Field experiments in which clip-caged apterae of Myzus persicae (Sulzer) (Homoptera: Aphididae) were monitored on sugarbeet revealed that leaf age, plant age and infection of the host with beet yellows closterovirus (BYV) had very large effects on aphid performance (development, reproduction and survival). On healthy plants performance was much better on young, expanding heart leaves than on older leaves. Performance on senescent leaves was poor. When apterae were kept on young heart leaves all their lives (by moving clip-cages to younger leaves every 4 days) there was a strong negative relationship between aphid performance and plant age (measured as plant leaf number at birth). On plants inoculated with BYV apterae showed large improvements in performance. These occurred so rapidly that nymphs born at the time of virus inoculation could benefit as much as those born later. The sizes and between-leaf distributions of M. persicae populations which developed from standard numbers of colonists on whole plants in cages were consistent with the results of the clip-cage experiments. Alatae were less sensitive than apterae to differences in plant and leaf age. Implications of the results for BYV epidemiology and control are discussed.

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The mechanisms and host plant responses that are indicative of tolerance to the African rice gall midge, Orseolia oryzivora Harris & Gagne were investigated in four promising donor rice cultivars, Cisadane, Bw 348-1, Tox 4093-17-1 and ITA 306 (local check), in a screen house at the International Institute of Tropical Agriculture (IITA), Ibadan, Nigeria. Results indicated that Cisadane was tolerant to infestations on the basis of higher (P 0n05) percentage seedling survival, production of significantly higher number of fertile tillers, productive panicles and significantly higher grain yield per infested plant than the control. Both single infestation at one larva per plant and adult infestation once at 14 days after seeding were positively correlated to higher fertile tiller production per infested plant (at 80 days after transplanting) and higher grain yield per infested plant at maturity. The grain yield per infested plant was significantly higher (P 0n05) in the tolerant variety (Cisadane) than in the control.

Host range experiments and field sampling in Nigeria produced no evidence that African rice gall midge, Orseolia oryzivora Harris & Gagné, can develop on plants other than Oryza species. Sampling in three outbreak areas during 1994 showed that the insect's annual cycle varied according to the agroecological zone and rice cropping pattern. In the humid forest zone, Orseolia oryzivora persisted through the short dry season on ratoons of cultivated rice Oryza sativa at a rainfed site and on dry season rice crops at an irrigated one. In contrast, at rainfed sites in the moist savannah zone the pest survived the longer dry season on the perennial wild rice O. longistaminata, while ratoons and volunteers of O. sativa provided 'bridges' between the wild host and wet season rice crops. Early in the wet season at rainfed sites, galls of Orseolia oryzivora were not found at high density on wild rice, ratoons or volunteers. The heavy infestations which developed by October resulted primarily from rapid multiplication on rice crops themselves during the wet season. At all 13 sites sampled, the large majority of galls were found on fallow or cropped rice fields, rather than in ditches, bunds or uncultivated wetland, irrespective of the time of year or the hosts involved. From gall dissections, the parasitoids Aprostocetus procerae (Risbec) and Platygaster diplosisae Risbec caused over 30% mortality at some sites by October but generally increased too late to prevent crop damage. Implications of the results for the management of Orseolia oryzivora are discussed.

Viviparae of Sitobion avenae (F.) in clip-cages on winter wheat in field plots in southern England were monitored throughout the 1980-81 winter to determine characteristics of development, reproduction and lifespan under winter conditions. Rate-temperature relationships for development and reproduction were well described by straight lines when temperatures were measured above 3°C. A simple thermal-time scale could therefore be used for both processes. The parental morph and the time of year modified thermal-time requirements for the production of an adult's first 20 nymphs but did not affect those for development. For both alatae and apterae, nymph production per day-degree was greater than at higher temperatures and the relationship between reproductive rate and age was different. The minimum temperature experienced by the aphids was -7°C; survival was good, with little low temperature mortality. Total fecundity of apterae was usually greater than at higher temperatures, though some apterae and alatae that were deposited in October had low fecundity, possibly because they were insufficiently acclimatized at the onset of cold winter weather. Overwintering alatae had lower development rates, reproductive rates and total fecundity than apterae and did not survive as long, but were restless as young adults and appeared ready to migrate. Oviparae were produced from late September to mid-October but not after this, suggesting that the clone used could 'switch off' sexual morph production in winter.