Nonrandom mating in the two-spot ladybird (Adalia bipunctata): the influence of weight on mating success

Maternal body size and age govern reproduction and offspring phenotype in the zig-zag ladybird beetle (Menochilus sexmaculatus)

Maternal effects are possible channels through which mothers provision their offspring differentially, thereby affecting offspring phenotype. We investigated maternal effects in the zig-zag ladybird beetle (Menochilus sexmaculatus (Fabricius, 1781) = Cheilomenes sexmaculata (Fabricius, 1781)) in response to body size (induced by different feeding regimes during larval development) and their age within the reproductive cycle. Different-sized females were permitted to mate and were provided with daily-replenished ad libitum prey. After mating, reproductive output and developmental duration of offspring from different oviposition days were recorded. We hypothesized that small females would lay smaller and fewer eggs than larger females, and that egg mass would also reduce with increased maternal age. In our study, the larger mothers laid more eggs per day. Small and large mothers oviposited maximally at middle age. Maternal age did not influence the egg mass, although it was slightly higher in the case of older, larger females. Offspring from old, small and large mothers developed rapidly. This nimble development could be an adaptive strategy for the use of ephemeral aphid patches. The results of the study are indicative of this ladybird species’ ability to adjust their offspring’s life-history traits, a feature more prominent in larger females.

THE FITNESS OF MANIPULATING PHENOTYPES: IMPLICATIONS FOR STUDIES OF FLUCTUATING ASYMMETRY AND MULTIVARIATE SELECTION

Phenotypic manipulation (or phenotypic engineering) that alters trait distributions provides a way to increase the statistical power of detecting relationships between traits and fitness. Manipulations relying on plastic responses, however, assume a specific relationship between the perturbation and the alteration of the traits when multiple traits are involved. We measured several traits, including condition measured as fluctuating asymmetry, in the ladybird beetle Harmonia axyridis under six different diets to examine how altered environments affected multiple traits and their distributions. Although diet affected fluctuating asymmetry, we found no consistent relationship between degree of asymmetry and other phenotypic measures. As expected, individual traits were altered by our treatments. Contrary to expectation, relationships among traits were not constant among diets. Our results suggest that assumptions about the relationship between condition and trait values, especially fluctuating asymmetry, cannot be made. Further, studies that use manipulated phenotypes to statistically determine the form of selection must first demonstrate that the pattern of the phenotypic correlation matrix is not itself altered by the manipulation. If the phenotypic correlation matrix is not constant, then experimental estimates of selection coefficients may not reflect selection that occurs in the wild.

Mate recognition in the two-spot ladybird beetle, Adalia bipunctata: role of chemical and behavioural cues

On encountering a mature female, a male of the two-spot ladybird, Adalia bipunctata (L.), first palpated her elytra with his maxillary palps, then mounted her, extruded his penis and mated. Copulation never occurred between active males but males copulated with dummies bearing male elytra as frequently as with dummies with female elytra of their own species. Similarly, males attempted mating with immobilised conspecifics of both sexes. However elytra washed in chloroform failed to stimulate mating. Analysis of the chloroform extracts of the elytra revealed that male and female ladybirds are coated by the same blend of hydrocarbons among which 9- and 7-methyl tricosane are dominant. Our results are consistent with a role of these cuticulars hydrocarbons in species recognition and show that behaviour, in particular movement, is necessary for discrimination between males and females.