Bollworm

Bollworm is any larvae of different moths (order Lepidoptera), including the cotton pink Bollworm (called Pectinophora gossypiella, the Gelechiidae family) and a few Helicoverpa species. While these larvae are most well-known for the damage they inflict on cotton bolls, a variety of plants are attacked by the bollworms, including alfalfa, peas, soybeans, beans, peanuts, flax, and other commercial crops.

The larvae of the corn earworm (called Helicoverpa zea, the Noctuidae family) generally feed on corn kernels but can also burrow into the tomatoes and cotton bolls and are therefore at times called tomato cotton bollworms or fruitworms. Bollworm pests can be managed with the help of biological control methods, such as natural parasites, or through trap crops, early crop planting, and insecticides.

Bollworms Resistance

The resistance to bollworm has been prevalent in TwinLink, Bollgard II, and WideStrike types since 2016, but not in TwinLink Plus, Bollgard 3, or WideStrike 3. As a result, control and scouting recommendations differ. All these varieties should be scouted regularly when if any of the sorts of adult moth activity is detected, but especially during the major flight events; these usually take place in mid-July (in the Southern counties) to late July or early August (in the Northern counties), but can be managed using the NC State Extension Light Trap Data network.

A good indication of when the primary flight begins can be confirmed by a significant increase in pheromone trap or light captures or the presence of freshly emerged bollworm moths either in the field or around the field edges.

Life Cycle and Appearance of Cotton Bollworm

Individual eggs are laid near flowers, buds, leaves, or fruits on young stems. Almost, they are spherical with a flattened base and a diameter of up to 0.5 mm. At first, they are a shiny yellowish-white, with their colour-changing to brown before the larvae emerge.

The cotton bollworm (called Helicoverpa armigera) generally goes through 6 larval stages, although under certain circumstances, 5 or 7 stages may take place. Young caterpillars are yellowish-white to reddish-brown, having dark spots. In later instars, the head is mottled; the body is marked with 3 prominent longitudinal dark bands having a number of lighter coloured wavy lines.

The colour of the older caterpillar is much variable and maybe straw yellow, green, pinkish or black or reddish-brown. Full-grown larvae are around 30 to 40 mm long. The first instars are defined as the most mobile, and they move like loopers. Often, the caterpillars are cannibalistic and aggressive. Pupation takes place in the soil, with pupae of around 14 to 18 mm long, with a brown, smooth body surface.

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Female cotton bollworms (Helicoverpa armigera) are brownish-orange moths with a wingspan of up to 40 mm and a length of about 18 to 19 mm. The grey-green males are very smaller. The forewings have a line of black dots along the border, while the hindwings are cream with a dark brown band around the outer margin in both sexes.

Damage Symptoms

First, instar larvae feed on the soft leaves by creating small holes. When they reach the second instar, then, they can penetrate fruit through a small hole, often bored near the stalk. During their development, caterpillars damage most fruits by mining. All fruits are damaged, although the preference is for the smaller specimens. The fruits will stop growing, mature rapidly, and then drop off. Flowers are attacked in legumes, and seed pods can be pierced.

Management

The impact of a bollworm infestation depends upon the larvae count present, the age of the larvae, and the timing of damage relative to the fruiting cycle of the crop. Although the large larvae do much damage, it is not possible to kill a significant proportion of them once they are older compared to the third instar. Therefore, control and monitoring should be aimed at the small larvae and eggs.

Natural enemies are most essential in managing populations of bollworms, especially in the San Joaquin Valley. In most cases, harmful populations do not emerge until late in the season, after previous pest treatments have disturbed natural enemies. Insecticides are required only if the population exceeds the treatment threshold, while the crop has a significant number of green bolls or squares that will have time to develop into mature bolls by the season's end.

There is no need to treat once bolls begin cracking because most bolls are too mature by that time to be susceptible, and squares still present will not have limited time to mature. With the difference that an injury may occur twice - once during each fruiting cycle - the same concept applies to long-season desert valley crops.

Monitoring and Treatment Decisions

Begin sampling plant terminals for bollworms as soon as bolls appear in the San Joaquin Valley and continue until the majority of the bolls have matured. As you move through the field, check 5 neighbouring plants at each stop. Then, at random, pick the first plant and inspect its mainstem terminal as well as those of the four plants nearby. It's best to choose at least 100 plants for standard sampling.

There exist two treatment thresholds for bollworms in the San Joaquin Valley. In fields, which have not been treated with broad-spectrum insecticides, treat when you find 20 small bollworms per 100 plants. Whereas, in fields, which have been treated previously, treat when you find 8 small bollworms per 100 plants. After that, instar larvae are the most destructive but are very resistant to insecticides; thus, aim treatments either at first or second instars.

