Cowpea, until recently the major food legume in Ghana (in terms of area under cultivation) has been witnessing a decline in production at the expense of groundnuts. Among the factors leading to the decline are the havocs caused by insect pests and diseases with the former being of major importance.

The paper presents some of the systematic screening and testing procedures of both exotic and indigenous materials with the aim of evolving varieties that are not only high yielding but also resistant to a broad spectrum of these prevalent insect pests.

Implantation of additional corpora allata (CA) into the last instar larvae of Corcyra cephalonica resulted in the production of extra larval instars, larval–pupal intermediates and adultoid larvae. Moths metamorphosed from the extra larval instars were externally normal.

The juvenoid, hydroprene, when given to last instar larvae, prolonged the larval life span, induced extra larval moult and the moths metamorphosed from such larvae were larvoid adults. Extra pupal instars were produced when this juvenoid was applied to early pupae. Moths obtained after pupal treatment were adultoids. Another juvenoid, methoprene, prolonged larval life and produced giant larvae without extra moult. Incompletely moulted extra larval instars, larval–pupal intermediates and adultoid larvae were formed. Moths obtained after both larval and pupal treatments were externally normal. The percentage of moths obtained after the application of both the juvenoids was small.

The antiallatotropin, precocene II, had no creditable effect on the duration of larval or pupal life. Application to both larvae and pupae, pretreated with hydroprene, largely minimized the hydroprene-induced effects. Pupal treatment, however, produced pupal–imaginal intermediates. The moths obtained from precocene-treated larvae or pupae, with or without juvenoid pretreatment, were externally normal.

Plant resistance research requires many entomological and plant breeding techniques and methodologies of which some are critical to the success of the programme. The entomologist has the responsibility of developing techniques for: providing sufficient numbers of insects for screening and other plant resistance studies; creating uniform and controlled populations of the test insect for identifying and developing resistant germplasm; measuring resistance; and determining mechanisms and bases of resistance. A thorough knowledge of the insect-host plant relationship is essential for developing efficient techniques. Some of the considerations involved in developing these techniques are discussed with selected examples to illustrate various techniques employed in plant resistance to insects research.

Wild-caught Xenopsylla brasiliensis from Kibwawa village, Chunya District, in south-west Tanzania were colonised in the laboratory and their offsprings used for determining the baseline susceptibility levels of the strain to DDT. A total of 120 fleas were tested with each concentration of DDT, using the techniques described by the WHO Expert Committee on Insecticides (1970).

Several filial generations were selectively raised, using DDT as a selection pressure. They were tested with various concentrations of the insecticide and their dosage–mortality regression lines compared with that of their parents.

About 5% of the parental generation (normal population) survived 1 hr exposure to 4% DDT. The LC50 of this population was 0.79% DDT. The mortality rates of the successive filial generations decreased gradually; their dosage–mortality regression lines moved to the right; and the slopes of these lines decreased. LC50s of F1, F2 and F3 were 1.6, 1.9 and 3.1% DDT, respectively. The highest DDT concentration (4%) killed 21.1 and 18.3%, of F4 and F5, respectively.

It was generally concluded that DDT resistance was present in the population, and that the selection pressure (DDT) was responsible for increasing the level of this resistance in the laboratory.

Lepidopterous stem-borers are the most serious and widely distributed insect pests of sorghum. Among the several avenues of their control, the use of resistant varieties holds good prospects. In order to identify the sources of resistance to stem-borers (Chilo partellus, Busseola fusca, Eldana saccharina and Sesamia calamistis), a large number of sorghum lines from Kenya, India and Texas, U.S.A. have been screened under a very heavy natural infestation in Western Kenya. Several promising lines with resistance potential have been identified for the stem-borer complex which include: IS 10370, L-2 (TX 2780), IS 10364, IS 1044, S-178, IS 3962, IS 4213, IS 12447, IS 18479, IS 18326, L-l (A&B TX 2756), IS 4405, IS 10711, IS 18676, IS 5613, IS 18517, IS 18323, IS 4881, IS 1151 and IS 18427.

