after marketable fruits have been harvested. If allowed to remain, the adults emerge and reinfest the standing crop.
Practice # 7. Crop Spacing:
The major objective in spacing crop plants is to obtain maximum high quality yield per unit area per unit time. But spacing may also influence the population and damage of many insect pests by modifying the micro-environment of the crop or affecting health, vigour and strength of the crop plants or pattern and duration of crop growth and development.
Closer spacing has been reported to increase the incidence of planthoppers, viz. brown planthopper and whitebacked planthopper, gall midge and leaf folder in rice crop. The percentage of plants showing hopper burn symptoms ranged from 100 in closer spacing (10 × 10 cm) to 7-67 in wider spacing (23 × 10 cm).
Detailed multilocation trials under All India Coordinated Rice Improvement Project showed that stem borer and gall midge were more in closer spacings (10 × 10 cm and 10 × 15 cm) than in wider spacings (20 × 15 cm, 20 × 20 cm and 30 x 30 cm). On the other hand, closer spacing resulted in lower incidence of green leafhopper, rice hispa and whorl maggot.
The closer spacing in cotton results in bushy growth of the crop that affects penetration of light, results in vertical growth of the plant hinders spraying operation and also results in higher relative humidity that favours higher incidence of sucking pests and bollworms. In closer plant spacing (45 × 30 cm), jassid population has been observed to be high as compared to wider spacing (60 × 45 cm).
Incidence of bollworms is also higher in closer plant spacing (15 cm) in hirsutum cotton varieties LH 886 and F 505 than wider plant spacing (30 cm). On the other hand, closer spacing resulted in lower incidence of green leafhopper, rice hispa and whorl maggot.
Closer spacing in groundnut lowered the incidence of thrips, jassids and leafminer, and increased parasitism in the latter. On the other hand, in cotton it increased the damage by jassid, whitefly and bollworms. In hirsutum cotton, the incidence of pink bollworm was 5.2 and 17.1 per cent at 75 × 20 cm and 50 × 20 cm, respectively.
Larval population of pod borer, H. armigerci on chickpea was 4 times as large at closest (33 plants/m2) as at widest (3 plants lm2) spacing. There are only a few advantages of more than 8 plants/m2, in chemically protected crop. In soybean, damage by a number of insect pests including Spilarctia obliqua (Walker), Melangromyzci sojae (Zehntner) and B. tabaci was greater in closely planted crop.
In sugarcane, closer spacing resulted in higher incidence of shoot, internode and stalk borers. In contrast to all these studies, population of Aphis craccivora Koch on chickpea was more in widely spaced crop (60 × 20 cm) than in closer spacing (30 × 10 cm).
Practice # 8. Crop Rotation:
There are two distinct systems of cropping, viz., specialized farming and mixed farming. Crops provide food for insect pests and, if food is abundant all the year round, they flourish and soon increase in number. Their abundance depends on the fecundity, hibernation, number of generations completed in a year, dispersion ability, etc. Therefore, pest problems in an area where there is specialized farming will be different from that in the areas of mixed farming.
Where there is a single-cropping pattern, the specific pests having a limited power of migration, breed slowly, particularly those spending a long time in the feeding stage and having only a few generations per year. They take number of years to become abundant and their multiplication can be checked by introducing crop rotation.
Some insects, however, which have greater mobility and which are omnivorous, can appear as pests in the very first year of planting a new crop. Crop rotation in that case does not materially affect multiplication. If the potato tuber moth, Phthorimaea operculella (Zeller), and nematodes become serious in a given locality, it is recommended to discontinue the cultivation of potatoes for three or four years.
In the case of the golden nematode of potato, Heterodera rostochiensis Woll., this is the only practical method of control known so far. The underlying principle in crop rotation is to starve the pest. It is, therefore, essential that there should be a discontinuity in the supply of food. In other words, the cropping scheme should be such that alternative hosts are eliminated in one season or another.
