The behaviour and pest status of the rice stem borers Diopsis longicornis Macquart and D. apicalis Dalman were investigated in the Republic of Guinée, particularly during the dry season. The variables were direct seeded and transplanted rice, and three (1988, 1989) and four (1987) planting dates. Transplanted rice was damaged nearly three times more than direct seeded rice in the dry season, but no significant difference was detected during the wet season. As previously observed in other West African countries, D.apicalis only occasionally damages rice in Guinée. Contrary to findings elsewhere, D. longicornis was not an important pest of Guinean rice, infesting only 5% and 3% of stems over the three dry and the two wet seasons, respectively. However, year round rice production could promote build up of the diopsid population if dry season planting is not sufficiently delayed (45 days) after the last rains of the wet season.

In surveys of cassava fields in the coastal savanna and rain forest zones of Ghana, eight species of ants, mostly in the genera Camponotus, Crematogaster and Pheidole (Hymenoptera: Formicidae), were found attending cassava mealybug, Phenacoccus manihoti Matile-Ferrero. Ant densities were much higher in the rain forest than in the savanna and, in most zones, positively correlated to mealybug population densities. Pheidole megacephala (Fabricius) was the only species more abundant in the interior of fields, whereas the other species preferred the edges of fields or, like Camponotus spp., did not respond to edge effects. Weediness of the fields did not clearly influence the presence of ants. Crematogaster sp. and P. megacephala spent most of the time on cassava tips, where they built carton tents. Ants of the three genera reduced parasitism rates by the exotic Epidinocarsis lopezi (De Santis) (Hymenoptera: Encyrtidae) to half, compared to the rates observed on mealybug colonies of equal size that were not attended. Hyperparasitism by Prochiloneurus insolitus (Alam) (Hymenoptera: Encyrtidae) and Chartocerus hyalipennis Hayat (Hymenoptera: Signiphoridae) and predation by the indigenous Exochomus troberti Mulsant (Coleoptera: Coccinellidae) were similarly reduced. Where ants were excluded experimentally by insectcide barriers, parasitism was 32%, compared to only 10% in the control, and predator densities were reduced three-fold. In a 24 h experiment, Pheidole megacephala was observed to remove about half the E. troberti larvae; Crematogaster sp. and Camponotus spp. were less efficient. The results show the locally strong interference of ants with biological control of the cassava mealybug. Despite this interference, mealybug populations in the rain forest were judged to be below damaging levels.

Pyrethroid resistance was examined in adult Helicoverpa armigera (Hübner) in both laboratory and field selected strains. A series of F1 and backcrosses was made between a susceptible and an isofemale resistant strain. Dose-responses were obtained using contact with a treated surface inside a scintillation vial. Inheritance of resistance was consistent with an incompletely dominant major gene and resistance factors were greater than 100-fold. Piperonyl butoxide synergized the effects of the synthetic pyrethroid, fenvalerate, 1000-fold and eliminated resistance entirely. Field trials examined the mortality of resistant and susceptible adults exposed to commercial application rates of synthetic pyrethroids. Adults were immobilized and placed in cotton fields prior to aerial applications of insecticide. Survival of resistant adults was significantly greater than susceptible ones in both trials. It was concluded that selective mortality in the adult life-stage contributes to the evolution of pyrethroid resistance.

Male Glossina morsitans morsitans Westwood and Glossina pallidipes Austen caught in artificial refuges in Zimbabwe had ca. six times as much haematin and up to 32% more fat than flies from odour-baited traps, but haematin-specific fat levels did not differ significantly between methods. G. pallidipes estimated to have fed < 9 h prior to sampling contained ca. 3.3 mg fat - only 10% less than peak levels. A differential equation model for blood-meal metabolism was developed which described well the changes in fat levels of laboratory G. m. morsitans and the relationship between fat and log haematin in field data. The model predicts a mean feeding interval (T) of 54 – 65 h and mean fat levels of ca. 2.8 mg for G. pallidipes at feeding. When haematin frequency data were analysed as suggested in the literature, estimates of feeding rates and intervals, and of the non-feeding phase, varied with sampling method. Published estimates of activity levels related to feeding suggest a model where feeding rates increase approximately linearly during the trophic cycle. For T = 58 h the model gives good predictions of mean fat levels and variances in both species, with starvation rates < 1% / day. Models with long non-feeding phases predict fat levels up to 40% lower than observed and with smaller variances. For constant feeding rates, fat levels were well predicted for T = 54 h, but predicted death rates (> 5% / day due to starvation alone) were impossibly high. It is suggested that a proportion of tsetse flies with high fat levels feed on mobile hosts early in the trophic cycle and that this effect is more marked for G. m. morsitans than for G. pallidipes. Over-estimates of T result from the failure to consider these tsetse flies and not to errors in the assumed time scale, nor failure to catch high-fat tsetse flies which visit stationary traps.

