KIDNEY-SPOT LADYBIRDS AND SCALE INSECTS

Kidney-spot ladybirds (Chilocorus renipustulans) are small black beetles with two red broad-bean shaped spots on the elytra. Characteristically they come out on fine days and several may be seen on a branch, motionless and apparently basking in the sun. Joan Westcott saw them on her spindle tree (Euonymus europaeus); she told Alison Paul who found them on her spindle trees and they told me.

As is usually the case, the more closely we watched them, the more intriguing it became. It turned out that the kidney-spot belongs to group of ladybirds that feed mainly on scale insects (Mills 1981), chiefly on the willow scale (Chionaspis salicis). Inspection revealed that all the spindle trees were heavily infested with a scale insect. David Nash (the SNS beetle recorder) has no records of the ladybird on spindle. We have still positively to identify the scale insect which may be Euonymous scale (Unaspis euonmyi), known on the south coast, and may be spreading northwards, rather than willow scale which occurs on sallow and ash.

When in March I first looked at the spindle twigs using a stereomicroscope I found only the adult female scales, an unappetising looking meal. Soon, however, small motile larvae called crawlers appeared; at about the same time the small black, spiny ladybird larvae were also seen. As the season progressed the crawler settled down and inserted its feeding tube into the cells of a twig or leaf. In the case of the females they attached themselves to a twig, lost their legs and covered themselves with a thick protective scale, never to move again. The males migrated mainly to the leaves where they also settled down and formed protective scales. In July/September the males, after having progressed through three larval stages, pupated in a strange, as yet not understood, series of changes, before becoming motile adult males, some with wings. Native scale insects appear not to have been studied in any detail and there are few papers in British journals. Searching for information on the worldwide web I found an electronic database ‘Scale net’, which contended, tongue in cheek, that some researchers believe that the life cycles are so strange that scale insects have an extra terrestrial origin.

As a growing season draws to a close, the recently emerged adult ladybirds are mating and will soon hibernate leaving a host of questions. How closely are the life histories of these two insects enmeshed, so that the ladybird larvae emerge at same time as the scale insect crawlers appear, providing them with suitable food? How do the ladybirds feed on scale insects? The females develop thick, waxy scales that soon become covered with algae and appear impregnable. Do the larvae of the ladybirds feed mainly on the male scale insects, which have thinner and more fragile scales than those of the female?

Next year we hope to find out more about these interesting insects, their feeding habits and life histories. In this we have been encouraged by the award of a Rivis bursary from the Suffolk Naturalists’ Society which will help us to pursue a more in-depth study.

References

Mills, N.J. (1981) Essential and alternative foods for some British Coccinellidae (Coleoptera) Entomologists Gazette. 32: 187-302.

Michael Kirby , The Studio, Blythburgh Road, Westleton, Saxmundham, IP17 3AS