From the entomological point of view, the following rotations are unsound:
(i) Guara-wheat-toria (Brassica campestris). Any delay in the burial of guara (Cyamopsis tetragonoloba) will attract termites and may jeopardize the chances of growing a successful wheat crop.
(ii) Sugarcane-maize-cotton. Termites are attracted to the dried roots of cotton and they attack sugarcane setts later on.
(iii) Maize should not be sown on grassland (both belong to the family Gramineae), because soil insects, white grubs, cutworms, wireworms, etc., are common to both.
The following rotations are considered useful:
(i) Maize-senji (Me I Hot us parviflora)-sugarcane is a good rotation from the entomological point of view.
(ii) For protection against cattle ticks, the pastures should be kept free from animals for a year, so that the ticks may be starved to death in 8-9 months.
(iii) The incidence of attack of the sugarcane black bug, Cavelerius excavatus (Distant) and tissue borers is greater on the ratoon crop than on the planted crop.
(iv) Rotation of groundnut with non-leguminous crops in recommended for minimising the damage by leafminer.
(v) Cotton should be rotated with non-preferred hosts like ragi, maize, rice, groundnut, cowpeas or soybean to minimise the incidence of insect pests.
Practice # 9. Intercropping:
A carefully selected cropping system (intercropping or mixed cropping) can be used to reduce pest incidence, and minimize risks of monocultures. Some of the examples of intercropping systems that help in preventing pest outbreaks are listed in Table 4.1.
Table 4.1 Selected example of intercropping systems that help to prevent insect pest outbreaks:
Intercropping System:
i. Beans grown in relay intercropping with winter wheat Brassica crops and beans.
ii. Cabbage intercropped with white and red clover.
iii. Intercropping of pigeonpea with red, black and green gram Cassava Intercropped with cowpeas.
iv. Maize intercropped with fava beans and squash.
v. Maize intercropped with soybean.
vi. Maize intercropped with sweet potatoes.
vii. Intercropping maize and beans.
viii. Cotton intercropped with forage cowpea.
ix. Intercropping cotton with sorghum or maize.
x. Cucumbers intercropped with maize and broccoli.
xi. Groundnut intercropped with field beans.
xii. Groundnut intercropped with maize.
xiii. Maize-bean intercropping.
xiv. Strip cropping of muskmelons with wheat.
xv. Oats intercropped with field beans.
xvi. Sesame intercropped with com or sorghum.
xvii. Tomato and tobacco intercropped with cabbage.
xviii. Tomato intercropped with cabbage.
Pest(S) Regulated:
i. Empoasca fabae (Harris) and Aphis fabae Scopoli.
ii. Brevicoryne brassicae (Linnaeus) and Delia radicum (Linnaeus) D. radicum, cabbage aphids, and imported cabbage butterfly. Pieris rapae (Linnaeus).
iii. Pod borers, jassids, and membracids.
iv. Whiteflies, Aleurotrachelus socialis Bondar and Trialeurodes variabilis (Quaintance).
v. Aphids; mite, Tetranychus unticae (Koch) and Macrodactylus sp. European corn borer. O. nubilalis.
vi. Leaf beetles. Diabrolica spp.: leafhoppers, Agallia lingula Van Duzee.
vii. Dalbulus maidis (Delong & Wolcott).
viii. Boll weevil, Anthonomus grandis Boheman.
ix. Corn earworm, Helicoverpa zea (Boddie).
x. Acalymma vittatum (Fabricius).
xi. Aphis craccivora Koch.
xii. Corn borer, Ostrinia furnacalis (Guenee).
xiii. S. frugiperda and Diatraea lineolata (Walker).
xiv. Mysus persicae (Sulzer).
xv. Rhopalosiphum sp.
xvi. Webworms (Antigastra sp.).
xvii. Flea beetles, Phyllotreta cruciferae (Goeze).
xviii. Diamondback moth, Plutella xylostella (Linnaeus).
