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Background  The harlequin ladybird,  Harmonia axyridis  Pallas (Coleoptera: Coccinellidae) is native to central and eastern Asia and was purposely introduced into Europe to control aphids. While it proved to be a good biological control agent, its rapid spread and buildup of large populations made it a nuisance, since it overwinters in homes, emits unpleasant odors, stains fabrics, occasionally bites humans and feeds on apples, pears and grapes. Aside from the above, the ravenous appetite of  H. axyridis  results in their consumption of harmless native insects, including even other ladybird beetles. A study of the natural enemies of  H. axyridis  in Denmark revealed the presence of nematodes. The present study describes this nematode parasite and discusses aspects of its development and ecology.    Methods  Adult harlequin ladybird beetles were collected from March to November from four localities in Copenhagen on different plant species. In addition, groups of last-instar larvae and pupae (n = 50) were examined for the presence of nematodes. Living and recently dead nematodes were removed from adult  H. axyridis  in 0.5% saline solution, the nematodes were then heat killed (at 75C), fixed in 5% formalin and transferred to glycerin on slides for further examination and measurements.    Results  A new species of Allantonematidae (Tylenchida),  Parasitylenchus bifurcatus  n. sp., is described from adults of the harlequin ladybird,  Harmonia axyridis  in Denmark. The new species is characterized by a straight stylet lacking basal thickenings, a bursa and a forked tail tip in the vermiform (infective) females and juvenile males. The new species is compared with  P. coccinellinae  previously described from ladybird beetles in France. Parasitism resulted in depletion of the fat body and partial or complete atrophy of the reproductive organs of the beetles. Infections occurred throughout the year with rates of parasitism reaching up to 35%. The rate increased to 60% when field-collected ladybirds were incubated for 30 days in the laboratory.    Conclusions  The production of subsequent generations within the host with only the fertilized females (not the males) leaving the hosts and the absence of parasitism of the larvae and pupae is an impressive developmental modification of  P. bifurcatus . It is proposed that the vermiform (infective) females pass from one adult host to another when the beetles are hibernating or in assemblage groups. Rates of parasitism show that  P. bifurcatus  could be a significant biological control agent of  H. axyridis .

Parasitylenchus bifurcatus n. sp. (Tylenchida: Allantonematidae) parasitizing Harmonia axyridis (Coleoptera: Coccinellidae)