A simple optical method for automatic recording and classification of a suction trap catch is described. The insects are illuminated against a dark background as they pass through a detection volume, and the amount of scattered light is used to measure insect size. The design centres on the detection volume, which is a volume through which the insects are made to pass, and within which they may be detected. The design is approached in four stages: 1. Delivery of insects to the detection volume. 2. Illumination of the detection volume. 3. Collection and detection of scattered light. 4. Signal analysis. The analysis could also be applied to related techniques. Results with a prototype demonstrate that classification into broad size categories is straight-forward (e.g. approximately three classes spanning body lengths of 2–7 mm), despite uncertainties of insect reflectivity, aspect and trajectory. Applications of the method are discussed, along with a brief mention of alternative techniques.

The effect of ivermectin treatment of cattle on the rate of dung beetle colonization of cow pats was assessed by pitfall trapping in Denmark, Tanzania and Zimbabwe. Ivermectin was injected subcutaneously into heifers at the recommended dose of 0.2 mg/kg body weight, and dung to be used as bait in pitfalls was collected at intervals after treatment. In one experiment in Denmark, species of the scarab beetle Aphodius and the hydrophilids Sphaeridium and Cercyon, preferred control dung from untreated cattle; no preference was found in two other experiments, involving some of the same species. In the Tanzanian trials, the overall tendency for scarabaeid beetles was to prefer control dung. In Zimbabwe, two scarabaeid species (Euoniticellus intermedius (Reiche) and Liatongus militaris (Castelnau), both belonging to Oniticellini) were particularly attracted to dung from treated cattle; two other scarabaeids showed no preference. It is concluded: 1. That ivermectin therapy can affect the rate of dung colonization by attracting or repelling beetles. 2. That the responses are probably not caused by ivermectin per se, but by some side effect, the nature of which is obscure and unpredictable with our present knowledge.

Daily light trap catches of Scotinophara coarctata (Thunberg) were analysed from mid-September 1986 to December 1990 and the physiological status of the trapped adults was examined. The catch size fluctuated synchronously with the lunar phase; large catches occurred around the full moon period and very few adults were trapped around the new moon period. Seasonally, the catches were large from January to March and from July to September, while they were small from May to June and October to November. The seasonal fluctuations of the catches seemed to be related to the growth stages of the rice plants on which the adults multiplied. The sex ratio of trapped adults was biased toward males, unlike that of the cage-reared insects which was 1:1. Most of the trapped females showed undeveloped ovaries and some of these females appeared to have oviposited previously. Light-attracted adults showed a considerable tolerance to starvation and survived for 20–30 days when they were given water, but for only two days in the absence of water. When the light-attracted females were supplied with food, their ovaries developed rapidly and females with mature eggs were produced after nine days. When the starvation period was prolonged, the ovaries remained immature and the fat bodies reduced in size. The nocturnal take-off of the adults was not triggered simply by the full moon illumination. Adult eclosion did not synchronize with the moon cycle.

