Every one knows the common ladybirds of gardens and childhood stories. Yet they are but a very few of the wide diversity of species assigned to the beetle family, Coccinellidae. The family name Coccinellidae probably derives from the diminutive of the Latinized Greek word 'Kokkos', a seed or berry in reference to their rounded and convex shape of the beetles. However, other authorities give the Latin Coccinus — scarlet colour, as the root of the name. Some of the commonly seen ladybirds are brightly coloured and patterned, readily attracting the attention of home gardeners and small children. Most species are predatory, particularly on insects that are often pests of agriculture.
Coccinellidae is the biggest family of superfamily Cucujoidea with about 6000 species classified in 370 genera worldwide. There are 57 genera and about 500 species in Australia with about half of them yet undescribed.
Various aspects of the biology and ecology of the Coccinellidae are covered by several thorough reviews and books that should be consulted. These include the works of Clausen (1940), Hagen (1962), Hodek (1967, 1973), Majerus (1994); Hodek and Honek (1996) and Kuznetsov (1997). Drea and Gordon (1990) reviewed the biology of ladybird species that prey on armoured scales (Hemiptera: Diaspididae), and Ponsonby and Copland (1997) reviewed the species feeding on soft scales (Coccidae). Dixon's (2000) book covers ecology of predatory Coccinellidae and their interactions with various prey groups. He also discusses consequences of various ecological traits in ladybirds for their successful application in biological control.
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Ladybird beetles are holometabolous insects, undergoing a complete metamorphosis with four discrete life stages: egg, larva, pupa and adult beetle. Of these stages, only the adult beetle is very mobile. This stage is capable of covering vast distances, using wings for flight to find new food sources or mating partners. The adult is also the stage that has a prolonged duration counted in months; the others last only few or more days. Larvae and adult are the stages that actively search and feed on various insects and other invertebrates, plant tissue or fungal hyphae and conidia.
Coccinellidae eggs are 0.2-2.0 mm long; white, yellow to red, oval or spindle-shaped and are laid singly or in batches of various sizes, always with the long axis perpendicular to the substrate. The chorion is smooth with distinct microsculpture visible only in Epilachninae. When freshly laid, an egg is usually white or creamy-yellow but during embryogenesis it changes colour, becoming darker — often very much darker. This stage lasts 2-18 days. Most Coccinellidae that feed on scale insects lay their eggs singly or in small groups close to or directly on or beneath the female or immature scale. True ladybirds, the Coccinellini are known to form larger batches of up to 100 eggs on leaf surfaces close to the colonies of aphids that are their primary prey. The total number of eggs produced by coccinellids varies considerably between species and is highly dependent on food supply and ambient climatic factors.
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Larvae emerge from eggs either by a rupture at one end of the egg or by a longitudinal split. Shortly before eclosion the chorion becomes thin and semitransparent, the stemmata, mandibles and hatching spines (egg bursters) on the head of the embryo becoming clearly visible. The freshly hatched larva remains stationary for about an hour for the hardening of its cuticle to take place. Then the larva moves around, often feeding on unhatched eggs (both fertile and unfertile).
There are four stages (instars) in larval life, separated from each other by a moulting period in which the larva casts off its old exoskeleton and builds up a new one, considerably increasing its size. Prior to moulting, the larva stops feeding and moving and usually attach itself to the substrate using the anal pad (cremaster on the terminal abdominal segment) and remains motionless for several hours. The ecdysis begins at the head and continues along the dorsal part of the thorax and abdomen. The larva liberates itself from the moult and walks away after its cuticle hardens.
The first instar larvae appear to be most restricted in their range of suitable host stages and suffer highest mortality rates. Once their cuticule has hardened the first instar larvae actively seek food. This consists mostly of the eggs of prey in case of coccidophagous species or small aphid nymphs for aphidophagous species. The newly hatched larvae have about one to one and a half days in which to locate their first prey. If unsuccessful in that time, the larva soon dies. Early authors assumed that all predatory coccinellid larvae use extra-oral digestion and that some larvae have perforated or grooved mandibles. In fact coccinellid larval mandibles are not perforated at all. In several species, however, there is a deep groove along the inner surface, used to inject digestive juices into the victim and to suck out juices in cycles until the victim is entirely dry; the prey's empty exoskeleton is then abandoned by the larva. Coccidophagous species feed on stationary prey that may be ingested as a whole if small or the coccinellid bites the scale covers, sometimes lifting them from the substrate first, and then chews the body. Cannibalism occurs frequently among all coccinellids. On emerging, first instar larvae will often feed on unhatched eggs of their own species. Early instar larvae and pupae are also preyed upon by later instars and adults of same species or of other species
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The fully grown fourth instar larva stops feeding and attaches itself using anal pad (cremaster) to the substrate. There it remains during a quiescent prepupal stage of several hours until it moults once again, this time transforming into a pupa. Pupation occurs in protected areas on foliage or bark in the immediate vicinity of the host. The habit of congregation by larvae of Chilocorini before pupation is quite common and widespread The pupal stage lasts about 7-14 days.
The adult beetle emerges from the pupal skin through a slit at the front of the dorsal surface. The body of the teneral adult is pale and the wings are extended. It takes several hours for the cuticle to harden and to develop the characteristic colour pattern. and freshly emerged specimens remain distinctly lighter than specimens from the previous generation for quite some time. Adults mate within a few days after emergence, and oviposition follows from 5 to 15 days after eclosion. Copulation typically lasts for 15-60 minutes but can extend for few hours or even days. Longevity in Coccinellidae is related to prey synchrony and dormancy mechanisms. Species in which the adults undergo long periods of aestivation may live up for two years. However, multivoltine species in warmer climates usually only live up to two months.
One of the most fascinating features of ladybird biology, especially common in the Northern Hemisphere, is their seasonal migration and aggregation. Similar, but smaller scale, aggregations occurring in summer and winter and lasting sometimes for several months have been described from Australia in Apolinus lividigaster, Harmonia conformis and Micraspis furcifera.