Introduction

For three or four years we, Joan Westcott & Alison Paul, noticed large numbers of kidney spot ladybirds in our gardens. They were seen mostly on two spindle trees (Euonymus europeaus), each about 50 years old and five metres high. The kidneyspot ladybird (Chilocorus renipustulatus) feeds mostly on scale insects (Hawkins, 2000) and when we first examined the trees in March 2004 we found large numbers of euonymus scale insects (Unaspis euonymi) on the twigs and trunks, well camouflaged with algae and lichens. At first sight these looked particularly unappetizing, the dirt encrusted waxy scales often in congested clumps of living and old, dead scales.

We wondered how the ladybirds, particularly the newly hatched larvae, coped with such unlikely, strongly armoured prey as scale insects, but could find little easily available published information about this specialised group of insects, other than how to kill them.

To find out more about how these two species of insects interact we watched them throughout their respective life cycles, JW and AP visiting their trees almost daily and noting the behaviour and stages of the ladybird while MK took frequent samples of the scale insects for microscopic examination.

Euonymous scale insect

Scale insects are a highly evolved group, related to aphids, and are quite different from the more familiar insects, most stages of the life cycle lacking legs, antennae and eyes. Male and female insects, apart from the first instar, differ in form; the male has four further stages, while the female has two. The crawler, the first instar, is the same in both sexes. It is about 0.25mm long, flat and leaf-shaped and yellowish green in colour. The most prominent features are the eyes, bristly antennae and two, dorsal, long, arching, anterior ‘filaments’ (see photo); on the ventral surface are six well developed legs.

As its name implies, after hatching from the egg, the crawler moved around freely before settling down. Some remained among the clusters of scale, often beneath the parent scale and developed into females. Others moved away, crawling up the young shoots and spreading out on the lower surface of leaves often in concentrations of c. 700 on a leaf of 800 mm² in area or 1.1mm² per crawler.

Male development. Having selected a site, the crawler moulted and inserted a long filamentous feeding tube deep into the stem or leaf tissue. It remained beneath the cast skin which was incorporated into a delicate waxy covering or scale secreted by the growing insect and completely covering it and the subsequent stages. It developed through four instars, the first two larval feeding stages followed by two non-feeding pupal stages when the insect developed the appendages of the final stage, the adult male.

Larval and pupal stages were orange in colour, spindle shaped, about 1mm long, with a soft delicate body. The final instar, the sexually mature male, was about the same size. It broke out from the rear of the scale complete with legs, wings and long bristly antennae but without mouthparts (see photo).

Female development.

The second instar larva remained beneath the cast skin of the crawler. This was incorporated into a strong waxy shield or scale secreted by the larva, together with a more delicate ventral scale. Scale formation continued during the next, final instar, completely covering and protecting it throughout its life. The second instar larva had no appendages, was shaped like a spinning top, and orange in colour, with a soft delicate skin. The final, third instar was similar, larval in form (photo p.17), but was sexually mature, mating and producing eggs. About 15 eggs were laid beneath the scale leaving only the dry husk of empty body of the female insect in the ‘roof’ of the scale.

Kidney-spot ladybird development

The first instar larva was c. 2mm long, stubby, with a black head and long simple spines. Subsequent instars developed black barbed spines (Hawkins, 2000) and compared with, e.g., seven spot ladybird larvae it was proportionately broader with shorter legs. The pupa remains partly invested in the skin of the final instar larva. The adult was about 5mm long with black wing cases which have kidney shaped red marks on the shoulders.

Life histories

Both species overwintered as adults. Many of the ladybirds remained on leafless branches of the spindle trees, usually congregating in groups of four or five individuals on the lower surface in a crotch formed by the nodes. On warm days they moved to bask on the sunny sites on the main stem.

The fertilised female scale insects remained beneath the scales and in March/ April they became fatter and darker in colour, and if punctured exuded developing eggs. Occasionally the swollen body appeared to loosen the dorsal scale which fell off leaving a naked insect which sometimes fell off the twig leaving it dangling by its feeding tube.