Fourth instar larvae of a new strain of Culex quinquefasciatus Say from Tanzania (TANGA) were tested for insecticide resistance. Initially, the resistance ratio (RR) to chlorpyrifos was 41.8, to temephos 30.8, to propoxur 3.7. After 2–3 years of laboratory culture, resistance to chlorpyrifos and propoxur had declined (chlorpyrifos 5.7, 3.8; propoxur 1.9, permethrin 1.9). Significant synergism was found between s, s, s-tributyl trithiophosphate (DEF) and chlorphyrifos, reducing the RR from 8.0 to 2.5. Synergism between piperonyl butoxide and permethrin was less than in a susceptible control strain. Twelve esterase isozymes of different relative mobilities (Rm) on polyacrylamide gel electrophoresis were identified, ten of which remained when the strain was reinvestigated two years (approximately 32 generations) later. Null activity for all but one of these bands was observed in some larvae. Four esterase bands (Rm 0.25, 0.27, 0.31, 0.34, designated A2, A3, B2, B3) showed polymorphism in activity, with very intense bands in some larvae. The mean frequency of bands with activity greater than standard, declined as organophosphorus (OP) resistance declined, but resistance was unconnected with the frequency of nulls at these positions. In mass larval assays of in vitro sensitivity of acetylcholinesterase (AChE) to propoxur, the I50 exceeded 10x10−4M, compared with 0.1x10−4M in a reverted resistant strain (RANGOON). Single larvel assays revealed heterogeneity, which was interpreted on the basis of an AChE resistance allele (AceR) with a frequency of 0.23.

Evidence from observing resistance in decline in fourth instar larvae of the multiple resistant MUHEZA strain of Culex quinquefasciatus Say from Tanzania (MUHEZA), showed that the major mechanisms for chlorpyrifos and propoxur resistance were different. Resistance to chlorpyrifos declined more than 400-fold (from an initial resistance ratio (RR) of 14285) while resistance to propoxur remained stable for at least 30 generations of laboratory culture. Significant synergism was found between propoxur and piperonyl butoxide (PB), propoxur and s,s,s-tributyl trithiophosphate (DEF), and permethrin and PB, but antagonism occurred between chlorpyrifos and PB, and no synergism between chlorpyrifos and DEF. Nine esterase isozymes active against naphthyl acetates on polyacrylamide gel electrophoresis (PAGE), were identified, four (A2, A3, B2, B3) showing polymorphism in activity, with very intense expression at one or other position in more than 32% of larvae, and null expression in less than 30%. The frequency of intense bands and of nulls both declined as resistance declined. A selected substrain (MUHEZA-fb), breeding true for A2, A3, B2 and B3 at the standard level of activity, showed almost stable chlorpyrifos resistance (RR=1428–1785) for approximately 35 generations. In mass larval assays of in vitro sensitivity of acetylcholinesterase (AChE) to propoxur, the I50 in MUHEZA was 950-fold greater than in a reverted resistant strain (RANGOON). Single larval assays indicated an AChE resistance allele (AceR) at frequency 0.43. PAGE of AChE revealed nine isozymes in MUHEZA and five in RANGOON, three of which were in common. It is concluded that propoxur resistance was due principally to AceR with a minor influence from oxidases and non-specific esterases, while chlorpyrifos resistance was due to an interaction between AceR and non-specific esterases, the latter exerting the dominant effect.

Pareuchaetes aurata aurata (Butler) from Chromolaena jujuensis in northern Argentina was tested as a biological control candidate for the composite weed C. odorata. The larvae feed voraciously and complete their development on C. odorata. The host-plant feeding range of P. a. aurata is limited to the genus Chromolaena. It is suggested that the egg-laying behaviour of P. a. aurata, which scatters its eggs around the base of the host-plant, will help to overcome the problem of ant predation which prevented the establishment of Pareuchaetes pseudoinsulata Rego Barros, which lays its eggs in batches, in South Africa. Any possible benefits of the new association between P. a. aurata and C. odorata may also contribute to the success of this biological control programme.

Bread and durum wheat varieties varying in wheat stem sawfly resistance were sown at different densities in northern Syria for two seasons to examine the effect of density on plant characteristics related to infestations of the wheat stem sawflies Cephus pygmeus (Linnaeus) and Trachelus spp. Plants sown at low densities had a longer interval between developmental stages and had higher stem solidity than those sown at high densities. Stem solidness was negatively correlated with the percentage of sawfly-infested stems. Plant height and grain yield were neither strongly nor consistently correlated with wheat stem sawfly infestation but were influenced by variety and density. These results differ from those of previous studies in North America where widely spaced plants suffered higher infestations than did closely spaced plants. Results of our study suggest that stand density should be considered when recommending wheat stem sawfly-resistant wheats to farmers in west Asia and North Africa and when designing screening trials for wheat stem sawfly resistance in the field.

