Differential winter mortality and seasonal selection in the polymorphic ladybird Adalia bipunctata (L) in the Netherlands

Abstract

Seasonal selection acting on the melanic polymorphism in the two-spot ladybird Adalia bipunrtala was investigated in The Netherlands. An increase in melanic frequency over the spring-summer reproductive period was quantified. The selective advantage gained by melanics averaged 9",,, but significant heterogeneity occurred between populations. Adult hibernation behaviour is described. The beetles when outdoors show a highly clumped distribution both between and within trees. The distribution of the morph classes between aggregations is random. Survivorship in a hibernating cohort (initial n = 1898) on a grid of 70 lime trees near Utrecht was monitored by making three counts over the winter of 1981-1982. Intense selection favouring each melanic morph occurred during December and January. The relative fitness of non-melanics was 0.55 (melanics = I ) . The discovery of dead beetles in late January (about 50, of total losses) and the absence of spatially density-dependent mortality were consistent with a climatic stress rather than selective predation. The period of selection was associated with very cold temperatures averaging up to 4°C below normal and an overall mortality of nearly 75O,. There was no change in morph frequency, near normal temperatures and a lower mortality from February to early April. Examination of groups of nearby trees in late January strongly suggested that similar differential mortality had occurred except on some willows. This difference was probably due to the more protected hibernation sites available on these trees. Samples of hibernating cohorts at three other sites showed no evidence of differential mortality. Laboratory experimrnts with hibernating beetles found no difference in survivorship or rate of weight loss between starved non-melanics and melanics in temperature regimes with and without periods of adult activity. I t is concluded that the intense winter selection on the study limes is probably exceptional. Examination of changes in morph frequency through the annual cycle suggests that at some sites the selection favouring melanics during reproduction is counterbalanced by selection against melanics in late summer or early autumn. The results are discussed in relation to mathematical models of cyclical selection and to other field studies including that of TimoReR-Ressovsky ( 1940), who found large drcrrascs in melanic frequency during hibernation in Berlin.

Chemical defence in ladybird beetles (Coccinellidae). I. Distribution of coccinelline and individual variation in defence in 7-spot ladybirds (Coccinella septempunctata)

“…= 5.6, p<0.05). The female 7-spot ladybirds were on average heavier than the males (43.7 mg vs 36.7 mg on day 8, 48.8 mg vs 36.0 mg on day 15 and 49.1 mg vs 35.5 mg on day 22 (F, , 09 >40, p<0.001 in all cases)), therefore this sex ratio bias could have been caused by differential mortality of (lighter) males (Brakefield 1985b). Females also produced on average more fluid than males (5.0 mg vs 3.9 mg on day 8, 3.6 mg vs 2.8 mg on day 15 and 3.3 mg vs 2.6 mg on day 22 (F, J0 9>8.5, p<0.004 in all cases)).…”

The Genetics and Cost of Chemical Defense in the Two-Spot Ladybird (Adalia bipunctata L.)

“…They were fed for 3 d to allow the cuticle to fully harden before being transferred to a refrigerator at 9°C. Adult two-spot ladybirds overwinter in temperate climates (Brakefield 1985b;Hemptinne 1988) and can often survive at 9°C for many months, because the low temperature induces a state ofdormancy. As the eggs were laid over a long time, the adults also emerged over an extended period.…”

“…Nearly all ladybird species (Coccinellidae) are warningly colored (Hodek 1973;Brakefield 1985a;Majerus and Kearns 1989), possibly as a consequence ofthe almost universal ability within the group to synthesize toxic or noxious defensive alkaloids (Pasteels et al 1973). When attacked, these genus-specific alkaloids are transported in a yellowish fluid through each of the six legs and to the surface via femero-tibial pores.…”

Polymorphic Müllerian mimicry and interactions with thermal melanism in ladybirds and a soldier beetle: a hypothesis

“…The tough cuticle, long lifespan, habit of laying eggs in batches and aggregative behaviour (e.g. during hibernation, see Brakefield, 1985) of adult coccinellids are all characteristic attributes of aposematic insects.…”