Response of a phytoseiid predator to herbivore-Induced plant volatiles: Selection on attraction and effect on prey exploitation

Bean plants infested with herbivorous spider mites emit volatile chemicals that are attractive toP. persimilis, a predator of spider mites. In Y-tube olfactometer tests we evaluated involvement of a genetic component in predator response to herbivore-induced plant volatiles. Replicated bidirectional selection resulted in a significant increase in attraction after one generation of selection, but no decrease even after three generations of selection, indicating significant, but unbalanced, additive genetic variation in predator perception of, or response to, herbivore-induced plant volatiles. Selected lines responded differently than an unselected population to food deprivation, pointing to an interaction between their internal state and response to plant volatiles. Selected lines also differed from unselected ones in behaviors associated with local prey exploitation, such as residence time, prey consumption, and reproduction. At lower prey densities,P. persimilis from both “+” lines left spider mite-infested leaves more rapidly and consumed fewer prey eggs than an unselected population. Defining olfactory components of predator search behavior is one step in understanding the effect of plant volatiles on predator foraging efficiency. By selecting lines differing in their attraction to herbivore-induced plant volatiles we may experimentally investigate the link between this behavior, predator foraging efficiency, and local and regional predator-prey population dynamics. The impact of significant additive genetic variation in predator response to plant volatiles on evolution in a tritrophic context also remains to be uncovered.     

The quantitative genetics of growth in a field cricket

Because of its relationship with both development time and adult size, the rate of growth in determinately growing organisms is an important aspect of their life histories. We reared sixty-nine families of Gryllus pennsylvanicus derived from a natural population and found significant genetic variation in growth rate as estimated by the slope of linearized growth trajectories. We found no evidence for a genetic tradeoff between rate of growth and survival, nor rate of growth and fecundity. In principle, adult size may be determined both by the rate of growth and the time taken by the nymphs to develop. Our data indicate that variation in adult size is explained by variation in growth rate, not by variation in development time. We conclude with a discussion of the plausible explanations for the presence of genetic variation in growth rate in this natural population.     

The cabbage aphid: a walking mustard oil bomb

The cabbage aphid, Brevicoryne brassicae, has developed a chemical defence system that exploits and mimics that of its host plants, involving sequestration of the major plant secondary metabolites (glucosinolates). Like its host plants, the aphid produces a myrosinase (beta-thioglucoside glucohydrolase) to catalyse the hydrolysis of glucosinolates, yielding biologically active products. Here, we demonstrate that aphid myrosinase expression in head/thoracic muscle starts during embryonic development and protein levels continue to accumulate after the nymphs are born. However, aphids are entirely dependent on the host plant for the glucosinolate substrate, which they store in the haemolymph. Uptake of a glucosinolate (sinigrin) was investigated when aphids fed on plants or an in vitro system and followed a different developmental pattern in winged and wingless aphid morphs. In nymphs of the wingless aphid morph, glucosinolate level continued to increase throughout the development to the adult stage, but the quantity in nymphs of the winged form peaked before eclosion (at day 7) and subsequently declined. Winged aphids excreted significantly higher amounts of glucosinolate in the honeydew when compared with wingless aphids, suggesting regulated transport across the gut. The higher level of sinigrin in wingless aphids had a significant negative impact on survival of a ladybird predator. Larvae of Adalia bipunctata were unable to survive when fed adult wingless aphids from a 1% sinigrin diet, but survived successfully when fed aphids from a glucosinolate-free diet (wingless or winged), or winged aphids from 1% sinigrin. The apparent lack of an effective chemical defence system in adult winged aphids possibly reflects their energetic investment in flight as an alternative predator avoidance mechanism.     

Phototoxicity of plant secondary metabolites: Insect and mammalian perspectives

Phototoxic chemicals produced by plants have been studied in a number of contexts, most notably as protective agents against mammalian and insect herbivores. Although there are commonalities in the responses of these two groups of herbivores to plant phototoxins, there are differences as well. Whereas a greater range of chemical classes has been demonstrated to display phototoxicity against insects, considerably more information is available on symptomology of phototoxicity and mechanisms of action in mammals. The commonalities include alterations in behavior following ingestion, notably photophobia, disruption of integumentary integrity following contact or ingestion, and metabolic detoxification following ingestion, in the case of furanocoumarins involving cytochrome P450 monooxygenases. Not yet known to exist in insects are phototoxin-mediated effects on sensory (particularly visual) systems and phototoxicity resulting from abnormal chlorophyll metabolism. In order to gain greater understanding of the ecological significance of phototoxin-mediated plant defense against both insects and mammals, there is a need for more studies centered on natural associations. © 1995 Wiley-Liss, Inc.     

Effects of plant trichomes on herbivores and predators on soybeans

Abstract Tritrophic interaction in soybean system has received increasing attention recently. However, few studies have investigated the influence of plant trichomes on the population dynamics of soybean herbivores and their natural enemies. We conducted a field survey to investigate whether soybean trichomes affected the abundance of herbivores and their predators. The results of this study show that moderately or densely pubescent trichomes have positive influences on the abundance of some herbivores (e.g., Stollia guttiger) and predators (e.g., Propylaea japonica and Orius similes) although the influence may change over time, while trichome types do not affect the density of soybean aphid, Aphis glycines.     

Identifying potential evolutionary relationships within a facultative lycaenid-ant system： Ant association, oviposition, and butterfly-ant conflict

Facultative associations are commonly encountered between ants and lycaenids, although the nature and patterns of associations are typically unclear. This study investigated a facultative symbiosis involving the lycaenid Theclinesthes albocincta （Lycaenidae）, its host plant Adriana quadripartita and Australian native ants. Ants in the genera Ochetellus and Iridomyrmex were most frequently found in association with T. albocincta larvae, although Iridomyrmex ants were found in much lower abundance than were ants in Ochetellus. The abundances of Ochetellus and Iridomyrmex were highly correlated with larval abundance, but not egg abundance. Observations and experiments recorded oviposi- tion on male inflorescences on more than 95% of occasions, but oviposition was not greater on inflorescences with ants present. Behavioral assays showed that Iridomyrmex ants were aggressive towards female butterflies on significantly more occasions than were Ochetellus ants. These findings indicate potential evolutionary relationships between T. albocincta and two genera of ants that were abundant within the habitat.