Role of trichome density in cassava resistance to mealybug, Phenacoccus manihoti Matile-Ferrero was evaluated on the apex, the first expanded leaf and the stem in pubescent and glabrous cassava varieties. Pubescence significantly retarded the rate of settling and establishment of P. manihoti. The weight of the developing larvae was also affected becoming more pronounced at the pre-ovipositing adult stage. However, the relationship between pubescence and P. manihoti weight was not significant. Moreover, the significant differences in weight at the preovipositing adult stage was not reflected in the corresponding fecundity on the glabrous and pubescent varieties. Pubescence accounted for only 18% of the differences in observed fecundity. There was also no significant difference in the development period of the mealybug on glabrous and pubescent varieties. Pubescence appeared to be a mechanical irritant only.

The development of a mass-rearing technique of Mantea testulalis by the ICIPE has allowed us to start extensive work on cowpea resistance to the legume pod-borer. A preliminary field observation showed that the cowpea resistance is on the low level. Therefore, our effort initially concentrated on the development of new methods and techniques which were sufficiently sensitive to separate lines possessing small differences of resistance. Eight different treatments were used to develop the methodology of cowpea screening under artificial infestation with Maruca larvae and eggs in the screenhouses and seven treatments in the field.

Plant growing stage has modified the expression of cowpea resistance to Maruca larvae. Five-seven shoots stage (not younger) was found to be most suitable for screening for resistance in pre-flowering period. By using five eggs/plant in pre-flowering stage on cowpea plants grown on the ground, it was possible to differentiate between the resistant and susceptible lines. The standard error between plants treated with 10 eggs/plant treatment was lower than those with five eggs/plant treatment. Therefore, if laboratory colony allows the higher infestation of 10 eggs/plant, it is recommended. Using 10 or 20 eggs/plant in flowering stage correctly segregated cowpea lines for resistance and susceptibility based on larval survival and damage level of flower buds, flowers and pods.

In West and Central Africa, panicle-feeding mirid bugs, particularly Eurystylus oldi Poppius (Hemiptera: Miridae), are major pests of improved sorghum cultivars. Population dynamics of panicle-feeding bugs were studied on the sorghum cultivar ICSV 197, sown on eight dates each year from 1992 to 1994 at Samanko, Mali. Relationships between weather factors and abundance of paniclefeeding bugs were examined for the three most commonly found mirids, E. oldi, Campylomma spp. and Creontiades pallidus Rambur (representing on average respectively 58, 39 and 2% of all mirid species). Although only one generation of E. oldi could develop between flowering and grain maturity on a particular panicle, there were three successive generations each year during the extended grainmaturing period in the experiment with eight sowing dates, with a major peak in abundance in early October, followed by a gradual decrease until late November. Abundance of Cr. pallidus peaked in late October, while that of Campylomma spp. showed no relationship with date of sowing.

An increase in abundance of E. oldi nymphs (y1) was favoured by high precipitation, reduced maximum temperature and high minimum relative humidity during and 2–3 weeks before the grainfilling period [a strong negative relationship was found with the number consecutive days without precipitation from 78–88 days after sowing (das) (x1): y1 = −11.7x1 + 149.3 (r2 = 0.67)]. The abundance of Cr. pallidus (y2) was correlated with wide temperature ranges and low precipitation during the same periods (notably precipitation frequency from 75–91 das: x2): y2 = −2.28x2 + 24.6 (r2 = 0.65). The abundance of Campylomma spp. (y3) was negatively correlated with amount of rainfall between 19–32 das (x3): y3 = −3.82x3 + 705.9 (r2 = 0.49). Reasons for these trends are suggested and their possible application for management of panicle-feeding bugs discussed.

The effects of infection by Trypanosoma brucei and DNA virus on the ultrastructure of the salivary gland cells in Glossina pallidipes Austen were investigated. Cytoplasm of uninfected cells contains a dense ribosomal population and rough endopiasmic reticulum, scattered Golgi areas and mitochondria. In infected ceils of salivary glands the ultrastructural integrity of the cytoplasm is profoundly changed due to cellular proliferation giving rise to a stratified epithelium and gland enlargement; cell degeneration characterized by formation of cytoplasmic vacuoles, chromatin margination, disorganization and elimination of cell organelles and gland hypertrophy. The virus particles are found in the nucleoplasm as well as the cytoplasm of cells. The ultrastructural evidence indicates that the virus are largely assembled in nuclei of cells and virions pass through the nuclear membrane. The trypanosomes are found within the degenerating cytoplasm and lumen of the cell.

