This review identifies key ways in which manipulations of the crop environment based on detailed understanding of tritrophic interactions can contribute to improvements in the control of insect pests. Such approaches are likely to be of particular benefit against those pests, notably certain species of Lepidoptera and aphid, which are difficult to control with insecticides because of insecticide resistance or suppression of natural enemies. Particular attention is given to the compatibility (or otherwise) of partial plant resistance and biological control, citing examples which support contrasting tritrophic theories. Other areas considered and supported with examples include the use or effects of allelochemicals, refugia, intercropping, crop backgrounds, fertilization regimes, parasitoid conditioning (by host plants) and transgenic crops. Examples of manipulations involving use of selective insecticides which show compatibility with biological methods are also included owing to their possible suitability in integrated crop management programmes.

The efficiency of miniature CDC light-traps in catching West African malaria vectors was evaluated during two rainy seasons in a village near Ouagadougou, Burkina Faso. Traps were employed both indoors and outdoors using human baits protected by an insecticide-free mosquito-net and different sources of light. Indoors, light from incandescent bulbs increased the catch of Anopheles gambiae s.l. (mainly A. arabiensis Patton and the Mopti chromosomal form of A. gambiae s.s. Giles) and A. funestus Giles c. 2.5 times as compared to traps whose light bulb was removed. Conversely, the difference was not significant when a UV ‘Blacklight-blue’ fluorescent tube was compared to the incandescent bulb. Protecting the bait with a mosquito-net increased the catch c. 3 times for A. gambiae s.l. and c. 3.5 times for A. funestus. A prototype model of double bednet gave intermediate yields. Outdoors, the addition of incandescent bulbs to unlighted traps did not significantly increase the number of vectors caught, but the addition of the mosquito-net to the unprotected human bait did so by c. 1.5–4 times. Thus, the CDC light-trap hung close to a human sleeping under a bednet and fitted with an incandescent bulb, was considered the most practical and efficient in terms of numbers of vectors caught, consequently its indoor efficiency was compared to human landing catches on single collectors and estimated to be 1.08 times and density-independent. Outdoor light-trap catches were either not significantly correlated to biting collections (as for A. gambiae s.l.), or density-dependent in their efficiency (as for A. funestus); thus, they were not considered a reliable means for estimating malaria vector outdoor biting densities in this area. No difference was found in the parous rate of A. gambiae s.l. samples obtained with CDC light-traps and human landing collections.

The damson–hop aphid, Phorodon humuli (Schrank), is a serious pest of hops in England. It is holocyclic (with obligatory sexual phase) and host alternating. From suction trap data, P. humuli aerial densities are known to be greatest in the main hop growing regions of Herefordshire and Kent (mid-west and south-east England, respectively), some 260 km apart. The aphid is now resistant to several insecticides. This is in part conferred by elevated carboxylesterase activity, ranging from low in susceptible to high in very resistant strains. Enzyme markers, including carboxylesterases (EST-4 to -7), separated electrophoretically from individual insects, have been used to examine the degree of genetic heterogeneity among P. humuli sub-populations on both its hosts – Prunus spp. (primary overwintering host) and hops, Humulus lupulus (secondary summer host). The esterase data revealed heterogeneity among subpopulations collected from wild, unsprayed hosts in regions less than 30 km in area, with a higher mean frequency of elevated esterase variants in the commercial hop growing regions of Herefordshire and Kent, compared with samples from a non-commercial region around Rothamsted. Esterase distributions remained similar over consecutive years. Similarly, allele and genotype frequencies for another enzyme (6-phosphogluconate dehydrogenase, 6-PGD) were also heterogeneous among subpopulations sampled at less than 30 km apart (especially from Prunus) in each of the three regions surveyed, whilst allele and genotype frequencies sometimes remained stable over a number of summers. In addition, 6-PGD genotype frequencies were mostly congruent with Hardy-Weinberg expectations, even for parthenogenetically-reproducing aphids colonizing hops. These data suggest that the 6-PGD alleles tested are selectively neutral; that gene flow (=migration) is restricted between aphid populations, even within a single region (≤ 30 km) and, that the autumn migration from hops to Prunus is probably of shorter range (perhaps less than 20 km) compared with the spring migration from Prunus to hops.

Argentine stem weevil (Listronotus bonariensis) (Kuschel) is a pasture pest of Lolium spp. in New Zealand and is the target of a classical biological control programme using the parasitoid Microctonus hyperodae Loan. However, laboratory experiments have indicated a strong density-dependent egg laying response which may have important implications for the biological control effort. Two field experiments were conducted using a manipulated range of weevil densities within 2.0–2.4 m–2 field enclosures and oviposition activity measured by weekly sampling of grass tillers within the enclosures. In both experiments, density had no effect on general oviposition behaviour although the percentage of tillers found to be infested with eggs increased significantly at higher densities. In experiment 1, L. bonariensis exhibited strong density-dependent oviposition with a significant decline in eggs/tiller/female with increasing adult density. An initial population of five L. bonariensis m−2 oviposited 32 times more eggs/tiller/female than 200 L. bonariensis m−2. By comparison, density-dependent oviposition was absent in experiment 2 with eggs/tiller/female comparable for all densities. The dramatic difference between the two experiments in the expression of density-dependent oviposition was attributed to prevailing spring temperatures during the sampling interval, which in the second experiment were much cooler and delayed the onset of egg laying. It is probable that in experiment 2, oviposition was not regulated by density-dependent cues but limited by temperature and age related mortality in the adult population. The mechanism for density-dependent oviposition, its occurrence in the field and impact on the effectiveness of the parasitoid M. hyperodae are discussed.

