Cotton

Cotton bollworm Helicoverpa armigera Hb.

Ubiquitous species. Damages many crops, including cotton. Overwinters as a pupa in the soil at a depth of 5–10 cm. Imagoes fly from early June to November. One female lays 500 to 2,700 eggs.

Eggs are laid singly, rarely 2–3 eggs on leaves, generative organs (buds, flowers, bracts, corn rachis, Apera), etc. Caterpillars live 15–22 days, passing through 6 larval instars. The optimum temperature for caterpillars is between 22–28 degrees. They feed on plants of 250 species.

First instar caterpillars damage the top leaves of cotton, corn, tomatoes, alfalfa.  From the second instar until the end of development, the caterpillars feed on the generative organs: buds, blossoms, seedbud, cotton boxes, grains in corn cobs, chickpea pods, etc. They also develop on weeds. In Bulgaria, 2–3 generations are developed. Females during egg laying are attracted by formic and oxalic acids secreted by the cotton in the extrafloral nectaries.

Egg-laying begins 3–4 days after the butterflies begin to fly. Eggs are laid on leaves, flowers, bracts.  In the case of tomatoes and chickpeas —on the leaves, buds, flowers, in the case of corn – on the cob. In the case of cotton — egg-laying is only at the beginning of budding.  This is why the first generation of bollworm develops on weeds and on alfalfa, chickpeas, tomatoes, and squash.  The first trichogramma displacement is made on the periphery of the fields on weeds up to 30 thousand individuals/ha or 0.4 g.

Overwintering pupae withstand temperatures down to — 13 degrees. But even with the death of 70–80% of the pupae, the number of the bollworm is rapidly increasing.

In corn, the bollworm feeds on the cob and then on the kernels of the cob. In tomatoes — with the leaves, buds and fruits, in alfalfa — with the leaves and buds. A caterpillar damages up to 20 fruiting bodies in cotton and up to 12 cobs in corn.

The economic injury threshold for cotton is 8–10 caterpillars per 100 plants or 3–5 for thin — fibred cotton. For tomatoes — 15–20 eggs per 100 plants in the first generation 40–90 eggs per 100 plants in the second generation. In the case of corn 12–17 eggs in the first generation and 30–80 eggs in the 2^nd generation.

Displacement of the trichogramma.  In areas where conditions are favourable for the development of the trichogramma (HTC = 0.9–1.2), the first trichogramma displacement is made at the beginning of egg-laying, the second displacement — in the period of mass egg-laying. If the flight period is extended, an additional displacement is made 5–7 days after the second.  Further control of the bollworm is at the expense of the reproduction of the dispersed trichogrammas.

In areas where conditions are unfavourable for the development of the trichogramma, an additional 1 or 2 dispersals are carried out.

Trichogramma capsules.  There are 500 trichogrammas per capsule.  200 capsules/hectare are needed for one displacement. One capsule should fall on an area of 50 m². This means one trichogramma per 10 m² of area. Not enough, but in another case too many trichogrammas are needed. According to our and foreign experience, trichogramma of the best quality parasitizes up to 70–92% of the bollworm eggs.