The implications of the features observed in the infected cells are discussed, and possible suggestions are made regarding alteration of the metabolic functions due to infection.

The location of olfactory and gustatory sense organs in the larvae of the American bollworm, Heliothis armigera, have been studied using the scanning and transmission electron microscopy. The olfactory chemoreceptors of the larvae are located on the antennae, mostly on the top of the second antennal segment in the form of two trichoid sensilla, two basiconic sensilla, two styloconic sensilla, campaniform sensilla and a uniporous peg sensillum. The third antennal segment bears one basiconic sensillum and two styloconic sensilla. The gustatory sense organs are located on the mouthparts including the labrum, mandibles, hypopharynx, maxillary palps and labium. Trichoid sensilla have been located on the labrum. Epipharyngeal setae as well as long short spines are located in the epipharyngeal membrane. The basal segment of the maxillary palp bears three trichoid sensilla and one campaniform sensilla. The maxillary palp also bears four styloconic sensilla on the distal portion of the lateral protuberance and one styloconic sensillum at the apex of the medial protuberance. Close to the bases of the protuberances, there are three basiconic sensilla and one campaniform sensillum. The third apical segment of the maxillary palp carries three campaniform sensilla on its lateral side, while on its top, eight small basiconic sensilla are distributed.

The shootfly, Atherigona soccata Rondani, is a major seedling pest of sorghum in many areas of tropical Asia, Africa and Mediterranean Europe. A chronological review of the progress made in various programmes on the screening techniques (natural and artificial) with artificial diets, sources, stability and mechanisms of resistance, bases (physico-morphological and biochemical factors) of resistance, inheritance and breeding for resistance is made. Many resistant varieties have been identified and sources of resistance utilized in breeding programmes to transfer resistance in high yielding background are listed. In addition, the glossy and trichome traits identified is sorghum cultivars are also reported. Glossy leaf surface and trichomes are frequently associated with shootfly resistance.

The host plant spectrum of Aphis gossypii Glover (Hemiptera:Aphididae) was assessed from 1991 to 1994 in the cotton-growing zone of northern Cameroon. The aphid is generally present on cotton (Gossypium hirsutum L.) and okra (Abelmoschus esculentus (L.) Moench) during the rainy season. During the dry season on the other hand, A. gossypii is observed on a very wide range of plants (63 families, 192 genera and 302 species). During this season, all types of plants, especially legumes, dry-season crops and various types of trees, are colonised. From the representativeness of the sampled plant species within the cotton zone of Cameroon, it is concluded that a part of the A. gossypii population survives the dry season in the Sudano-Sahelian African zone, thus forming numerous reservoirs scattered among plants and in very varied habitats.

Studies were carried out to compare the efficacy of insecticide schedules based on calendar intervals with those based on plant growth stages for the control of cowpea insect pests. Spray schedules based on calendar intervals and plant growth stage gave reasonable control of the pest complex and increased grain yields significantly. However, spray schedules based on plant growth stages alone were more efficient for two of the three cowpea varieties tested. This was attributed to differences in maturity periods which made spraying by regular calendar intervals slightly out of phase with attack by key insect pests. Thus, spray schedules based on plant growth stages are recommended for the control of insect pests of cowpea.

An integrated approach to the management of the sorghum shootfly (to keep its population below economic injury level) has been a major thrust of sorghum research workers during the last one and a half decades. The necessary foundation has been laid by detailed studies on the biology of the pest and a fairly thorough understanding of the effect of different ecological factors on the shootfly population. Based on seasonal incidence studies, sowing immediately after the onset of monsoon has proved to be effective for avoiding shootfly damage.

A large number of high yielding and shootfly-resistant derivatives have been developed by crossing agronomically desirable exotic lines with identified resistant sources. The new hybrids and varieties released recently possess low to moderate levels of resistance.

