Host–instar selection in the aphid parasitoid Monoctonus paulensis (Hymenoptera: Braconidae, Aphidiinae): assessing costs and benefits

Abstract

Females of Monoctonus paulensis (Ashmead), a solitary parasitoid of aphids, generally select the relatively smaller over equally available larger instars of the pea aphid, Acyrthosiphon pisum (Harris) (Hemiptera: Aphidoidea: Aphididae). Large hosts contain more resources for parasitoid development and hence have presumably higher quality; however, they require more time to subdue and are more likely to escape. We tested the hypothesis that a female’s choice among first (L1), second (L2), third (L3), and fourth (L4) instars of the pea aphid is based on the optimal balance between fitness costs in terms of time and fitness gains in terms of offspring number and size. Prepupal mortality did not vary with host instar, but pupal mortality was greater among parasitoids developing in L4 than in any younger instars. Offspring mortality was not influenced by clutch size in that mortality risk did not differ between parasitoids developing alone and counterparts developing in a clutch. The sex ratio, measured as proportion of daughters among offspring, was female-biased on all four host instars; the degree of bias increased from 0.70 (in L1) to 0.92 (in L4). Parasitoid body size was a function of aphid size at parasitism. Females were larger than males; the magnitude of the difference in body size was constant and independent of host and hence parasitoid size. A female’s potential fecundity as measured by the number of ovarial eggs at eclosion varied with her size and larval ontogeny. The four instars of the pea aphid were ranked in the order L1 > L2 > L3 > L4 both in terms of the number of offspring produced per encountered host and in terms of a female’s time costs; first instars are easier to handle and are more abundant in the field than older instars. The four host types were ranked in the order L2 > L3 > L1 > L4 in terms of the proportion and potential fecundity of daughters among offspring. The observed preference pattern (L1 > L2 > L3 > L4) suggests that, in choosing hosts, females of M. paulensis maximize the number of offspring per unit of search time rather than simply offspring quality.

Covariance of phenotypically plastic traits induces an adaptive shift in host selection behaviour

“…Per cent parasitism of A. ervi exposed to instars of A. pisum (20 individuals) resulted in second instar hosts producing the highest reproductive returns, with declining returns from younger and older instars (McBrien 1991). Similar patterns have been demonstrated with Lipolexis oregmae and Lysiphlebia mizari using Toxoptera citricida (Tsai & Wang 2002;Walker & Hoy 2003), Monoctonus paulensis using A. pisum (Chau & Mackauer 2001) and A. ervi using Myzus persicae (Colinet et al 2005). Intermediate instars represent a high-quality pool of future resources for koinobiotic parasitoids, because the larva continues to grow and develop with their hosts after parasitization.…”

“…In contrast, adult aphids are large enough for parasitoid development; however, the larvae must compete with the host's embryos for resources, and adult aphids are thought to possess greater physiological resistance to parasitism (Walker & Hoy 2003;Colinet et al 2005). This can result in high larval or pupae mortality when parasitoid larvae develop in older aphids (Chau & Mackauer 2001;Walker & Hoy 2003;Colinet et al 2005;Henry et al 2005). For koinobiotic aphid parasitoids (i.e.…”

Size-mediated adaptive foraging: a host-selection strategy for insect parasitoids

“…Previous studies using a variety of aphid parasitoid species have shown that many species typically prefer the small and intermediate host instars for oviposition even though the larger host instars have more resources for the developing larvae (Kouame and Mackauer 1991;Mackauer et al 1996;Chau and Mackauer 2001;Tsai and Wang 2002;Colinet et al 2005). However, none of these studies have considered these results in light of optimal foraging theory.…”

Qualidade de diferentes espécies de pulgões como hospedeiros do parasitóide Lysiphlebus testaceipes (Cresson) (Hymenoptera: Braconidae, Aphidiinae)

“…Alguns modelos assumem que o desempenho (tamanho, desenvolvimento, razão sexual, fecundidade e longevidade) de um parasitóide está relacionado com o tamanho do hospedeiro no momento do parasitismo (Nicol & Mackauer 1999, Chau & Mackauer 2001. Entretanto, a relação entre as características do hospedeiro no momento do parasitismo e o ganho no desempenho do parasitóide não é linear e depende da combinação de vários fatores relacionados ao hospedeiro tais como, fi siologia e comportamento, qualidade da planta e ecologia nutricional (Mackauer et al 1996, Colinet et al 2005.…”

“…Entretanto essa característica pode não ser necessariamente verdadeira para os parasitóides cenobiontes, como L. testaceipes, cujas larvas se desenvolvem enquanto os hospedeiros ainda se alimentam e crescem (Sequeira & Mackauer 1994, Chau & Mackauer 2001. Assim, o efeito do tamanho do hospedeiro no desempenho do parasitóide é mais difícil de predizer para cenobiontes do que para as espécies idiobiontes (Jenner & Kuhlmann 2006).…”