Factor(S) Involved:
i. Impairment of visual searching behaviour of dispersing aphids.
ii. Higher predation and disruption of oviposition behaviour.
iii. Interference with colonization and increase of ground beetles.
iv. Delayed colonization of herbivores.
v. Changes in plant vigour and increased abundance of natural enemies.
vi. Enhanced abundance of predators.
vii. Differences in corn varietal resistance.
viii. Increase in parasitic wasps.
ix. Interference with leafhopper movement.
x. Population increase of parasitic wasps (Eurytoma sp.) increased abundance of predators.
xi. Interference with movement and tenure time on host plants.
xii. Aphids trapped on epidermal hair of beans.
xiii. Abundance of spiders (Lycosa sp.).
xiv. Lower oviposition rates, trap cropping.
xv. Interference with aphid dispersal Interference with aphid dispersal.
xvi. Shading by the taller companion crop.
xvii. Feeding inhibition by odours from non-host plants.
xviii. Chemical repellency or masking.
In central and southern India, intercropping of cotton with black gram, green gram, onion, cowpeas, etc. is reported to divert the population of sucking pests and American bollworm from cotton. Monoculture of cotton was also found to harbour more insect pests than cotton intercropped with groundnut, cowpea and soybean.
The intercrop of cowpeas in cotton helped in the colonization of coccinellids and also enhanced the parasitism of spotted bollworm. On the other hand, okra, mung and pigeonpea as intercrops with cotton increased the population buildup of jassid, whitefly, spotted bollworms and American bollworm.
Intercropping of groundnut with pearlmillet reduces the incidence of thrips, jassid and leaf- miner. Intercropping of taramira in raya reduces the incidence of mustard aphid in the latter crop due to allelopathic influence of the former. Intercropping of chickpea with barley, wheat, linseed, mustard and sunflower is also known to reduce infestation of H. armigera in chickpea.
The damage by sorghum shoot fly, Atherigona soccata Rondani and midge, Stenodiplosis sorghicola (Coquillett) is reduced when sorghum is intercropped with leguminous crops. Intercropping sorghum with cowpea or lablab reduced the damage by spotted stem borer, Chilo partellus (Swinhoe) by 50 per cent and increased the grain yield by 10-12 per cent over a single crop of sorghum. Intercropping sorghum with pigeonpea reduces the damage by H. armigera in pigeonpea. Intercropping red clover with maize also reduces the damage by the European corn borer, Ostrinia nubilalis (Hubner).
Intercropping of groundnut with pearlmillet reduced the incidence of thrips, jassid and leaf miner whereas intercropping with sunflower and castor increased the incidence of thrips and jassid, respectively. When pearlmillet was grown as an intercrop in groundnut, the parasitic activity of Goniozus sp. was considerably enhanced. The pollen grains of the millet were preferably used as food by the adult parasitoids.
Tomato intercropped with cabbage has been reported to inhibit or reduce egg laying by diamondback moth. A planting pattern of one row of cabbage and one row of tomato (cabbage planted 30 days later than tomato), caused maximum reduction of diamondback moth and leafwebber larvae on cabbage.
Reduction in insect incidence was attributed to possible release of volatile substances from late crop growth stages of tomato which inhibited opposition by incoming moths. Even the female moths which entered the mixed crop laid fewer eggs probably because of tomato foliage spread over every row of cabbage.
The cowpea intercropped with sorghum significantly decreased the incidence of aphid, Aphis cractivora Koch and thrips, Megalarothrips sjostedi Trybom. Two varieties of cowpea, Kanannado white and Kanannado brown were sown solo and intercropped with sorghum. This reduction in population may be due to the micro-environmental effect of the associated crop which may attract predators and or disruption of insect visual search for preferred host.
Carrot intercropped with lucerne has been shown to suffer less damage by the rust fly, Psila rosae (Fabricius). Intercropping bean with collards decreases flea beetle, Phyllotreta cruciferae (Goeze) densities on collards and minimizes the leaf damage. Intercropping of taramira in raya reduced the incidence of mustard aphid in the latter crop due to the allelopathic influence of the former.