Parasites and predators affecting different stages of the shootfly have been identified but utilization of these natural enemies for the biological control of the pest is yet to be undertaken.

Carbofuran seed treatment has proved to be highly effective for protecting sorghum from shootfly damage. Some other insecticides have also been found to be promising.

Based on the control methods available at present, an integrated package has been recommended for implementation in traditional sorghum growing areas of the country.

The leaf surface treatment given to nymphal instars of Lipaphis erysimi with methoprene (1–10,000 ppm) prolonged the developmental period only in the second instar. The percentage emergence decreased significantly in the treatments of first and second instar and the alate production increased in the treatment of the latter instar only. In addition, dose-dependent morphological abnormalities were also observed. The longevity and fecundity were affected adversely when adults were given both topical and leaf surface treatments. However, these effects were not transmitted to the F1 generation.

The prepupal stage of Spodoptera littoralis was affected by Bacillus thuringiensis kurstaki HD-1 (Dipel) at high concentration (5%), when sprayed or kept in soil treated with the pathogen. The emerged moths showed a short longevity associated with low egg production and low fertility. Increase in malformed individuals was observed. Treatment of the pupae of S. littoralis with B. thuringiensis during different ages showed an increased effect when treatment was applied, immediately after pupation.

The stemborers Busseola fusca (Fuller) and Chilo partellus (Swinhoe) cause the highest economic damage to maize and sorghum in Zimbabwe. A number of studies on stemborer bioecology and management in Zimbabwe have been conducted, but most are still incomplete. The more important of these are studies on life history, incidence and distribution patterns, host plants, yield losses, chemical control, cultural control, host plant resistance, the use of sex pheromones, and biological control. These are discussed here in detail, and areas where further research is needed are pointed out. As part of an integrated strategy for managing stemborers in Zimbabwe, an exotic braconid larval parasitoid, Cotesia flavipes Cameron, has been released in the lowveld area and prospects for its establishment appear to be good. However, this establishment might be curtailed mainly by the inability of Co. flavipes to adapt to the seasonal carryover mechanisms utilised by the indigenous Cotesia sesamiae (Cameron).

The differential attraction by 2 pigeonpea cultivars has been tested through the oviposition behaviour of Heliothis armigera females in a standardized laboratory assay. Seedlings of a susceptible variety (PPE-50) are under choice conditions significantly more attractive at a distance, and without contact allowed other than those of the resistant variety (PPE-45). The volatile signal is contained in the hexane extract from PPE-50 leaves, as shown in the oviposition assay, and after vacuum distillation is present in the more volatile fractions of the mono- and sesquiterpenoids. These compounds, which are also contained in the hexane extracts of 2 more pigeonpea cultivars, a check of medium susceptibility (T-21) and a wild relative (Atylosia scarabaeoides), were compared by capillary gas chromatography. The 4 pigeonpea varieties were characterized by their specific aromagrams, a method which may also advantageously be used in pigeonpea breeding programs.

Graminaceous crops are attacked by lepidopterous stem-borers all over the world causing severe crop loss. Chemical control of stem-borers is difficult and biological control is preferable for peasant farming systems. The most important genera are Diatraea, Chilo, Eldana, Sesamia and Busseola. Their natural enemies include egg, larval and pupal parasites and predators. The parasites are cued by different stimuli, some to host plant, others to borer tunnels or frass. Biocontrol was first attempted against Diatraea spp. in the New World where D. saccharalis is now under fair control in several countries. In the Old World attempts have been made to control C. sacchariphagus and E. saccharina without much success. In East Africa releases of exotic parasites against B. fusca, S. calamistis, C. partellus and E. saccharina have not been very successful and further work is suggested. Leguminous crops suffer from attacks of lepidopterous pod-borers. The most damaging species are Etiella zinckenelia, Maruca testulalis, Cydia spp. and Lampides hoeticus. There are relatively few records of important parasites and fewer still of predators, however, large parts of the world are still unsurveyed for natural enemies. Successful control using introduced parasites has been reported from Canada against C. nigricana and Mauritius against E. zinckenelia and M. testulalis. There appears to be scope for introductions of Caribbean parasites into Africa.

