Abstract. Sexually transmitted diseases (STDs) have rarely been reported in insects and other invertebrates. The majority of those reported involve organisms where sexual transmission is augmented by either vertical (i.e. inherited) transmission, or horizontal transmission, independent of host sexual activity. We here demonstrate the existence of an STD in the coccinellid beetle Adalia bipunctata. This species bears a parasitic mite of the genus Coccipolipus. We show that, like many other podapolipid mites, this mite is transmitted between host individuals at a high rate during copulation. It also appears to be transmitted at a low rate between non‐copulating individuals. We show that infected female A.bipunctata produced eggs at a reduced rate, and that the eggs produced by infected females have highly decreased viability. However, no effect of infection upon host longevity was observed. The results are discussed in relation to the incidences of sexually transmitted disease in invertebrates in general, the causes of disease symptoms, and the importance of this disease in the evolution of A.bipunctata.

A wild sample of Ada/ia bipunctata larvae and pupae were collected from Surrey, and F1 virgin adults mated. The sex ratio produced by these pairs was found be be variable, 22 pairs produced a sex ratio consistent with a 1:1 sex ratio, but four produced a strong female bias. Daughters and males from biased broods were mated to adults from non-biased broods of different parentage.Males and females from unrelated families were crossed. Where the female was taken from a biased clutch, the biased sex ratio trait recurred in a high proportion of cases. Where males from such broods were crossed, or females from broods with no history of bias were mated, sex ratios consistent with 1:1 were obtained. Pairs producing a strong female bias showed hatch rates which were approximately half that of crosses producing non-biased sex ratios. Treatment with tetracycline in golden syrup cured females who showed the biased sex ratio trait. Daughters conceived after this treatment produced normal sex ratios. Females fed purely on golden syrup, as a control, continued to produce female biased sex ratios until treated with tetracycline. We therefore suggest that an unknown bacterium, transmitted through egg cytoplasm but not sperm, is responsible for the early mortality of male embryos and thus a bias in the secondary sex ratio. The contribution that the study of the Coccinellidae can make to theories concerning the incidences and evolution of male killing elements is discussed in relation to egg cannibalism.

Inherited symbionts which selectively cause the death of male hosts are found widely across the Insecta. Previous studies have shown a single, but different micro-organism to be responsible for male-killing in each taxonomic group studied. We here produce evidence that within a group of insects, the Coccinellidae, there is more than one causal agent of male lethality. We report a novel observation of a male-killing trait in the species Coleomegilla maculata. Six of 26 crosses were found to produce a female-biased sex ratio associated with a low egg hatch-rate. The trait was matrilinearly inherited and was observed to be tetracycline-sensitive. However, tests which indicate the presence of a Rickettsia, previously found to cause male-killing in another member of the Coccinellidae, Adalia bipunctata, proved negative. We therefore conclude that the phenomenon of male-killing is multicausal, within, as well as between, taxonomic groups of the Insecta.

Adalia bipuncfafa, the two spot ladybird, is polymorphic for a cytoplasmically inherited element which produces female-biased sex ratios by effecting the death of male offspring during embryogenesis. The levels of this element were assessed in Cambridge populations. Six out of 82 females tested showed both a female-biased sex ratio and low egg hatch rates consistent with the presence of this element. Population sex ratios were assessed by collecting pupae from the Cambridge area. The population sex ratio was found to be 1.15 : I, female biased, significantly different from 1 : 1, and consistent with predictions based on a model incorporating the observed level of the sex ratio element in the population.

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