Similarly, trials conducted under the All India Coordinated Pulses Improvement Project at several locations demonstrated that the sole crop of chickpea attracted more H. armigera compared to intercrops with wheat, barley, linseed, mustard and safflower. On the other hand, lentil and field peas as inter-crops enhanced infestation in chickpea. Crop mixtures were more effective than row plantings.
In Kenya, intercropping maize with the non-host molasses grass, Melinus minutiflora Beauv decreased levels of infestation by stem borers, C. partellus and Busseola fusca Fuller, and also increased larval parasitization of stem borers by Apanteles sesamiae Cameron.
In field trials, M. minutiflora planted in alternate rows with maize significantly reduced stem borer infestation of the main crop (damaged maize plants: single crop 39.2%, intercropped with M. minutiflora 4.6%). There was also a significant increase in parasitization by the larval parasitoid, A. sesamiae (parasitized larvae in maize: single crop 5.4%, maize with M. minutiflora intercrop 20.7%). Volatile agents produced by M. minutiflora repelled female stem borers and attracted foraging female A. sesamiae.
Practice # 10. Trap Cropping:
Trap cropping is the planting of a trap crop to protect the main cash crop from a certain pest or several pests. The trap crop can be from the same or different family group, than that of the main crop, as long as it is more attractive to the pest. Even early or late plantings of the same crop in the main crop may also serve as traps. Thus trap crops are plant stands that are grown to attract insects or other organisms so that the target crop escapes pest attack.
Protection is achieved either by preventing the pests from reaching the crop or by concentrating them in certain part of the field where they can easily be destroyed. The attractiveness of trap crops may be enhanced by use of insect pheromones, plant kairomones or insect food supplements. Depending on the seasonal cycle, insects may be left to develop in the trap or killed with an insecticide.
There are two types of planting the trap crops; perimeter trap cropping and row intercropping. Perimeter trap cropping (border trap cropping) is the planting of trap crop completely surrounding the main cash crop. It prevents a pest attack that comes from all sides of the field. It works best on pests that are found near the borderline to the farm. Row intercropping is the planting of the trap crop in alternating rows within the main crop.
The major benefit of trap cropping is that insecticides are seldom required to be used on the main crop and this enhances the natural control of pests. Moreover, trap crops may also attract natural enemies thus enhancing natural control. The overall use of pesticides is clearly less than in conventional farming, making the strategy environmentally attractive.
However, it needs to be emphasized here that the trap crop may also serve as a pest nursery for the target pest or some other pest(s). Some highly mobile natural enemies may be attracted to and aggregate on the trap crop just as on their host or prey. They are likely to be destroyed by insecticide sprays. A thorough understanding of the agro-ecocystem is, therefore, essential for recommending trap cropping as a means for minimising pest damage.
Globally, trap cropping has proved successful in experimental studies in a number of crops and has been recommended for large scale utilization. Trap cropping has been successfully employed on a large scale in four crop ecosystems, viz. cotton and soybean in USA, potatoes in CIS (erstwhile USSR) and Bulgaria, and cauliflower in Finland.
In cotton/sesame intercrop trials in USA, row strips of sesame, constituting 5 per cent of the total acreage, were used as a trap crop to attract Heliothis spp. from the main crop of cotton. Sesame, which is highly attractive to Heliothis species from the seedling stage to senescence, attracted large numbers of insects away from the cotton. It also attracted the parasitoid, Campoletis sonorensis (Cameron) which ultimately parasitized large numbers of Heliothis insects.
Sesame, sunflower, marigold and carrot can be used as trap crops for H. armigera. In rice, trap cropping of rice for green leafhopper control resulted in 12 per cent higher economic return than chemical control and 29 per cent higher than the untreated control. The possibility of use of Indian mustard as preferred crop of Plutella xylostella (Linnaeus) was investigated. Indian mustard was found to be suitable trap crop for P. xylostella.
