The most desirable form of insect resistance is one which is stable across location and seasons. However, the level and nature of insect resistance are influenced by several climatic and edaphic factors. Several environmental factors are known to influence the inherited characters, especially those involving physiological characteristics of crop plants.
Following are the factors which affects the host plant resistance:
Factor # 1. Temperature:
Temperature is one of the most important physical factors of the environment affecting the behaviour and physiological interactions of insects and plants. Temperature-induced stress can cause changes in plant physiology and affect the expression of genetic resistance, resulting in changes in morphological defenses and/or changes in the levels of biochemicals or nutritional quality of the host plant.
Temperature also has pronounced effect on plant growth and indirectly influences the extent of damage. Temperature also affects the biology and behaviour of insects and thus influences insect growth and population build-up. In general, low temperatures cause a negative effect on resistance.
Differences between resistant and susceptible genotypes of sorghum to green bug increase with an increase in temperature. The level of resistance to pea aphid and alfalfa aphid in alfalfa is greater at higher temperatures. On the other hand, progressive loss of resistance to Hessian fly in wheat has been reported at temperatures higher than 18°C.
Factor # 2. Soil Moisture:
The level of plant resistance is influenced by water stress. High levels of water stress reduce the damage by sorghum shoot fly, Atherigona soccata Rondani. The populations of Aphis fabae Scopoli have lower rates of reproduction on water stressed plants. However, water stressed plants suffer greater damage due to aphid feeding.
Thus, moisture stress can alter the plant reaction to insect damage leading either to increased or lower susceptibility to insect damage. Atmospheric humidity also influences the insect-plant interaction. High humidity increases the ease of detecting of odours which may influence host finding and antixenosis mechanism of resistance to insects.
Factor # 3. Photoperiod:
Photoperiod affects the development of both the crop plants and their insect pests. It may also bring about changes in the physico-chemical characteristics of crop plants and thus influence the interaction between insects and plants. Intensity and quality of light have been reported to influence biosynthesis of anthocyanins and phenylpropanoids.
Continuous high intensity light is reported to induce susceptibility in resistant PI 227687 soybean plants to cabbage looper, Trichoplusia ni (Hubner). Reduction in light intensity leads to loss in resistance of some wheat cultivars to wheat stem saw fly by decreasing stem solidness and density.
Shade-induced loss of resistance has also been reported in sugarbeet genotypes resistant to the green peach aphid. Similarly, shading has been known to reduce resistance in potato genotypes to Colorado potato beetle, which is due to reduced levels of steroidal glycosides, some of which are known to limit growth and development of beetle.
Factor # 4. Nutrients:
Plant nutrition exerts pronounced effect on resistance of plants to insects. In several cases, high levels of nutrients increase the susceptibility, whereas in others they increase the level of resistance. Application of nitrogenous fertilizers decreases the damage by shootfly and stem borer, Chilo partellus (Swinhoe) in sorghum.
Application of phosphatic fertilizers is reported to decrease the damage of shoot fly. Similarly, application of potash decreases the incidence of sugarcane top borer, Scirpophaga excerptalis (Walker), and several rice insect pests like yellow stem borer, brown planthopper, green leafhopper, leaf folder, whorl maggot and thrips.
However, high levels of nitrogen are known to increase the damage by cotton jassid, Amrasca biguttula biguttula (Ishida); rice stem borer, Scirpophaga incertulas (Walker) and many other insect pests of agricultural crops. In alfalfa, excess levels of nitrogen and magnesium, and low levels of calcium and potassium, have been associated with loss of resistance to the spotted alfalfa aphid.
Factor # 5. Soil pH:
Plants can grow in soils over a pH range of 3.0 to 9.0, but extreme conditions stress the plants. Foliar damage to whorl-stage sorghum by the fall armyworm, Spodoptera frugiperda (J.E. Smith) was greater in acidic soils (pH 5.4) than in plants grown in soils with a pH higher than 6.0. Salinity induces plant responses that can alter the suitability of a plant as a host for insects.
Salinity stress at 1.25 m-1 increased nitrogen, decreased potassium and reduced the production of allelochemicals in rice-plant leaf sheaths. The salinity-stressed rice plants were conducive to the feeding and survival of the whitebacked planthopper, Sogatella furcifera (Horvath). The salinity stress led to increase in insects’ growth and development, longevity, fecundity and population build-up.
Factor # 6. Air Pollution:
Air pollution refers to the presence in the outdoor atmosphere of one or more contaminants such as dust, fumes, gas, mist, odour, smoke or vapour in such quantities and duration as to be injurious to human, plant, or animal life or to property. Air pollution has been demonstrated to influence insect-plant interactions. When Mexican bean beetles were fed on soybean fumigated intermittently with SO2, the mean number of beetle progeny was 1.5 times greater than that of beetles feeding on the control plants.
Mexican bean beetles under laboratory conditions preferred to eat soybean leaves pre-fumigated with ozone-enriched ambient air. Substantial modifications in the form and content of plant nitrogen and sugars have been reported following plant exposure to moderate levels of ozone. Such nutritionally enriched plants are better hosts for insect herbivores, especially aphids.
Factor # 7. Plant Factors:
Insects have often exhibited feeding preferences for particular plant organs or for foliage of certain specific age. Moreover, variations in physical growth conditions such as plant canopy and plant height may influence host selection and subsequent population development.
For example, Myzus persicae (Sulzer) was relatively more successful in feeding on older leaves of Brussels sprouts than Brevicoryne brassicae (Linnaeus) and vice-versa. The early-maturing genotypes of soybean show greater defoliation by soybean caterpillar, Anticarsia gemmatalis (Hubner); soybean looper, Chiyspdeixis includens (Walker) and beet armyworm, Spodoptera exigua (Hubner).
The damage by the sorghum midge, Stenodiplosis sorghicola (Coquillett) on sorghum hybrid CSH5 was higher in plots with low plant densities. In contrast, infestation by wheat stem sawfly, Cephus pygmeus (Linnaeus) was higher in low plant densities.
Wheat plants sown at low densities (40 × 40 cm, 2.5 kg seed ha-1) had a longer interval between plant development stages and had higher stem solidness than those sown at higher densities (10 × 3 cm, 133 kg seed ha-1 ). Stem solidness was negatively correlated with the percentage of sawfly-infested stems.
Factor # 8. Insect Factors:
The age, sex and levels of infestation of insects affect the preference for host plants with varying levels of resistance. Young (3-day old) female Mexican bean beetles showed no preference for leaves of susceptible Deane soybean over resistant soybean leaves.
In contrast, 14-day old Mexican bean beetle females showed a marked preference for leaves of susceptible plants. In feeding bioassays, the female beetles demonstrated a distinct preference for leaves of susceptible soybean over the resistant soybean leaves, but males did not.
The size of sorghum shoot fly, Atherigona varia (Meigen) population had a pronounced effect on both absolute and relative rates of opposition in various sorghum genotypes. The resistance was partially dominant when evaluated under low shootfly populations, however, when evaluated under high populations, susceptibility appeared dominant.