This paper reports the results of a survey of parasitism of budmoths on larch in Europe as part of a programme to determine the potential of European parasitoids for the classical biological control of the North American defoliator, Zeiraphera canadensis Mutuura & Freeman. The parasitoid complexes of Zeiraphera diniana (Guenée), Exapate duratella (Heyden), Ptycholomoides aeriferanus (Herrich-Schäffer) and Spilonota laricana (Heinemann) are determined from field-collected samples of host larvae and pupae. The parasitoid complexes are characterized by their component guilds and the mean apparent parasitism of the individual species. Host phenology appears to affect the parasitoid complex of the larch budmoths. Parasitoids that attack the young larval instars are better represented from hosts that overwinter as larvae, while parasitism is dominated by the guilds that attack the late larval instars in hosts that overwinter as eggs. The study identifies the ichneumonids, Phytodietus spp., Chorinaeus cristator (Gravenhorst) and Phaeogenes osculator (Thunberg) for further experimental investigation as potential candidates for the biological control of Z. canadensis.

A new field-based method for sorting insects collected in suction samplers is proposed. Test sampling on grassland showed that for most taxonomic groups it was over 80% efficient. The efficiency of sorting for certain insect groups was influenced by the amount of debris in the sample. However, it was possible to determine a relationship between these two variables which enabled a correction factor to be calculated and used to compensate for this. It is suggested that popular use of this technique would radically increase the amount of research performed in the field.

Field and laboratory trials were undertaken to determine effects and efficacy of neem extracts on the Sahelian grasshopper, Kraussaria angulifera (Krauss). Formulations of aqueous extracts were sprayed in field experiments. Concentrations of 0.5% and 1% were effective in reducing grasshopper feeding. Neem kernel powder applied systematically, partially protected millet against K. angulifera. The residual activity of the botanical insecticide was at least seven days. Analysis of plant selection behaviour showed that neem is primarily an antifeedant rather than a repellent for this insect. On first to third instar nymphs, growth regulating activity, expressed as difficulty in moulting or an incompleteness of sclerotization, may occur.

Culex quinquefasciatus Say from Peliyagoda, Sri Lanka, has elevated esterases A2 and B2. In vitro insecticide metabolism studies on the temephos-selected Sri Lankan Cutex quinquefasciatus strain, Pel RR, showed that fenitrothion was hydrolysed more rapidly than malathion. A significantly larger amount of fenitrothion was metabolized in Pel RR compared to the insecticide susceptible Pel SS strain, but the amount of malathion metabolized in the two strains was not significantly different. There was more esterase hydrolysis of fenitrothion than oxidase or glutathione S-transferase metabolism in Pel RR, which suggests that the elevated esterases in this strain have both metabolic and sequestering functions and that the relative importance of the two roles depends on the insecticide used. Genetic crosses of the temephos resistant and susceptible strains, showed that both resistance and elevated esterase activity were inherited as semi-dominant multi-genic characters. Hydrolysis of 2-naphthyl acetate was highest in the Pel RR strain, lowest in the Pel SS and intermediate in the F1 cross of the two strains. A minor maternal effect on resistance occurred with backcrosses involving resistant (F1) females, which gave consistently and significantly lower mortalities than those involving resistant (F1) males.

An experimental hut trial of bednets was undertaken in The Gambia to compare six different permethrin treatments, including a wash-resistant formulation, and to test the effect of thorough hand washing of the nets. The wash-resistant formulation on nylon and cotton nets deterred Anopheles gambiae Giles (sensu lato) from house entry. All treated bednets killed a significantly higher number of A. gambiae (sensu lato) than untreated nets. The ability to kill mosquitoes was not diminished with the wash-resistant and Wellcome formulations after washing unlike the normal ICI formulation, this result being consistent with the permethrin residues found by gas liquid chromatographic (GLC) analyses to persist on the nets after washing. The mortality on treated cotton nets was significantly less than on treated nylon nets. Of the unwashed nets only the cotton net was significantly repellent. Both washed and unwashed treated nets significantly reduced the proportion of human bloodfed mosquitoes that survived and were potentially able to transmit malaria. Although the wash-resistant formulation might prove more efficient at killing mosquitoes with time, as compared to the other treatments, in The Gambia, where women wash their nets frequently, we suggest that the cheapest and most readily available formulation is used on nylon nets where possible.