Planting Sudan grass (a commercial fodder grass), Sorghum vulgare sudanense (Piper) Hitchc around maize fields reduced infestation on maize by C. partellus and B. fusca in Keyna, as considerable number of stem borers were trapped on Sudan grass. There was increase in maize yield (4.45 t/ha) due to reduction in stem borer damage as compared to control (3.44 t/ha).
Planting grass around maize fields also increased parasitization of borers by Apanteles flavipes (Cameron) and A. sesamiae Cameron. As compared to maize mono field where only 4.8 per cent C. partellus and 0.5 per cent B. fusca larvae were parasitized, 18.9 per cent C. partellus and 6.17 per cent B. fusca larvae were parasitized in maize surrounded by grass.
Okra can be used as a trap crop around cotton for cotton jassid, American bollworm and spotted bollworms. The infested fruits that have large population of bollworms should be removed periodically and destroyed, and jassid can be easily controlled by spraying with any systemic insecticides. Marigold and Nicotiana rustica Linnaeus grown as trap crops are also preferred hosts of H. armigera. Planting of castor as trap crop diverts the population of Spodoptera litura (Fabricius) from cotton.
Some early crops are sown in narrow strips around a major crop to serve as a trap for the pests that might be common to both. The trap crop can either be harvested early or otherwise cut and used as fodder. The preferred host plants can also be grown around valuable crops, and when the pest has appeared, they can be cut and destroyed.
Sesame can be sown around the cotton field to attract the red hairy caterpillar, Amsacta moorei (Butler) and the Bihar hairy caterpillar, Spilarctia ohliqua (Walker). The cotton grey weevil, Myllocerus undecimpustulatus Faust, has a marked preference for arhar (Cajanus cajan), which can be sown as a trap crop.
Bold seeded mustard (Brassica juncea), when used as a trap crop in cabbage, attracts 80-90 per cent diamond back moth population. The use of African marigold (Tagitus erecta) as a trap crop is quite effective for the control of tomato fruit borer.
The major benefit of trap cropping is that insecticides are seldom required to be used on the main crop and this enhances the natural control of pests. Moreover, trap crops may also attract natural enemies thus enhancing natural control. The overall use of pesticides is clearly less than in conventional farming, making the strategy environmentally acceptable.
However, it needs to be emphasized that the trap crop may also serve a pest nursery for the target pest or some other pests. Some highly mobile natural enemies may be attracted to and aggregate on the trap crop just as on their host or prey and they are likely to be destroyed by insecticide sprays. A thorough understanding of the agro-ecosystem is, therefore, utmost essential for recommending trap cropping as a means for minimising pest damage.
Practice # 11. Presence of Weeds:
The presence of diverse vegetation within or near the field may add essential resources for predators or parasitoids and so enable them to find all their requirements near the pest population. Such resources include food, cover or alternate prey. Conversely, weeds may also adversely affect the orientation of predators and parasitoids to their prey.
Many common weeds also act as hosts for opposition, and provide a better ecological niche for the insects to hide, thus shielding them from natural enemies and insecticide sprays. The common vetch or garden vetch, Vicia sativa Linnaeus is a common leguminous weed associated with chickpea in northern India.
Removal of the weed at a time when maximum eggs are laid substantially reduces the incidence of pod borer, H. armigera. Weeds may also aid in chemical repellence, interference in movement of insects or provide alternate host for natural enemies.
Practice # 12. Pruning and Thinning:
Some pests are normally carried from the old crop to the new one. It is particularly so in the case of perennial plants, such as the fruit-trees. Proper pruning of the undesirable portions of citrus plants is useful for keeping under check the citrus leaf-miner, Phyllocnistis citrella Stainton; citrus psylla, Diaphorina citri Kuwayana; the citrus red scale, Aonidiella aurantii (Maskell); San lose scale, Quadraspidiotus perniciosus (Comstock); woolly apple aphid, Eriosoma lanigerum (Hausmann) and peach leafcurl aphid, Brachycaudus helichrysi (Kaltenbach) of stone-fruits.
