Production of maize in western Kenya is severely constrained by the parasitic weed striga. Although productivity of maize can be improved through adoption of improved varieties, adoption of such varieties remains low in the region, as the majority of smallholder farmers still grow unimproved open-pollinated varieties (landraces). The performance of two improved hybrid varieties was evaluated against six landraces in striga-infested soils in western Kenya. The varieties were planted in plots under natural striga infestation and were supplemented with pot experiments under artificial infestation. Striga emergence was lower in landraces than in the hybrid varieties in both field and pot experiments. Similarly, height of maize plants at harvest and grain yields were higher in the landraces than in the hybrids. After three continuous cropping seasons, in all treatments, striga seedbank density increased two to seven times. Seedbank increase was higher with hybrids and two of the landraces, ‘Rachar' and ‘Endere'. These results provide an insight into the potential role landraces could play in efforts toward an integrated management approach for striga in smallholder cropping systems. They also highlight the need to develop hybrid maize lines with local adaptation to biotic constraints, specifically striga.

The effects of various levels of cow manure (equivalent to 0–50 kg nitrogen/ha.) or ground limestone (equivalent to 0–5 tons calcium/ha.) on the expression of resistance of cassava, Manihot esculenta Crantz to the cassava mealybug, Phenacoccus manihoti Mat. Ferr., were investigated under semi-controlled conditions in the laboratory. The study was made on two varieties of cassava, Moudouma and MM79, characterized by different levels of resistance in terms of antibiosis and tolerance. The antibiotic component was evaluated by determining the intrinsic capacity for increase rc and tolerance by quantifying morphological and physiological modifications in the growth of plants induced by 2 months infestation by 100 mealybugs.

The antibiotic resistance of the two varieties was reduced by both fertilizers. However, the reduction was only significant at levels of 10 and 20 kg N/ha. and 1 and 1.51 Ca/ha. for Moudouma and MM79 respectively.

In both varieties, tolerance was increased by the addition of nitrogen and not modified by the addition of lime.

The integration of these results into traditional cassava crops systems has also been discussed.

Antixenotic and antibiotic components of resistance to the cassava mealybug, Phenacoccus manihoti Matt. Ferr. were evaluated in different varieties of cassava (Manihot esculenta Crantz), in Poinsettia (Euphorbia pulcherrima Wild), Talinum (Talinum triangulare Jack) and Faux-caoutchouc (hybrid of M. esculenta and M. glaziovii Mull. Arg.). Resistance, was estimated in the field on 25 varieties of cassava by means of varietal screening. Although we were unable to identify varieties ofcassava totally resistant to P. manihoti, there was evidence of partial resistances. Thus the Incoza variety is the most resistant, followed by the Moudouma and Zanaga varieties. On the other hand we found very susceptible varieties, such as Dikonda, Kataoli, 3M8 and 1M20. Laboratory evaluations of the antibiotic component of resistance, made by estimating the intrinsic capacity for increase rc, showed that the host-plantshave a considerable effect on the multiplying capacity of the mealybug. Indeed rc ranges from 0.038 (Poinsettia) to 0.160 (Ganfo cassava variety), i.e. a ratio of 1 to 4 between the two extreme values. If the cassava varieties alone are considered, the percentage of maximum variation is 20% between the Incoza (rc = 0.133) and Ganfo (rc = 0.160) varieties. The Incoza cassava variety is the most resistant, in terms of both antixenosis and antibiosis. Classification of the other cassava varieties, from the most to the least resistant, differs according to the resistant component under consideration. Poinsettia and Talinum both have a very strong antixenotic component, but their antibiotic component differs (rc = 0.038 and 0.150 respectively). Our results suggest that the resistance mechanisms of the host-plants of P. manihoti intervening in the fixation of the pest (antixenosis), are different from those acting on the development of the mealybug (antibiosis). We established significant correlations between the size of the mealybugs and their demographic characteristics for all the plants studied: the duration of the prereproductive period is shorter and the net reproduction rate is higher when the mealybugs are large-sized. These results are discussed in the context of an integrated monitoring programme.

