Coccinellidae - Biology

Biology

See also : List of Coccinellidae genera

Coccinellids are best known as predators of Sternorrhyncha such as aphids and scale insects, but the range of prey species that various Coccinellidae may attack is much wider than that. A genus of small black ladybirds, Stethorus, presents one example of predation on non-Sternorrhyncha; they specialise in mites as prey, notably Tetranychus spider mites. Stethorus species accordingly are important in certain examples of biological control.

Various larger species of Coccinellidae attack caterpillars and other beetle larvae. Several genera feed on various insects or their eggs; for example, Coleomegilla species are significant predators of the eggs and larvae of moths such as species of Spodoptera and the Plutellidae. Larvae and eggs of ladybirds, either their own or of other species, can also be important food resources when alternative prey are scarce. As a family the Coccinellidae used to be regarded as purely carnivorous, but it is now known that they are far more omnivorous than used to be thought, both as a family and in individual species; examination of gut contents of apparently specialist predators commonly yield residues of pollen and other plant materials. Besides the prey that they favour, most predatory coccinellidae include other items in their diet, including honeydew, pollen, plant sap, nectar and various fungi. The significance of such non-prey items in the diet is still under investigation and discussion.

Apart from the generalist aphid and scale predators and incidental substances of botanical origin, many Coccinellidae do favour or even specialise in certain prey types. This makes some of them particularly valuable as agents in biological control programmes. Determination of specialisation need not be a trivial matter though; for example the larva of the Vedalia ladybird Rodolia cardinalis is a specialist predator on a few species of Monophlebidae, in particular Icerya purchasi, which is the most notorious of the cottony cushion scale species. However, the adult Rodolia cardinalis can subsist for some months on a wider range of insects plus some nectar.

It is thought that certain species of Coccinellids lay extra infertile eggs with the fertile eggs. These appear to provide a backup food source for the larvae when they hatch. The ratio of infertile to fertile eggs increases with scarcity of food at the time of egg laying. Such a strategy amounts to the production of trophic eggs.

Some species in the subfamily Epilachninae are herbivores, and can be very destructive agricultural pests (e.g., the Mexican bean beetle). Again, in the subfamily Coccinellinae, members of the tribe Halyziini and the genus Tythaspis are mycophagous.

While predatory species are often used as biological control agents, introduced species of coccinellidae are not necessarily benign. Species such as Harmonia axyridis or Coccinella septempunctata in North America out-compete and displace native coccinellids and become pests themselves.

The main predators of coccinellids are usually birds, but they are also the prey of frogs, wasps, spiders, and dragonflies. The bright colours of many coccinellids discourage some potential predators from making a meal of them. This phenomenon is called aposematism and works because predators learn by experience to associate certain prey phenotypes with a bad taste. A further defence known as "Reflex bleeding" exists in which an alkaloid toxin is exuded through the joints of the exoskeleton, triggered by mechanical stimulation (such as by predator attack) in both larval and adult beetles, deterring feeding.

Coccinellids in temperate regions enter diapause during the winter, so they often are among the first insects to appear in the spring. Some species (e.g., Hippodamia convergens) gather into groups and move to higher elevated land, such as a mountain, to enter diapause. Most coccinellids overwinter as adults, aggregating on the south sides of large objects such as trees or houses during the winter months, dispersing in response to increasing day length in the spring.

Predatory coccinellids are usually found on plants which harbour their prey. They lay their eggs near their prey, to increase the likelihood the larvae will find the prey easily. In Harmonia axyridis, eggs hatch in 3–4 days from clutches numbering from a few to several dozen. Depending on resource availability, the larvae pass through four instars over 10–14 days, after which pupation occurs. After a teneral period of several days, the adults become reproductively active and are able to reproduce again, although they may become reproductively quiescent if eclosing late in the season. Total life span is 1–2 years on average.