Practice # 13. Time of Sowing and Harvesting:
By adjusting the time of planting, infestation by some pests can be prevented ; the egg-laying period of a particular pest can be avoided; young plants can be established before the attack starts; short duration crops can allow the minimum possible time for pests to multiply or they can mature before the pest appears.
Similarly, by adjusting the time of harvesting, a crop can be saved from attack of the pest which might become abundant rather late in the season or the pests can be killed before they have completed their life-cycle. Where it is desirable to have a crop mature early, the purpose can be achieved by fertilizer application, by adjusting irrigation or by making a varietal selection for early maturity.
Early sowing can be used to minimise the damage to chickpea from Helicoverpa armigera (Hubner) in northern India. Two peaks of H. armigera occur during December and March in the rabi season. During the second peak, the pest inflicts severe damage to chickpea crop. Early (October) sown crop escapes with least damage. Late-sowing (December and January) matures during late March to April and suffers heavy damage. November sown crop also suffers moderate damage.
If maize meant for seed is sown after the 15th of August in the Punjab, it escapes a heavy attack of the maize-borer. Moreover, the borers in August and September are heavily attacked by their parasites and, as a result, the pest remains under control. The early sowing of rice in the Punjab between the 3rd week of May and mid-June is helpful in protecting it from the attack of rice-borer, Scirpophaga incertulas (Walker). The incidence of whitefly, Bemisia tabaci (Gennadius), is also severer on the early sown cotton than on the late-sown crop.
Another example to illustrate the importance of the time of sowing is that of wheat in America to save it from attack of the Hessian fly, Mayetiola destructor (Say). Hessian fly eggs are laid on the upper surface of young winter wheat leaves. By delayed planting, suitable hosts are not available until most of the overwintering adult flies emerge and die.
It is recommended that sugarcane should be harvested before mid-February, when moths of the top-borer appear on wings. If it is not possible to harvest the crop, the terminal portions of cane should be cut and fed to cattle. The maize borer, Chilo partellus (Swinhoe), which hibernates in the stalks of the crop, should be killed before its emergence in spring, by chopping up stalks which are kept for use as fodder.
The attack of mustard aphid on the seed pods of sarson, toria (Brassica campestris) and other cruciferous crops is severe in January-February. In certain years, the attack is so heavy that seeds are not formed at all. Early maturing varieties or the early sowing of these crops is the best solution for protection against this fast-reproducing pest. Damage of pod-borer, H. armigera, in chickpea and pigeonpea can be reduced by growing early maturing varieties.
Practice # 14. Destruction of Crop Residues:
A great majority of insect pests, particularly those which hibernate, are found in various portions of the plants remaining in the fields or orchards. After spending their inactive period in such crop residues, they emerge as adults, more or less at the same time as the new crop is grown. Therefore, it is most logical to destroy such remnants of the crops thoroughly and systematically.
After the crops are over, the stubble of rice and sugarcane should be ploughed up, collected and burnt, so that the hibernating borers are destroyed.
The cotton stem borer, Sphenoptera gossypii Cotes, hibernates in the lower portion of the stalks. If cotton sticks are cut below the ground level and are used as firewood, the pest can be destroyed. Deep harvesting of sugarcane fields which are to be ratooned, provides protection to the crops from root borer and scale insect damage.
The old and dry cucurbit creepers and other debris are sometimes allowed to remain in the fields. A number of insects, like the brinjal hadda beetle, Epilachna spp.; red pumpkin beetle, Raphidopalpa foveicollis (Ludas); the dusky cotton bug, Oxycarenus laetus Kirby, and a host of other pests hibernate in crop remnants. Their destruction, immediately after the crop is over, will go a long way in reducing insect population.
Infested fruits of guava, melon, peach, plum, apricot, etc. which contains maggots of fruit flies and the fruits of walnut which contain grubs of the walnut weevil should be collected and burnt or buried deep in the soil.