Lepidoptera (butterflies and moths) have a caterpillar (larval) stage that causes the most damage by chewing and boring, while the adult, fruit piercing moth may be a pest on some ripe fruits. Most adult Lepidoptera have long, siphoning, tube like mouthparts to feed on plant nectar. Larval (caterpillar) stages have chewing mouthparts; most have three pairs of thoracic legs and five or less pairs of abdominal prologs. Most larvae feed on leaves by leaf mining or bore into stems and fruits. Some Lepidoptera have successfully used to control weeds, such as some cactus species. Some pupae forms are distinctive of the species or family. The adults are active at night and often are attracted to light.
Moths include common pest such as
- Cabbage looper
- Diamondback moth
- American bollworm
Diamondback moth (Plutella xylostella)
Diamondblack moth (DBM) is a major pest of the Brassica family of all species and a wide range of wild range of wild and cultivated cruciferae plants.
Eggs are usually laid on the upper leaf surface singly or in groups. The newly hatched larvae caterpillars crawl to the underside of the leaf and penetrate the epidermis and during the 1st instar they mine in the leaf tissues and feeds on the leaf. The 3rd instar feed on the underside of the leaf making large holes right through the leaf, leaving a windowing effect. Serious attack usually occurs during the dry season when the stress and its not able to able outgrow the damage caused by the pest. The adults are small grey moths with a wing span if 15mm. They have three diamond-shaped pale spots down the middle of the wing at the back hence the diamondback moth. The adult lay from 50 to 180 eggs. The life cycles vary depending on the weather. During the hot season, the cycles are short while during the cold season they are long. In a year 20overlapping generations have been recorded.
The freshly laid eggs are oval in shape, light yellow in colour and measures approximately 0.49mm in length by 0.26mm in width. The eggs are laid singly or in groups of three or four on the leaves of the host plant. The eggs darken just before hatching and the young larvae can be seen coiled beneath the chorion. The first instar larva hatches after 3 to 5 days wanders for a short distance in search of suitable leaf tissue in which to mine. The larva mines into the spongy mesophyll leaf tissues where feeding continues until the second instar. The second instar exists from the mines and starts feeding on the underside of the leaf. Third and fourth instar larvae, which are pale green and about 9 mm long, also feed on the lower leaf surface. The larvae can be distinguishes from other species their habit of actively dropping from the leaf on silken thread when disturbed. The larval period lasts 8-15 days depending on the temperature. The pupa is brown and encased in a delicate, netlike cacoon on the leaves and the pupal period lasts 3-6 days. Female adult longevity ranges from 7 to 47 days, with an average 16.2 days while male longevity ranges from 3 to 58 days and an average of 12.1n days. Thus, the development from egg-adult from 12 to 20 days depending on environmental conditions.
DBM is difficult to control because of its intrinsic biology and ecology. Control by chemical or biological dead equate levels of control, growers can also grow on bacterial insectcides, variety selection, parasitoids and planting schedules. Interrmittent overhead irrigation provides effective economic control of diamondblack moth by disrupting adult flight, mating and oviposition, and to some extent washes off the larva. The best time for overhead irrigation is late in the afternoon when the mothsâ€™ activities are high. Unfortunately, diseases tend to become a major problem to crops. Crop residue destruction immediately after harvest helps prevent the bulid up of diamondblack moth and subsequent migrations to younger plants in the adjacent fields once they are planted. Parasites and predators play a dominant role in the biological control of DBM. Biological control methods usually used include classical, biological control, augmentation, inoculation, inundation and natural enemy conservation. Some of the parasitoids used in the control of DBM include the Diadegma, Cotesia and Oomyzus species.
The pests can be controlled by the following insectcides
American bollworm (Helicoverpa armigera )
It is a serious world-wide pest. It attacks wide range of crops namely maize, tomatoes, tobacco, pigeon peas, cowpeas. Heavy attacks occur during flower period.
Eggs are laid singly on the upper leaf surface and flowers buds. Eggs are roughly spherical in shape and about 0.5 mm in diameter. They are yellowish when fleshly laid and turn brown as the embryo develops. Hatching takes place after 2-4 days.
Larvae are variable in color but are generally greenish brown and the body is marked with longitudinal bands in alternatively dark. Young larvae feed on flower buds and terminal buds. They feed with the head inside the boll while the rest of the body is outside.