In desert valleys, start sampling in mid-July, up to 1 to 2 weeks after peak squaring. Continue sampling until most of the bolls have matured. Continue until the top crop bolls have developed in crops with a second fruiting cycle. For the standard samplings, check for larvae on the terminal growth of a minimum of 100 plants chosen at random. Divide fields of around 80 acres into quarters and check 25 plants in every quarter. Divide the larger fields into more areas and check 25 plants in every area. The treatment threshold ranges from 10 to 12 small budworm or bollworm larvae per 100 plants.

Pink Bollworm

Description

Pink Bollworm (which is also called P. gossypiella) adults are small, dark-brown moths measuring up to 12-20 mm across the wings (by USDA 1948). The head is in reddish-brown colour with pale, iridescent scales. Antennae are brown and the basal segment bears a pecten of either 5 or 6 long, hair-like and stiff scales. The labial palpi are long and bent upwards; the second segment has a wrinkled hairy brush on the underside that becomes smooth distally, and the terminal segment is shorter than the second, and the proboscis is scaled.

The forewings are elongated-oval in shape, with pointy ends and a broad fringe. The forewings have a brown ground colour and are covered in tiny black scales that create hazy patches towards the wing base and in the medial cells. The apical portion of the wing looks dark brown with a light-coloured and transverse band. Sometimes the wing also bears a medial round spot.

The hind wings are larger than the fore wings, trapezoidal in shape, silvery grey in colour, and have a darker, iridescent rear border. The wing fringe is darker and ochreous at the apex and base.

Legs look brownish-black with transverse, which are ochreous bands in the form of rings. The abdomen is ochreous toward the upper side, laterally dark brown and covered with ochreous-brown scales on its underside.

The male uncus is broad at the base, tapering to a point and the aedeagus has a hooked tip in the genitalia. The female ovipositor is sclerotized weakly.

Eggs are white and elongated-oval, with 0.4-0.6 mm in length are 0.2-0.3 mm wide. Usually, they are laid singly or in groups of 5-10. Larvae are pale-coloured ones and 1-2 mm long when they first hatch. Mature larvae are about 12-15 mm long, having a prominent pinkish colouration. Pupae are reddish-brown and measure around 8-10 mm in length.

Distribution

Pink Bollworm (P. Gossypiella) is found in tropical America, Asia, Africa, and Australasia, as well as subtropical locations such as Egypt, Pakistan, and Mexico. It has been eradicated from all the cotton-producing areas of the continental USA (USDA, 2018).

The distribution map includes the records based on the specimens of P. Gossypiella from the collection in the Natural History Museum (in London, UK).

American Cotton Bollworm or American Bollworm

Description

Egg

Eggs are radially ribbed, subspherical, 0.59 mm in diameter and 0.52 mm high, stuck singly to the plant substrate, green when laid, turning red and finally grey before hatching. Egg maturity takes 2-3 days at a temperature of 20-30°C. Usually, eggs are located on the silk of maize or on the fruiting structures.

Larva

On hatching, the tiny grey caterpillars hold a blackhead; they grow through 6 instars usually, but five and 7 instars are not uncommon, and the final body size is nearly 40 mm long. In the third instar, two colour phases can develop green (the less frequent) and brown (the predominant phase).

Longitudinal white, yellow, or cream lines can be seen, with the spiracular band being the most prominent. The pattern becomes better defined as the larvae develop, but in the final instar (6th), the colouration abruptly changes into a bright pattern, often pinkish and with extra striations. Larvae contain 5 pairs of prolegs.

Pupa

Pupae are light to dark brown depending on the maturity and nearly 20 mm long, having two distinct terminal cremaster spines. They are located 5-15 cm below the soil surface in the earthen cells.

Adult

A stout-bodied (from 20-25 mm long) brown moth of the wingspan 38-43 mm; forewing pale brown (the female) to greenish (the male) with darker transverse markings, underwings pale having a broad dark marginal band. 

Spotted Bollworm in Cotton

Now, in several Old World cotton-growing areas. In India, Earias insulana is observed in the Northern part of the country, whereas E. Vitella is a common pest in the Southern region. The pest feeds on the alternative host plants like Holyhock, Hibiscus; China rose, Sunhemp, pundi, Bhendi, Abutilon Indicum, etc.

Some Types of Bollworms in Cotton are Given As

• Spiny bollworm, Earias insulana: Africa

• Red bollworm, Diparopsis castanea: central-southern Africa

• Helicoverpa armigera - Old World bollworm

• Helicoverpa zea, tomato grub or the American bollworm