Learn about the pests that damage pulse crops and its control.
1. Bean Fly, Ophiomyia Phaseoli (Tryon) (Diptera: Agromyzidae):
The bean fly has been found to infest the streak bean (Pliaseolus vulgaris), cowpea (Vigna sinesis), lima bean (Phaseolus lunatus), soybean (Soja hispida), Cajanus, Canavalia and Dolichos in Sri Lanka, India, China, Indonesia, Malaysia, the Philippines, Singapore and many African countries. It is a very minute insect, body length of the male being about 1.9 mm and that of the female 2.2 mm. The general colour is shiny black except for eggs, wing veins and antennae which are light brown. The maggot is creamy in colour and apodous in form.
The flies are seen as soon as the host crops are available in the field. The slender, white eggs are laid singly in holes made on the upper surface of young leaves, especially near the petiole end of the leaf.
On hatching, the maggot forms a short linear leaf mine and further on it tunnels underneath the epidermis of the leaf until it reaches one of the veins which leads it to the midrib and then to the leaf stalk and the stem. Pupation takes place inside the stem. The barrel shaped pupae are black and about 3 mm long. The total life cycle takes 2-3 weeks. As many as seven generations of this pest have been reported during the active season of the pest infestation.
As a result of severe infestation, the leaves turn yellow, giving the plants a dry appearance. The stems turn brown, become swollen and break down. The spring crops usually suffer less infestation than the late summer crops in which infested plants may constitute over 70 per cent of the plant population. The attacked plants bear less pods which are mostly empty or else their seeds may be very small.
(i) Apply 625 ml of monocrotophos 40EC or 750 ml of oxydemeton methyl 25EC in 625 litres of water per ha at 15-day intervals during flowering stage,
(ii) Soil application of 10 kg of phorate 10G is effective upto 40 days of sowing.
2. Green Nettle Slug Caterpillar, Thosea Aperiens (Walker) (Lepidoptera: Cochlidiidae):
In India this insect is a pest of sorghum, pigeonpea, red gram, Eleusine coracana (finger millet), Cassia auriculata (Tanner’s cassia), cowpea and tamarind. In Sri Lanka, it is reported to feed on Dunbaria heynei, an herbaceous climber.
The greenish larvae which are 30 mm in length, feed on leaves. They have stinging hairs on their body and are thus a nuisance to the field workers, in addition to damaging the plants. Fore wings of the adult moths are dark, cinereous-brown, whereas the hind wings are pale in colour. The wing expanse may vary from 30 to 35 mm.
The moths are active during October-November and lay eggs on host plants. The caterpillars on emergence feed on leaves till the end of December. When full-grown, they bore into the soil, construct a cocoon and hibernate there in the larval stage. During the following August- September, they pupate and subsequently emerge as moths. The period from the onset of pupation and the emergence as moths takes about 30 days. Normally, one life-cycle is completed in a year.
The larvae defoliate the plants. The field workers when accidentally touch the stinging hair of the caterpillars may develop painful rashes on their hands and arms.
3. Girdle Beetle, Obereopsis Brevis (Gahan) (Coleoptera: Cerambycidae):
This is an important pest of soybean and also attacks lablab and cowpea. The adult is a small black beetle.
The ovipositioning female beetle girdles the stem twice and makes 3 punctures just above the lower ring before inserting a single egg through the largest whole into the pith. This results in dropping of the upper part of the stem. A female beetle lays 7-13 eggs and they hatch in 4-5 days.
The larva tunnels upwards and downwards within the stem and a single larva can destroy the whole plant. The larval period lasts 34-47 days. Over wintering takes place as the full- grown larva within the feeding tunnel in a gall-like chamber near the base of the plant in the girdled portion of the stem which has fallen out or under plant debris. The pupal period is 8-11 days.
The female of this insect feeds on the xylem of the stem. The larvae further damage the stem and make tunnels inside and fill these with excreta. The leaves and the growing points dry up. The broken stems can be seen in the field.
4. Leafhopper, Empoasca Kerri Pruthi (Hemiptera: Cicadellidae):
Leaf hoppers belonging to genus Empoasca comprise over 100 species, but E. kerri is the most common species feeding on various pulse crops like pigeonpea, cowpea, mungbean, ricebean, etc. Adults are small green insects, 2.5 mm long and fly when disturbed. The nymphs and adults have a similar shape and colour, but the nymphs do not have wings and run sideways when disturbed.
Eggs are laid singly within the leaf veins on the upper surface of the leaf which hatch in 3-10 days. Nymphs feed on the lower surface of the leaves. Nymphal period lasts for 7-20 days. Adults live up to 3 months. Life-cycle is completed in 15-45 days and up to 10-12 generations are found in a year. Intermittent rainfall, cloudy weather and high temperature favour the multiplication of this pest and hence the damage caused during kharif season is severe than rabi or summer/spring crop.
Both nymphs and adults suck the sap from the plants which results in characteristic yellow discolouration of leaf edges and tips followed by cupping of the leaves. In case of heavy infestation, the leaves roll down at the edges, dry and fall down. Seedlings that have sustained considerable feeding by jassids may be stunted and have red-brown leaflets followed by defoliation.
5. Groundnut Aphid, Aphis Craccivora Koch (Hemiptera: Aphididae):
Several species of aphids are known to attack pulse crops, among which A. craccivora is the most prevalent species on pigeonpea, chickpea, lentil, peas and beans. Aphids colonize the young shoots, flowers and pods. Young leaves of seedlings become twisted under heavy infestation. Seedlings may wilt, particularly under moisture-stressed conditions.
In case of severe infestation, leaves and shoots get deformed and stunted, and sticky honeydew may be deposited over the leaf surface. Chickpea stunt disease caused by pea leaf-roll virus is transmitted by A. craccivora. Stunt disease limits plant growth, rendering leaflets small and reddish brown.