Abstract
The duration of the increase, peak and decline in abundance of the immature stages of sycamore and cereal aphids each year is ephemeral. These temporary resources are exploited by a sequence of aphidophagous insect predators. The temporal sequence in the appearance of the immature stages of coccinellids and syrphids in the sycamore and cereal aphid systems is defined. In spring in the sycamore aphid system and early summer in the cereal aphid system the immature stages of syrphids consistently appeared before those of coccinellids. In the case of the sycamore aphid the autumnal peak in abundance was on average larger than the spring peak, and although attacked by more syrphids, it was not exploited by coccinellids. These temporal patterns in the attack sequence are associated with a difference in the lower developmental thresholds (LDT) of these two groups of predators. The LDT of syrphids (4°C) enables them to be active at lower temperatures and to develop faster between 10° and 27°C than coccinellids, whose LDT is 10°C. As a consequence, early in the year, when temperatures are low but increasing, syrphids appear before and complete their development more quickly than coccinellids, and in the latter half of the year, when temperatures are generally lower and decreasing, only syrphids are likely to be able to complete their development before the aphids disappear. Thus, the niche shift between syrphids and coccinellids is possibly more a consequence of a phylogenetic constraint than a response to competition and or intraguild predation. The relevance of these findings for the ecology of intraguild predation is discussed.
Highlights
Resource utilization is usually viewed in terms of food species size (Schoener, 1974) with each species in a predator guild adapted to exploit a particular sized species of prey
On the nine occasions (9 tree-years) when the immature stages of both syrphids and coccinellids were found, those of the syrphids were recorded on average 35 days earlier than those of the coccinellids and on none of these occasions did ladybirds appear before syrphids (Fig. 2)
This study has revealed that for coccinellids and syrphids there is evidence of temporal structuring in aphidophagous guilds
Summary
Resource utilization is usually viewed in terms of food species size (Schoener, 1974) with each species in a predator guild adapted to exploit a particular sized species of prey. Species in a guild are able to displace other species from a particular portion of the resource space by virtue of their being better adapted to exploit that particular species of prey in that resource space. Increasing size in predatory coccinellids is associated with increase in size and/or mobility of their prey. The smallest species of predatory coccinellids feed on mites, and the largest on caterpillars and beetle larvae (Dixon & Hemptinne, 2001). There appears to be no association between the size of an aphidophagous predator and that of the species of aphid it exploits
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