Three species of Chilo, namely C. auricilius, C. suppressalis, and C. polychrysus are predominant in Asia; and the other three, C. zacconius, C. partellus, and C. diffusilineus are predominant in Africa on rice crop. Various screening methods to identify resistance donors in cultivated as well as wild rices have been used over the last 30 years. The level of resistance in available donors of cultivated species is moderate, except in some accessions of Oryza nivara/O. rufipogon, O. officinalis, O. brachyartha and O. minuta. Genetics of resistance is complex as appears from the review. Efforts to incorporate the resistance into new developing breeding lines have been sporadic and discontinuous in African countries. At IRRI and in many Asian countries, systematic approaches have been undertaken. Once donors with superior levels of resistance become available, systematic breeding approach has to be made with the help of multidisciplinary teams at suitable test sites. African breeding programmes need close linkage with IRRI's stem borer programme and International Rice Stem Borer Nursery.

Stem borers are major insect pests of rice in Africa. The utilization of varietal resistance in the management of these pests, has great economic potentials, as it involves no extra cost to the farmer and is also free from environmental hazards. Investigations into the sources of resistance to the African rice striped borer, Chilo zacconius, some possible factors associated with host plant resistance and the benefits derived from growing resistant varieties are briefly reviewed.

The spotted stem borer, Chilo partellus (Swinhoe) is an economically important pest in many sorghum growing areas of Asia and Africa. A chronological progress made in Indian and other programmes on natural and artificial screening techniques is reviewed. Numerous germpiasm accessions were evaluated and resistant sources identified and documented for various parameters of resistance such as foliar damage, “dead hearts” and stem tunnelling.

Studies on the mechanisms of resistance showed predominance of tolerance, variable degrees of antibiosis and antixenosis as only a factor for oviposition. The morphological characters and biochemical factors associated with resistance are discussed. The nature of resistance is polygenic and partially dominant over susceptibility. The inheritance patterns of resistance to foliar damage and/or “dead hearts”, and stem (stalk and peduncle) tunnelling damage are different and independent. Pedigree breeding with single and multiple crosses has been useful to transfer resistance in more economic background. Continuous screening for stem tunnelling over several generations improves the level of resistance. Recurrent selection was suggested alternatively with and without infestation for resistance and yield, respectively to increase the level of tolerance. The resistant sources utilized in developing commercially released high yielding varieties, hybrids and derivatives with low to moderate levels of resistance are listed. Efforts to develop cultivars with levels of resistance higher than presently grown cultivars and combining cross resistance to stem borers and shootfly are under way. Current progress has been reviewed and future strategies are suggested.

Major rice stem borers in W. Africa were discussed and scientific investigations relating to varietal resistance were reviewed. Impediments militating against a faster rate of development of this area of research in W. Africa were highlighted. A call was made to accord greater emphasis and priority to research on varietal resistance since in the long run it will become the most important rice crop management tactic feasible for the majority of West African small scale rice farmers.

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 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.

About 20 species of borers are rice pests. Of these, five—yellow borer, striped borer, white borer, dark-headed borer, and pink borer—are of economic importance. Yellow borer and striped borer occur most widely in Asia and cause serious yield losses.

Varietal resistance to these two species has been investigated during the last 30 years. Field and greenhouse screening techniques have been developed and numerous germplasm entries have been evaluated for resistance. Several hundred varieties with low to moderate levels of resistance have been identified. Except for several wild species of Oryza, germplasm with a high level of resistance has not been found.

Information available to date indicates that the resistance is under polygenic control. Moderate levels of resistance from several donor parents have been incorporated into improved varieties. Some of these moderately resistant varieties, for example, IR20, IR36, IR50, Ratna and Chandina, are widely grown and play an important role in integrated pest management programs for stem-borer control. Efforts to develop cultivars with levels of resistance higher than presently grown varieties are under way using the diallel selective mating system and the male sterile facilitated recurrent selection schemes.

Orseolia oryzae, the rice gall midge, is a major pest of rice in many areas of tropical Asia and is becoming an important pest in Africa. A chronological review of the progress made in various national programmes on varietal resistance, sources of resistance and breeding for resistance is given. Many resistant varieties have been identified and have been utilized in breeding programmes to develop high yielding varieties with multiple resistance to the gall midge and other insect pests and diseases. Mechanisms and inheritance of resistance in rice varieties are discussed. Rice varieties resistant in various countries and sources of resistance used in breeding programmes are listed. Biotype variations in different countries and within the countries are revealed, and a preliminary classification of gall midge biotypes based on varietal reactions is proposed.