NONRANDOM MATING IN THE 2-SPOT LADYBIRD (ADALIA-BIPUNCTATA) - THE INFLUENCE OF WEIGHT ON MATING SUCCESS

In some populations of the two-spot ladybird, Adalia bipunctata, there is evidence that melanics of both sexes have a general mating advanta ge over the typical morph. There is also evidence that some female lad ybirds possess a simple mating preference for melanic males. We have d etermined the influence of body weight on mating success in the two-sp ot ladybird and assessed whether weight differences might influence th e mating success of the melanic morph. We found that the ''formal mati ng tests'' used in previous studies of the two-spot ladybird did not d etect any influence of weight on mating success. Using more sensitive ''singleton'' tests, however, a significant mating advantage to heavie r males was detected, irrespective of morph. There was also evidence i n favor of a similar advantage to heavy females, but this was not pres ent in all ladybird groups that were tested. Heavy individuals neither produced more eggs in matings nor showed higher activity rates. We su ggest that some form of mate competition favors large individuals: for example, the larger an individual, the greater is the chance of an en counter with a potential mate. There is no evidence in this study that either sex chooses heavier mates. Effects of weight might contribute to the general melanic mating advantage found in some populations but cannot account for all the data in favor of a female mating preference for melanic males.