Scale insect eggs and crawlers were seen during late May and early June by which time the spindle tree leaves were fully expanded. At this time, also, the first instar ladybird larvae were seen on the leaves which were being colonised by crawlers. As the season progressed more third and fourth ladybird larvae were seen and from late June onwards all adult ladybirds had vanished and ladybird pupae appeared in increasing numbers, some in small congregations on leaves but in mostly large groups high up in the tree. The next generation of adult ladybirds began to appear in late July, orange-coloured at first, darkening within a few hours to the familiar black and orange livery.

During the summer, the male scale insects grew and developed and winged adults emerged during August, a few leaving it until September. After emergence they walked around, occasionally flitting short distances but were not seen to fly for more than a few millimetres. Some found adult female scales and were seen to mate, standing on a scale and inserting a long penis under the scale to locate the female insect beneath it. Unable to feed, they lived only a short time and died within 12-24 hours. By October all ladybird pupae had hatched and the spindle leaves had started to senesce and drop from the tree.

Feeding behaviour

When no other stages were present the adult ladybird was seen to get at the female scale insect by standing with its four hind feet on the scale, inserting its strong, hookshaped mandibles (Dixon, 2000) beneath the rim of the scale and prising it off using its front legs. With the advent of crawlers the adult ladybirds started to feed on them and were often seen with crawlers on the wing cases (a possible way in which the scale insect is spread to other hosts?). The larvae were seen to feed only on the crawlers and male larvae and pupae, moving steadily over heavily infested leaves or stems and ‘grazing’ the sedentary, lightly protected scale insects (see photo).

Postscript

After following the changes in the scale insect through the season we have been astonished both by its unusual morphology and life history (a sentiment shared by experts in this group of insects). The female third instar is particularly strange having none of the features of most insects, remaining larval in form although sexually mature.

After more than two years of watching we have begun to understand the complex alliance between the spindle tree, the kidney-spot ladybird and the euonymus scale insect; as one of us remarked “I never knew there was so much in it”. The life cycles are so intermeshed that egg laying and hatching of the crawlers coincides with leaf expansion of the spindle tree providing feeding sites for male crawlers. This in turn provides a plentiful supply of easily found, sedentary, soft, juicy male insects, particularly suitable for the small newly hatched ladybird larvae. The life style of the kidney-spot ladybird is adapted to the sedentary nature of its prey and unlike, for example, the active agile larvae of the seven-spot ladybird, the kidney-spot ladybird larva is short legged and slow moving but well protected by stout spines. The life cycles of both insects are complete before leaf fall, leaving the adult ladybirds with adult female scale insects, well armoured with a stout covering scale but when broached providing a soft skinned, easily eaten meal as the main source of nutrient for egg formation.

While we now feel that we have some understanding of the complexity of ‘ladybirds in the garden’ there are still some details of behaviour that we want to continue watching and hope to publish a more complete account later.

Acknowledgments

Mr David Walker made microscope slides and photographs of some of the scale insect stages and we also thank him for helpful discussions about the study.

Mr David Nash (Beetle recorder, Suffolk Naturalists’ Society) confirmed our identification of the kidney-spot lady bird.

The Suffolk Natur alists’ Society awarded a Rivis bursary which helped with incidental expenses, particularly those incurred borrowing obscure books and papers.

The Suffolk Library Service was very helpful, arranging loans of rare and foreign publications from the British Library.

References

Hawkins, R. D. (2000). Ladybirds of Surrey. Surrey Wildlife Trust. Woking.

Dixon, A. F. G. (2000). Insect predator-prey dynamics: ladybird beetles and biological control. Cambridge University Press, Cambridge.

Michael Kirby, Alison Paul and Joan Westcott

Scale insect crawler

Adult male scale insect with male scale.

Ventral view of sexually mature female still enclosed in overturned scale

Third or fourth instar ladybird grazing on male scales. Pieces of broken scale are visible amongst its spines.