Each larva can attack an average of 14 buds. The larva lasts for 14-24 days and this causes considerable damage to crops. Full grown larva is usually about 4cm long. When the larva is full grown, it burrows into the soil and pupates.
Pupa is found on the soil and it is brown in colour and 15 mm long. Pupal period lasts 10-14 days depending on weather.
Adult is brown, night flying moth with a wingspan of 40 mm. egg laying starts 4 days after emergence and may continue for up to 10 days. The female can lay up to 1000 eggs.
The severity of the damage varies between crops, regions and locations and between seasons. Caterpillars feed on leaves, buds, growth can points, flowers and fruits. Leaf damage reduces leaf area, which can slow plants growth. Feeding on flowers and fruits causes the main damage. Flower feeding can prevent fruit formation. Caterpillars usually bore clean, circular holes through fruits/pods. Excrements (feaces/waste) of the feeding caterpillar are placed away from the damaged plant parts. The holes serve as entry points for secondary infections by diseases causing fruit decay. One caterpillar can damage several fruits/pods. Once they burrow into the fruits/pods they are difficult to reach and control using insecticides.
Crops losses at farm level in Kenya have been estimated at over 50% on cotton pigeon pea. Over 20% on sorghum and millet, and over 2 million stems on cut flowers. In addition, the African bollworm is a quarantine pest. This is important for export crops. If a caterpillar of this pest is detected in a consignment of an export commodity (e.g. flowers, vegetables….) shipped to Europe, the whole consignment may be rejected
Affected plant stage
- Vegetable growing stage
- Flowering stage
- Fruiting stage
Affected plant parts
- Growing points
Larvae feed on leaves, flowers buds, flowers,grains and bore into pods and fruits
Excrements (feaces/waste) of the feeding caterpillars are evident on damaged plant parts.
Cotton, maize, sorghum, zucchini Bean, Plums, Lemon, Orange, Tomatoes and Tobacco. Ornamental plants and flowers attacked include: Pinks, Geranium, Lettuce, Dianthus, Carnation, Nasturtium, Rose, Snapdragon and Zinnia.
The pest can be monitored using the following
- Plant sampling
- Random sampling: one should walk through the crop area and randomly examines plant for eggs,larvae, damage or frass.
- Thoroughly inspect plant from soil and roots to the top of the newest shoot (leaves, buds and flowers).
- During sampling select different spots each time.
- Point sampling where damage is noticed for presence of larvae.
- Regular sampling from pre-fixed point or bays.
Management and control
The pest can be easily managed through scouting of plants, use of light traps for adults and pesticides. Effective chemicals depend on the toughness and the time of application. Thus proper timing is crucial. Application of chemicals should be done when the larva is small (1-8 days). On beans, apply chemical when flowering while on maize when silking.
Some of the pesticides one can use include
Potato tuber moth (Phythorimae operculella)
The pest attacks wide range of crops including tobacco, tomato and Solanaceae family. Infestations arise in the field and continue during storage of tubers. There is a serious risk of transportation from one area to another or from one country through infested tubers.
Eggs are laid on the underside of the leaf or in the tubers around the eye. Females may lays between 150-250 eggs during her lifespan. Incubation period last 3-5 days.
Larvae. Up to hatching the 1st instar larva bore into the leaf where they make mines. The caterpillar is greenish in colour. The attacked leaves have silvery blotched caused by the young larvae which mines in the leaves, left veins and petioles and stems. The mines increase in size as they approach the base of the stem. This is followed by wilting of the plants and plants become affected by fungi or bacteria. In tobacco the mined leaves have blotches and become unusable or the grade of the crop is lowered. Full grown caterpillars vary in size ranging from between 8-10mm. larval period lasts 9-26 days. The larvae may fasten two leaves and then feed between them. In tubers the larvae makes black tunnels which are generally filled with faeces.
Pupation. Takes place in the cocoon at the surface of the litter or just under the surface of the tuber.
Pupa Period lasts between 6-26 days.
Adult are short lived and have wingspan of about 15mm. one generation takes 3-4 weeks and there can be up to 12 generation in a year depending on the weather
Cultural control– Plant potato deeply in the soils. Harvest potatoes early in the morning because the pest lays eggs last in the afternoon. Potatoes in the field should be covered with soil to prevent the moth from laying eggs on them.
Chemical insecticides should be applied after every 14 days after the mines have been spotted on the leaves. Use the following insecticides.