The relationship between primary parasitoids and hyperparasitoids of cereal aphids: an analysis of field data

1. We analysed data collected in 1985-89 from cereal fields to elucidate the relationships between the primary parasitoids and hyperparasitoids of cereal aphids. The analysis aimed to establish whether or not hyperparasitoids interfere with the impact of primary parasitoids on cereal aphid populations. 2. From more than 10000 parasitized aphids, 14 primary parasitoid and 18 hyperparasitoid species were reared. The five most common hyperparasitoids attacked the five most common primary parasitoids, which, in turn, parasitized most frequently the most abundant aphid species, Sitobion avenae (F.). 3. Levels of aphid primary parasitism never exceeded 33%, whereas levels of hyperparasitism often reached 100% in the latter part of the season. However, the low rates of aphid primary parasitism could not have been solely the result of hyperparasitism: if primary parasitoid females which emerged early continued to deposit eggs before the collapse of the aphid populations, those females should have been numerous enough to parasitize large numbers of their hosts before hyperparasitoid pressure became high. 4. We hypothesize that primary parasitoid females leave areas which develop high hyperparasitoid densities. A multiple linear regression model was used to test the hypothesis. The observed declines of aphid primary parasitism were quantitatively related to hyperparasitoid density. 5. The results of this study and additional theoretical considerations are in favour of the above hypothesis. However, other factors such as mortality other than that due to hyperparasitoid attack, and aestivation acting together may also contribute to the low abundance of primary parasitoid females later in the season.