The green spruce aphid Elatobium abietinum (Walker) is an introduced pest in the United Kingdom and more recently in New Zealand. In outbreak years this aphid can cause severe defoliation and sometimes death of spruce trees (Picea spp.). As chemical control is not financially viable, other options including host-plant resistance and biological control are currently being investigated. An understanding of the genetic variation of this pest is imperative in fully utilizing these control strategies. To examine this, E. abietinum was collected from Sitka spruce Picea sitchensis from four locations in the UK that were up to 240 km apart. Of these, 40 aphids were analysed via two alternative polymerase chain reaction (PCR) analyses using primer pairs. The first analysis used 10-mer random primers, whilst the second analysis used primers designed to amplify across the intergenic spacer region of rDNA. Combining results from the two analyses allowed the 40 UK aphids to be separated into 28 different genotypes. The genetic variation was also high within each UK site, with 77 to 89% of the aphids sampled being of a different genotype. The two PCR analyses were subsequently used to examine 40 aphids across six sites in New Zealand up to 1200 km apart. No genetic variation was identified. Further analysis of several of these New Zealand aphids with 87 individual 10-mer primers and two polymerase enzymes, still did not detect any genetic variation. The high degree of genotypic diversity in the UK populations was presumably due to a longer period of establishment, multiple introductions and/or sexual reproduction. The contrasting lack of genetic variation in New Zealand populations was probably due to a very limited founder population, continued isolation and lack of sexual reproduction. Reduced genetic diversity can seriously decrease the ability of a population to adapt to control strategies. Therefore the durability of certain control methods may be more readily maintained in such an isolated population in New Zealand.

In an outdoor experimental set-up, the number of Prostephanus truncatus (Horn) flying from maize cobs was recorded over 38 observation weeks. Flight activity in the field was recorded for 50 weeks with three pheromone traps, each placed at c. 100–300 m from the first experimental set-up. Multiple regression analyses revealed that both flight initiation and flight activity were partly influenced by mean temperatures, but were not directly related. Flight initiation was mainly dependent on population density. An additional experiment showed that sex ratios among pheromone trap catches were not correlated with the number of beetles caught; sex ratios were female-biased throughout the year. Seasonal fluctuations in flight activity recorded with pheromone traps are mainly dependent on changes in the number and sizes of beetle populations in a given area, as well as on breeding site availability and suitability.

Genetic variation within triatomine bugs was investigated by amplification of a 400 bp portion of the mitochondrial 16S ribosomal RNA gene by polymerase chain reaction (PCR), using evolutionarily conserved primers, from a selection of species representative of the genera Rhodnius, Triatoma and Panstrongylus. Amplification products were subsequently screened for sequence variation using single strand conformational polymorphism analysis (SSCP) and also subjected to direct sequencing. Single strand conformational polymorphism analysis could detect variation within and between genera; intraspecific variation was also detected and SSCP profiles appear to be useful for identification purposes at the inter- and intraspecific levels. A 290 bp multiple alignment of 15 sequences obtained from nine species was generated, phylogenetic inference subsequently used three methods; a distance estimate, maximum parsimony and maximum likelihood. This 16S region exhibited considerable variation which ranged from intergeneric to intraspecific levels. Phylogenies from these three methods of inference were in broad agreement. Triatoma and Panstrongylus were more closely related to each other than either was to Rhodnius, in keeping with the current taxonomic appraisal.

In a rural area of southeastern Tanzania, studies were undertaken on the dispersal and survival of Anopheles funestus Giles and A. gambiae Giles s.l. during the rainy season. Blood fed, resting mosquitoes were collected indoors, marked with fluorescent powder and released on the same day from two different sites in the study area. For two weeks indoor resting mosquitoes were collected from 11 houses in the release area. Additional collections were made with a light trap from a sentinel house in the centre of the study area. Anopheles funestus was more abundant than A. gambiae s.l. Of 4262 A. funestus and 645 A. gambiae s.l. released, 4.3% and 7.4%, respectively, were recaptured. Dispersal of mosquitoes was not random: one of three areas was favoured significantly more than the other areas, as shown by the recapture and movement rates of marked mosquitoes. Based on the regression of the recapture rate, estimated daily survival rates of A. funestus and A. gambiae s.l. were 0.63 and 0.78, respectively. These were significantly different. The differences in dispersal and survival rates between the two species are discussed in view of local topography and species-specific characteristics.

The effect of temperature on the rate of consumption of the lipid reserves in Ixodes ricinus Linnaeus was characterized in constant temperature experiments. There was a consistent linear relationship between temperature and lipid consumption in nymphs and adult females and males between 5 and 37°C. Nymphs and adults were able to withstand extended periods of exposure to temperatures within the range of 2–37°C, although at 2°C there was evidence of thermal stress. The variance about the median rate of lipid consumption was found to increase with increasing rate of lipid consumption. Predictions of longevity were made using the rates of lipid consumption for the three stages studied, which ranged from 16 weeks to more than two years depending on temperature and stage. The predictions were consistent with reported survival times for I. ricinus and followed patterns reported for other ixodid ticks.