Abstract
Monoctonus paulensis is a solitary parasitoid of several species of aphids, including the pea aphid, Acyrthosiphon pisum. We evaluated host-instar selection by comparing the parasitoid's preference for the four nymphal instars of the pea aphid, presented two at a time in dichotomous choice tests. Females parasitized more, and laid more eggs in, the relatively smaller aphids among those available. This preference was independent of aphid instar: L1 > L2 > L3 > L4. Preference was not influenced by female size or age. Normal and anaesthetized aphids were accepted equally. The total time needed by a female to capture, position, and parasitize an aphid varied among host instars, with fourth instars requiring nearly twice as much time as first, second, and third instars. The probability of an attacked aphid escaping or avoiding parasitism increased with aphid instar, from ~10% in first and second instars to ~50% in fourth instars. Although fourth-instar pea aphids contain more resources for offspring development than smaller counterparts, it may not be profitable for a female to invest opportunity time in attacks on large aphids.
Highlights
Most species of aphidiine parasitoids (Hymenoptera: Braconidae, Aphidiinae) attack and successfully develop in all four nymphal instars of their aphid hosts (Mackauer, 1973; Liu et al, 1984; Sequeira & Mackauer, 1987; Weisser, 1994)
The numbers of aphids parasitized (F5, 78 = 6.309, P = 0.0001) and eggs laid in total by females of M. paulensis (F5, 78= 6.581, P < 0.001) varied between the six instar combinations (Table 2)
Pooling within instars across combinations, we found that the mean number of aphids parasitized by each female declined with host instar: L1 (x ± SE = 8.05 ± 0.25 aphids, n = 44), L2 (7.45 ± 0.42 aphids, n = 40), L4 - apteriform first (L3) (6.86 ± 0.32 aphids, n = 43), and L4 (4.22 ± 0.41, n = 41), in that order
Summary
Most species of aphidiine parasitoids (Hymenoptera: Braconidae, Aphidiinae) attack and successfully develop in all four nymphal instars of their aphid hosts (Mackauer, 1973; Liu et al, 1984; Sequeira & Mackauer, 1987; Weisser, 1994). Second and third instars are often preferred, aphid embryos and adult aphids may be suitable hosts (Mackauer & Kambhampati, 1988; Volkl & Mackauer, 1996). Patterns of preference are not fixed, but often vary with the availability and relative abundance of different host types (Chow & Mackauer, 1991) and the female’s physiological state, including her egg load, previous reproductive success, and age (Weisser, 1994; Michaud & Mackauer, 1995; Mackauer et al, 1996). Ecological theory predicts that a female’s reproduc tive success reaches a maximum ifhost choice is based on the optimal balance between her costs in finding and cap turing a host and the latter’s quality for the development and growth ofher offspring (reviewed in Godfray, 1994)
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