Aphids are plant sucking bugs which occur throughout the world. The greatest number of species are in the temperate regions, where few higher plants are free from aphids. They differ from other plant sucking bugs of the Aphidoidea in that the females of at least a few generations are parthenogenetic and viviparous. Although many species are small and inconspicuous, they frequently become abundant. As many as 2000 million per acre (0.4 ha) may live on the above-ground parts of plants, and the roots may support a further 260 million (46, 92, 117, 148). Aphids, like many other insects, are capable of migrating great distances (up to 1300 km) by means of wind (25, 48, 70). Polymorphism is characteristic of aphids. Asexual aphids of some species can either possess wings (in which case they are termed alatae), or lack wings (these morphs are called apterae). Typically there are several structurally different morphs in a species, including both sexual and asexual forms.

Aphid ecology: life-cycles, polymorphism, and population regulation.

Simulation and systems management in crop protection.

The biology and pest status of the common species of Sitobion, Metopolophium and Rhopalosiphum found on wheat, barley and oat crops in Europe is critically reviewed. After preliminary sections on species and sampling, the factors that induce the production of the different morphs of cereal aphids are considered. Basic life-cycles are described and the importance of migration and dispersal considered. Host-plant preferences, survival and reproduction on different Gramineae, and feeding site preferences are considered, particularly in relation to the type and amount Of damage caused by the aphids. The effects of constant temperatures on the biology of some species have been investigated, but little is known about the effects of weather in the field. There have been few quantitative studies on the effects of natural enemies on cereal aphids, but the evidence suggests that they may be important in reducing their numbers. Side-effects of pesticides and fertilisers on aphids are also discussed. Aphids may reduce grain yield and quality. Direct damage may result from reductions.

The biology and pest status of cereal aphids (Hemiptera: Aphididae) in Europe: a review

The relationships to population density of predation, intraspecific competition and female fertility are the major components in the population dynamics of many species. The way in which these relationships interact, and the resulting effect on the population, is conveniently illustrated by a comparison of densities in successive generations using a population growth curve (Ricker 1954). This may intersect the 450 line at several points, each of which corresponds to a stable or unstable equilibrium (Takahashi 1964; Holling 1973; Southwood 1975). An extension of such an approach to include habitat stability has been developed by Southwood (1975). This views a population's dynamics as a landscape along the axes of population growth, population density and habitat stability (Fig. 1).

A synoptic population model.

Insect parasitoids play a major role in terrestrial food webs as they are highly diverse, exploit a wide range of niches and are capable of affecting host population dynamics. Formidable difficulties are encountered when attempting to quantify host-parasitoid and parasitoid-parasitoid trophic links in diverse parasitoid communities. Here we present a DNA-based approach to effectively track trophic interactions within an aphid-parasitoid food web, targeting, for the first time, the whole community of parasitoids and hyperparasitods associated with a single host. Using highly specific and sensitive multiplex and singleplex polymerase chain reaction, endoparasitism in the grain aphid Sitobion avenae (F) by 11 parasitoid species was quantified. Out of 1061 aphids collected during 12 weeks in a wheat field, 18.9% were found to be parasitized. Parasitoids responded to the supply of aphids, with the proportion of aphids parasitized increasing monotonically with date, until the aphid population crashed. In addition to eight species of primary parasitoids, DNA from two hyperparasitoid species was detected within 4.1% of the screened aphids, with significant hyperparasitoid pressure on some parasitoid species. In 68.2% of the hyperparasitized aphids, identification of the primary parasitoid host was also possible, allowing us to track species-specific parasitoid-hyperparasitoid links. Nine combinations of primary parasitoids within a single host were found, but only 1.6% of all screened aphids were multiparasitized. The potential of this approach to parasitoid food web research is discussed.

Endoparasitism in cereal aphids: molecular analysis of a whole parasitoid community.

SUMMARY

Field-caged, post-anthesis populations of the aphids Sitobion avenae and Metopolophium dirhodum reduced grain weight of wheat by 14 and 7% respectively and induced changes in the senescence of the flag leaf. Spikelet number and grain number were unaffected as they are normally determined by pre-anthesis factors. Percentage grain protein was significantly reduced by both aphid species.



The pattern of grain weight reduction within the ear was consistent with known limitations on the distribution of flag leaf assimilates among the grains. The relative effects of the two aphid species apparently resulted from the degree of nutrient drain imposed at particular feeding sites and the reduction in the leaf area duration of the flag leaf.

S. D. Wratten

Papers

The nature of the effects of the aphids Sitobion avenae and Metopolophium dirhodum on the growth of wheat