Once a resistant cultivar is developed, the hope is that the resistance which has been selected is durable. One way of making sure that the resistance obtained lasts long is the development of multiple resistance.

The International Institute of Tropical Agriculture (IITA), located in Ibadan, Nigeria, developed a maize population (TZBR) resistant to the stem-borer, Sesamia calamistis. The International Centre of Insect Physiology and Ecology (ICIPE), Nairobi, Kenya crossed this population with some of their lines resistant to Chilo partellus. The resultant crosses and lines derived from them were screened under natural infestation for multiple resistance to C. partellus and Eldana saccharina. Since the population had previously been screened for resistance to S. calamistis, lines identified in this study to have resistances to either C. partellus or E. saccharina or both, will be assumed to have maintained their resistance to S. calamistis unless proved otherwise.

Therefore, new sources of multiple resistance to stem-borers particularly S. calamistis, C. partellus and E. saccharina have been identified. Crosses between them would certainly offer a wide scope in the multiple resistance approach towards pest management.

Sorghum shootfly, Atherigona soccata, oviposition and infestation on eight resistant sorghum cultivars (IS nos: 2122,2123, 2205, 2291, 4660, 5092, 5480 and 18551) and two susceptible cultivars (Serena and CSH-1), were studied under field conditions (natural infestation). The eight resistant cultivars and CSH-1 were also tested for preference for oviposition in a two-choice situation in the screenhouse. The eight resistant cultivars received considerably less shootfly eggs and showed fewer “dead hearts” than the susceptible cultivars. The results indicate that non-preference for oviposition is an important mechanism for resistance to shootfly in eight test cultivars. There was also evidence for a certain degree of antibiosis in IS 2291.

A new technique of mass-screening rice seedlings for resistance to the crab, Sesarma huzardi, developed by the West Africa Rice Development Association (WARDA) at Rokupr is described.

About 500 mangrove swamp local collections and a few exotic varieties have been screened for resistance to crab damage. Only 2% of the varieties have been identified as highly tolerant. Generally, the local collections were tolerant, while none of the foreign introductions have been tolerant.

The basis of resistance appears to be morphological, which physically affects feeding mechanism; thus, some varieties were more susceptible to crab damage than others.

Degree of the heritability (hb2 = 61%) estimate indicated that there is genetic variability for tolerance. In a team-oriented research, some of the varieties are being used in the WARDA breeding programme at Rokupr.

The success in streak virus resistance breeding at the International Institute of Tropical Agriculture (IITA) has depended mostly on developing suitable techniques for mass rearing Cicadulina triangula as a vector. Since 1977, IITA scientists have continuously improved maize streak virus (MSV) screening techniques. Recent innovations in mass rearing Cicadulina have made it possible to infest experimental maize fields at IITA with as many as 180,000–200,000 viruliferous leafhoppers each week. The 10–14 day old maize seedlings have been replaced with millet plants for oviposition and rearing of nymphs. The effect of temperature on the biology of C. triangula was studied under growth-chamber and screen house conditions. Monitoring to find out the percentage of active virus transmitters in the research population helps to indicate the number of viruliferous leafhoppers which should be released per experimental plant to secure 90–95% efficiency of MSV infections. Three-leaf stage plants produce clear streak virus symptoms after 5–7 days. Older plants require a higher number of Cicadulina adults to produce symptoms.

A method for steady production of Chilo partellus has been established. An efficient simple and easy method of collection of first instar larvae from incubation chambers has been developed. It has been established that pupae of Chilo partellus can be held at a chilling temperature between 5 and 15°C without any adverse effect on the emergence of adults. This finding has been used in synchronizing the emergence of males and females for mating or longer storage of the pupae.

Studies on the effect of high and low humidities on egg development have shown that low humidities affect the hatchability of Chilo partellus egg but did not effect the actual development of the embryo adversely. It has also been shown that Chilo partellus oviposits best under high humidity.