Localization of Defensive Chemicals in Two Congeneric Butterflies (Euphydryas, Nymphalidae).

Many insect species sequester compounds acquired from their host plants for defense against natural enemies. The distribution of these compounds is likely to affect both their efficacy as defenses, and their costs. In this study we examined the distribution of sequestered iridoid glycosides (IGs) in two congeneric species of nymphalid butterfly, Euphydryas anicia and E. phaeton, and found that the pattern of localization of IGs differed between the two species. Although IG concentrations were quite high in the heads of both species, the relative concentrations in wings and abdomens differed substantially. Euphydryas anicia had relatively high IG concentrations in their abdomens and low IG concentrations in their wings, whereas the reverse was true in E. phaeton. We interpret these results in light of two current hypotheses regarding where sequestered chemicals should be localized: that they should be found in wings, which would allow non-lethal sampling by predators; and that their distribution is constrained by the distribution of tissue types to which sequestered compounds bind. We also offer the third hypothesis, that costs of storage may differ among body parts, and that the localization of compounds may reflect a cost-reduction strategy. Results from E. phaeton were consistent with all three of these non-mutually exclusive hypotheses, whereas results from E. anicia were only consistent with the notion that tissue bias among body parts plays a role in IG distribution. The finding that these two congeneric butterflies exhibit different patterns of IG localization suggests that they have been shaped by different selection regimes.

Arbuscular mycorrhizal fungal species suppress inducible plant responses and alter defensive strategies following herbivory.

In a greenhouse experiment using Plantago lanceolata, plants grown with different arbuscular mycorrhizal (AM) fungal species differed in constitutive levels of chemical defense depending on the species of AM fungi with which they were associated. AM fungal inoculation also modified the induced chemical response following herbivory by the specialist lepidopoteran herbivore Junonia coenia, and fungal species varied in how they affected induced responses. On average, inoculation with AM fungi substantially reduced the induced chemical response as compared with sterile controls, and inoculation with a mixture of AM fungi suppressed the induced response of P. lanceolata to herbivory. These results suggest that AM fungi can exert controlling influence over plant defensive phenotypes, and a portion of the substantial variation among experimental tests of induced chemical responses may be attributable to AM fungi.

The importance of sequestered iridoid glycosides as a defense against an ant predator.

We reared larvae ofJunonia coenia Hubner (Nymphalidae) on artificial diets with trace concentrations of iridoid glycosides and on leaf diets with higher concentrations of iridoid glycosides. We offered these caterpillars to predacious ants and observed the effects of the following on predation: diet (artificial vs. leaf), site (ant colonies in dry vs. wet areas), instar (early vs. late), and time (changes in predation over five days). Diet and site were consistently significant predictors of the ants' propensities to reject prey and the caterpillars' abilities to escape predation. Leaf-diet caterpillars escaped more frequently than artificial-diet caterpillars, and ants from dry sites were more likely to reject prey than ants from wet sites. The percentage of iridoid glycosides found in individual caterpillars was also a good predictor of the probability of rejection by predators and prey escape. Caterpillars with higher levels of iridoids were more likely to be rejected and to escape, suggesting that sequestered iridoid glycosides are a defense against predaceous ants.

Effects of cages, plant age and mechanical clipping on plantain chemistry.

In plant-insect herbivore field studies, effects of cages, plant age, and mechanical clipping on host plant chemistry are often postulated but not well documented. We examined the effects of cages (for the purpose of restraining insects on experimental plots), plant age over the course of the experiment and mechanical clipping on plantain (Plantago lanceolata) chemistry. Leaf age affected the concentrations of nitrogen and iridoid glycosides (IGs; specifically aucubin and catalpol), with higher levels in newer leaves. Caged plants had higher levels of IGs and lower concentrations of nitrogen than uncaged plants. The IG concentrations were greater in new leaves of caged plants than uncaged plants, whereas the concentrations in mature leaves were unaffected by caging. Plants that were 5 weeks older had higher levels of IGs and lower nitrogen than plants harvested 5 weeks earlier. Comparison of three studies suggested that over the summer IG concentrations increase during dry years but decrease during wet years. Plants with above-ground parts clipped and then allowed to regrow for five weeks had similar concentrations of IGs and nitrogen compared to control plants; but the regrowth plants had a lower catalpol to total IG ratio. We conclude that cages and time can have significant positive effects on iridoid glycoside concentrations and significant negative effects on leaf nitrogen concentration. But our results also indicate that the direction and magnitude of the effects of cages, time and mechanical damage are not easily predicted. Therefore, it is advisable to determine and/or control for such effects in field experiments on plant-insect interactions.

Effects of genotype, habitat, and seasonal variation on iridoid glycoside content of Plantago lanceolata (Plantaginaceae) and the implications for insect herbivores.

We investigated the effects of genotype, habitat, and seasonal variation on production of the iridoid glycosides, aucubin and catalpol, in leaves of the common weed Plantago lanceolata. Two genotypes, one each from a lawn and an adjacent abandoned hayfield population, were clonally replicated in the greenhouse, and then planted back into the two habitats. One quarter of the plants from each treatment were harvested on each of four dates, at approximately two-week intervals. Over the course of the growing season, and in both habitats, we found a significant increase in the concentration of both aucubin and catalpol in P. lanceolata leaves. The genotypes differed in their response to environmental variation, both in time and between sites, as indicated by significant genotype x date and genotype x site interactions. Early in the season, habitat (lawn or field) had a greater effect on iridoid glycoside concentration than did plant genotype, but later in the season, plant genotype was more influential in determining the iridoid glycoside concentration. Thus, the relative palatability of Plantago genotypes to specialist and generalist herbivores may vary in time and space.

Response of generalist and specialist insects to qualitative allelochemical variation.

We examined the effects of a set of four biosynthetically related iridoid glycosides, aucubin, catalpol, loganin, and asperuloside, on larvae of a generalist,Lymantria dispar (Lymantriidae), the gypsy moth, and an adapted specialist, the buckeye,Junonia coenia (Nymphalidae). In general,L. dispar grew and survived significantly less well on artificial diets containing iridoid glycoside, compared to a control diet without iridoid glycosides. In choice tests, previous exposure to a diet containing iridoid glycosides caused larvae subsequently to prefer iridoid glycoside-containing diets even though they were detrimental to growth and survival. In contrast,J coenia larvae grew and survived better on diets with aucubin and catalpol, the two iridoid glycosides found in the host plantPlantago lanceolata (Plantaginaceae), than on diets with no iridoid glycoside or with loganin and asperuloside. The results of choice tests of diets with and without iridoid glycosides and between diets with different iridoid glycosides reflected these differences as well. These results are discussed in terms of (1) differences between generalists and specialists in their response to qualitative variation in plant allelochemical content, (2) the induction of feeding preferences, and (3) the evolution of qualitative allelochemical variation as a plant defense.