The American bollworm, Heliothis armigera Hüb. is the major cotton pest in Tanzania. The commercial cotton varieties have the ability to compensate for early loss of fruiting points caused either by physiological stress or by bollworm damage; and therefore early sowing, together with a blanket spraying regime, is recommended to minimize the damage caused by this bollworm. To extend this integrated approach, studies to incorporate Heliothis -resistant characters, viz. frego bract, nectariless, high gossypol and glandless into the Ukiriguru cotton varities, were started in the early 1970s and are still in progress. To date, frego bract has given consistently good results; and intensive work is under way.

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.

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.

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.

Stem-borer incidence on upland and irrigated rice varieties is compared under natural or artificial infestation, and with simulated damage by sectioning tillers. In the early stages, Diopsids lay more eggs on irrigated varieties, but these are less suitable for the development of Diopsids larvae than upland varieties, whose stems are thicker.

Some varieties of Oryzae glaberrima show a high level of resistance to stem-borers and Hispids. Little difference is noticed for antibiosis of the Oryzae sativa studied, and losses depend on the tolerance of varieties. Irrigated varieties have a better tolerance because of their high tillering, preventing the attack, and of their ability to compensate for early losses by producing new tillers. This compensation does not exist for upland varieties. Their tolerance depends on tillering rate, the ability of infested stems to produce fertile panicles, and the difference in productivity between primary and secondary tillers, the former being more frequently infested. For the same population of stem-borers, losses observed on upland rice are three times lower on tolerant varieties than non-tolerant ones.

The advantages and disadvantages of host plant resistance in crop protection are considered. The role of plant resistance as a primary tactic in integrated pest management and its interactions with other tactics, i.e. biological, chemical and cultural are discussed.

The agroecosystem is the basic unit of consideration in the integrated pest management approach. The target crop within this ecological system and its level of susceptibility to insect pest species is of fundamental importance, and can be altered genetically through breeding for pest resistance. The relative level of pest resistance as influenced by pest density, demands a knowledge of pest density and plant damage (yield) relationships. An understanding of these relationships leads to the determination of economic injury levels and subsequently to the establishment of dynamic economic threshold levels. Comparing the economic injury level of insect resistant cultivars to that of susceptible cultivars defines the resistance level and provides the base-line data on which to quantify the effects of other direct control tactics and natural pest density suppressing agents.

Insect resistant sorghums are used as a model to examine the unique values of this tactic as a component in an integrated pest management strategy; the role they play, and how they influence other direct control and support tactics.

The paper stresses the fact that an integrated pest management system is not a new concept leading to a more stable agroecosystem, but was practiced by the farmers for centuries; host-plant resistance is one of its major components. The system was destabilized by the introduction of new sorghum varieties and hybrids not resistant against the major sorghum insect pests like shootfly Atherigona soccata (Rondani); stem borer, Chilo partellus (Swinhoe); midge, Contarinia sorghicola (Coquillet); and headbugs, Calocoris angustatus (Lethiery). In order to balance the agroecosystem again, an integrated insect pest management system for sorghum has to be developed based on traditional pest management practices. These are host-plant resistance, cultural control and biological control. Insecticides should be used only if absolutely necessary. For a better understanding of such a control approach, a summary of the biology and population dynamics of the major insects is given, together with a brief account on the levels and mechanisms of host-plant resistance so far known. Based on this information, the rainfall pattern, plant duration to maturity, time of planting, natural enemies and insecticides, a sorghum based integrated pest management system is proposed for the monsoon and post-monsoon season in which host-plant resistance alone or in combination with the above mentioned control methods could be used.

Among several species of insect pests that attack the millet crop, the earhead caterpillar, Raghuva albipunctella de Joannis and Acigona ignefusalis Hmps., are considered the major pests. The constraints against the successful implementation of an integrated pest management programme of these insects are examined. The relative importance and significance of host plant resistance in such a programme is compared against other modalities.

The use of insecticides is negated by lack of cash value returns, water supply, trained personnel and poor delivery system. Cultural practices, such as the synchronisation of early planting, destruction of harvest residues and end-of-season ploughing, have the potential for reducing carry-over populations. However, the difficulties in their implementation range from farmer education and efficiency to changes in traditional practices and introduction of new labour intensive practices after harvest. While it appears that natural enemies are taking a gradual toll on R. albipunctella, they appear inefficient in controlling A. ignefusalis. A range of plant characters have been identified: tillering in certain varieties as a form of tolerance to borers; maturity cycles that result in escape (pseudo-resistance) from Raghuva infestation, hairiness in relation to preference for oviposition and head compactness in deterring larval penetration into millet heads. Most of these characters are common in landrace types. The real problem appears to be one of incorporating and utilising the identified resistance into germplasm sources which possess other desirable agronomic characters that the farmer requires. This implies multi-disciplinary effort which is the basis for integrated pest management. In most of Africa, and in the Sahel in particular, this basis is remotely present.

The origin of African rice, Oryza glaberrima Steud and Asian rice, O. saliva L., in West Africa is briefly reviewed. Major pests of rice in this region have been listed and the present status of crop protection with reference to rice is discussed. Major constraints to utilization of varietal resistance as a component in crop protection are mainly research-related. The need for manpower development, provision of adequate facilities, standardization of techniques as well as greater emphasis on studies on varietal resistance are highlighted.

Distinct differences in Panonychus ulmi population densities on various apple cultivars have been observed under natural field infestations. Various laboratory techniques may be used to determine the differences in susceptibility or resistance to P. ulmi of different apple cultivars. The lowest variation in P. ulmi behaviour, and fecundity between replications was observed for females reared on leaf discs 20–25 mm in diameter and for females reared in the small plastic cages placed on the leaves of potted young apple trees. The leaves used for bioassays should be collected from the same part of the trees. The density of P. ulmi populations and their fecundity should be observed on leaves growing on short spurs. Preconditioning P. ulmi on various cultivars did not influence the female's fecundity on the new plants. There is no individual, singly-acting mechanism responsible for the moderate resistance of apples to P. ulmi. The resistance is caused by a number of morphological, anatomical and biochemical leaf properties, expressed differently in different cultivars.