Lepidosaphes ulmi

Biology

The adult female oystershell scale is up to four millimetres long, elongated, tapering to a point at the posterior end and often slightly curved, somewhat resembling a mussel shell. The upper side is a banded, brown, waxy scale and the underside is cream coloured. There are no eyes or legs and the short antennae have only a single segment. The mandibles are lengthened into a stylet adapted for sucking sap. The female lays about one hundred oval white eggs, retaining them under her body, and then dies. Her scale darkens in colour and stays in place, protecting the eggs over the winter. They can survive temperatures as low as -32 °C. They hatch in the spring at about the time the host plant's buds are bursting. The crawlers are tiny and disperse on the host, each one looking for a suitable protected site with thin bark in which to settle, remaining in that place permanently after sinking the stylet into the host plant's vascular tissues. The crawler moults twice before becoming an adult female, forming a protective scale from larval exuviae and secretions. Some crawlers may develop into males. These undergo four moults and the adult males have eyes, three pairs of legs, one pair of wings, a head and a body divided into a thorax and abdomen. Not all colonies produce males and bisexual and parthenogenetic populations are known.