Time related factors in Heliothis control on cotton

Most factors in Heliothis control on cotton are crucially related to time but all are interrelated one with another and form two complex interactions hinging on the pest life cycle. One is the crop-pest interaction and the other the control strategy. This paper reviews most of the factors and reports work carried out in Swaziland to elucidate some of them. In the crop-pest interaction the timing of the initial attack and a cue for spraying can be indicated by development of an alternate host and, in Swaziland, by reference to the average date of maize tassellation. Later infestations are measured by scouting and a comparison is made of the two main systems, the first based on a small part of many plants and used in the USA, and the second based on the whole of a few plants in Africa. Their respective purposes are discussed and economic spray thresholds based on them are presented. The compensatory ability of the plant diminishes as the season progresses but in Swaziland manual bud/flower removal experiments over 3 years on raingrown and irrigated cotton showed that prior to 12 January (9-12 weeks from planting) removal increased the yield in half the cases whilst later removal could cause a decrease. The natural Heliothis infestation was allowed to remove fruits in an experiment by leaving spray gaps and the results confirmed those of the previous manual experiments except that damage continued for about 2 weeks after spraying restarted. This advanced the critical date to 31 December but up to three sprays could be saved without loss of yield and possibly with a yield bonus. In the control strategy the different stages of the life cycle are considered as targets but the most obvious is normally the first instar larvae. Rapid plant growth can dictate frequent spray applications regardless of deposit persistence which may need to be longer when using aircraft in spite of Heliothis oviposition occurring predominantly on exposed surfaces. Temperature-independent or fumigant insecticides or those affecting the adult may best be sprayed in the evening when cooler and calmer post-treatment conditions occur and adults are starting to fly.