Sweetpotato weevil, Cylas formicarius formicarius F is the most destructive pest of sweetpotato, Ipomoea batatas in tropical and subtropical regions. The larvae and adults feed on sweetpotato roots and crowns. For the past 12 years we have been screening sweetpotato germplasm for resistance to this pest in order to breed an agronomic cultivar with weevil resistance. Resistance screening is conducted at AVRDC and Penghu island in autumn and summer, respectively. Germplasm is planted between heavily infested source rows maintained in a weevil nursery. Roots are harvested, sliced into thin pieces, and the number of insects (larvae + pupae + adults) per unit weight, as well as the weight of damaged and healthy root slices, are recorded. The resistance rating is based mainly on the number of insects found in the root. Considerable differences in resistance rating are observed from location to location and season to season. Even at one location, tremendous differences are observed in weevil infestation of a particular entry among the replicates. Although over 1000 sweetpotato accessions have been screened, we have not found any that is consistently less damaged over several seasons at either location. Our recent screening of a population of I. trifida × I. batatas crosses show certain hybrids with high yield and low weevil infestation. These hybrids need to be evaluated further to confirm their resistance.

Vertical distribution of different larval instars and pupae of Culicoides oxystoma in different depths in the mud and sandflats of Hooghly estuary were studied in relation to soil temperature and moisture at three stations in and around Sagar Island during March 1983 to February 1984. Temperature recorded higher in 0–3 cm depth and gradually decreased in subsequent depths. During the last part of the pre-monsoon (May and June) season, moisture was recorded minimum in 0–3 cm depth and gradually increased in greater depths, but in other seasons (monsoon and post-monsoon) the results were just opposite. Larval and pupal population showed a peak during post-monsoon then gradually declined in pre-monsoon and early monsoon. They were mostly encountered throughout the year from 0–3 cm depth except during summer months (May, June and July). First and second instar larvae were only recorded from 0–3 cm depth. Third and fourth instar larvae were distributed in all the depths, maximum in 0–3 cm. Pupae were never encountered from deeper depth (6–9 cm). Temperature and moisture act as important factors to control the vertical distribution of larvae and pupae of this species.

The influence of host presence, age and mating on calling behaviour of the female spotted bollworm, Earias vittella, was studied under a 14L:10D photoperiod. Onset of calling was earlier in the presence of a host and 93% of the females initiated calling in its presence on day one after emergence, compared to 67% in its absence. A majority of the females initiated calling during the 10th hour of the first scotophase and thereafter the initiation shifted to earlier hours as age advanced. Females generally called longer in the presence of a host. The mean time spent in calling, from the 1st to the 10th day increased from 56 to 152 min in the absence of a host, and from 77 to 234 min in its presence. Most of the females resumed calling after mating, but the mated females exhibited a significant reduction in calling performance when compared with the virgin females.

Gorging is stimulated in some insects by plasma components alone. In other insects engorgement is maximal only when blood cell components are present in addition to the plasma. Plasma components recognized by insects are NaCl, NaHCO3 (some anopheline mosquitoes) and albumin (Anopheles dirus and Pediculus humanus). Those requiring blood cells recognize this fraction by its adenine nucleotides which, for optimal engorgement, have to be offered in the presence of isotonic NaCl solution buffered by NaHCO3 (i.e. Rhodnius, culicine mosquitoes, Simulium, tsetse and Tabanus). The presence of albumin in the feeding medium may lower the level of nucleotides required to induce gorging (i.e. Aedes).

For most haematophagous insects requiring the cellular fraction, ATP is the most effective phagostimulant. It may however be replaced quite effectively by non-hydrolyzable analogues, indicating that ATP is not essential as a source of energy for the gustatory receptors involved in blood meal recognition.

In this study, Coccinella septenipnnctata was efficient in controlling Aphis gossypii populations on cucumber in the greenhouse. This efficiency was a function of the predator/prey ratio and the initial aphid density at the time of release of the predator. Prey density had a large influence on the fecundity and establishment of this coccinellid in the greenhouse.

Worldwide, food grain is lost due to damage by insects, mites and micro-organisms. Any additional approaches towards preventing farm-storage losses could contribute to saving more grain. In one paper of this issue, the authors assess the effect of a lectin-like protein on lepidopteran larvae causing loss in stored food.