The causes and processes of the mid-summer population crash of the potato aphids Macrosiphum euphorbiae and Myzus persicae (Hemiptera: Aphididae).

Populations of many phloem-feeding aphid species in temperate regions increase exponentially in early summer and then 'disappear', usually over a time-scale of a few days, in July. To understand these dynamics, empirical investigation of the causes and modelling of the processes underlying population change are required. Numbers of the aphids Myzus persicae(Sulzer) and Macrosiphum euphorbiae (Thomas), monitored over three years in commercial potato fields in the UK, increased to a maximum of 2-2.5 per leaflet on 16 July in 1999 and 2001, and then declined to < 0.25 per leaflet by 26 July. In 2000, aphid numbers remained very low (< 0.25 per leaflet) throughout the season. The onset of the crash in aphid numbers (16-19 July in 1999 and 2001) was consistently associated with changes in the phloem amino acid composition of potato leaflets. Natural enemies, including syrphids, parasitoids, coccinellids, chrysopids and entomopathogenic fungi, increased in abundance throughout the sampling period. The incidence of winged emigrant aphids prior to the crash was low (< 10%). Experimental manipulation during 2001 demonstrated that, during the crash period, the fecundity of aphids (caged on leaves to exclude natural enemies) was depressed by 25-45% relative to earlier in the season, and that presence of natural enemies reduced aphid numbers by up to 68%. Using these data, an excitable medium model was constructed, which provided a robust description of aphid population dynamics in terms of plant development-induced changes in aphid fecundity and temporal change in natural enemy pressure.

Amino acid composition and nutritional quality of potato leaf phloem sap for aphids.

To define plant 'nutritional quality' for aphids, the causal basis of the variation in aphid performance between host plants of different developmental ages was explored using the aphids Myzus persicae and Macrosiphum euphorbiae on potato plants (Solanum tuberosum). Both aphid species performed better on developmentally young ('pre-tuber-filling') plants than on mature ('tuber-filling') plants. Aphid performance did not vary with leaf phloem sucrose:amino acid ratio but could be related to changes in the amino acid composition of the phloem, which included a developmental shift from high glutamine levels in pre-tuber-filling plants to low glutamine levels in tuber-filling plants. Aphid performance on chemically defined 'young' and 'old' diets, with amino acid composition corresponding to that of phloem amino acid composition in pre-tuber-filling and tuber-filling plants, respectively, confirmed that phloem amino acid composition contributed to low aphid performance on tuber-filling plants. The relatively poor performance on 'old' diets could be accounted for, at least in part, by depressed feeding rates. These data suggest that amino acid composition of the phloem is one factor shaping the nutritional quality of plants for aphids.