Diet change in the cotton stainers, Dysdercus cardinalis and D.fasciatus, occasioned by their migration from Gossypium hirsutum (cotton) to alternate hosts Sterculia rhynchocarpa and Adansonia digitata, was overall beneficial to their bionomics. Only the host Abutilon mauritianum appeared to possess antibiotic effects against the fecundity, fertility and longevity of the stainers. The occurrence in tropical natural ecosystems of both suitable and unsuitable hosts appears to constitute a vital and a viable stainer population-regulatory mechanism. Overall, D. cardinalis exhibited superior reproductive parameters on a variety of diets than did D. fasciatus. The latter apparently suffered profound negative effects on its bionomics when feeding on certain hosts.

In laboratory studies on the nymphal and adult performance of the cotton stainers Dysdercus cardinalis and D. fasciatus (Pyrrhocoridae: Hemiptera) on seeds of five different host plants, two categories of hosts with significantly (P < 0.05) different biological effects were identified. Gossypium hirsutum L., Ceiba pentrandra Gaertn and Sterculia rhynchocarpa (K. Schum) were found to be suitable hosts (category A) while Adansonia digitata L. and Abutilon mauritianum (Jacq. Medic.) were unsuitable (category B). Category A hosts that grew in the wild appear to serve as perennial breeding reservoirs for the cotton stainers and thus play a significant role in initiating and perpetuating their infestation of adjacent cotton crops. The converse appears to be true for category B hosts, whose long-term effect would be to gradually decrease the population of the cotton stainers.

Relative susceptibility of laboratory-established cultures of four populations of Dysdercus fasciatus Sign. and one population each of D. nigrofasciatus Stal., D. cardinalis Gerst, and D. superstitiosus (F.) to topical applications of cypermethrin, fenvalerate and permethrin was investigated at Kibos, Kenya. The LD50 values of the three chemicals to populations of D. fasciatus were 0.0656–0.0757, 0.335–0.469, 0.442–0.505 μg/insect, respectively, while those for D. nigrofasciatus were 0.1690–0.2541, 0.481–0.504, 0.721–0.851 μg/insect. The LD50 values for D. cardinalis and D. superstitiosus were 0.1507, 0.714, 0.798 μg/insect and 0.0991 and 0.824 μg/insect of each of the three chemicals respectively. Based on D. fasciatus from Kibos as the reference strain, the relative susceptibility values of all populations of Dysdercus species studied were small ( < four-fold) indicating that resistance was not apparent in the cotton stainers. Of the three insecticides tested, cypermethrin was the most effective judging from the high toxicity ratios between it, fenvalerate and permethrin.

A field evaluation of nine chemicals was conducted at Cotton Research Station Kibos for their efficacy against bollworm and cotton stainer species attacking cotton in Kenya. Based on the yield and percentage stained seed cotton, Decis'Triazophos, pp 321 Karate and Baythroid and Decis were the most effective insecticides in 1985 and 1986 respectively. However, there was no consistency in results of performance of insecticides tested against cotton insects in the 2 years. The apparent poor performance of most insecticides tested against cotton insects was attributed to fixed insecticide spray schedule which resulted in superfluous and often mistimed chemical applications. It is therefore necessary to establish damaging threshold(s) of major cotton pests in this country so that chemicals can be efficiently used to control them with minimum adverse effects on our environment.