Introduction

Beetles comprise the largest order of insects, with over a quarter of a million described species. They have four wings, with the front pair thickened, leathery, or hardened, and usually covering the hind wings. Their chewing mouthparts are well developed in the adults and larvae. Mandibles range from slender and sharp use for predaceous habits, to large and toothed, used to crush plant and animal material, or gnawing wood. In weevils, the front of the head is produced into an elongated snout, with the mandibles at the end. Developmentinthis order includes egg, larva, pupa, and adult stages. Larvae vary considerably inform, butmostare elongate and flattened or grub-like. The life cycle varies in length to provide several generations a year to one generation in several years. Overwintering generally occurs as a larva or pupa in protected sites, but some species overwinter as adults in large aggregations.

Wood isasource offood and harborage for several species of beetles. However, cellulose and lignin that give wood strength and durability are not easily converted to food. Only a small number of insects actually derive nutrition from wood. Others, such as carpenter ants and carpenter bees, have mouthparts strong enough to tunnel into sound or slightly decayed wood, but their gut is not equipped with the enzymes or microorganisms necessary to convertitinto food components. Utilization of wood as food or shelter requires strong mouthparts to bite off small pieces, and a digestive system capable of breaking down cellulose. The larvae ofwood-infesting insects have these essential features, and this stage creates the powder-fill tunnels characteristic ofinfested wood.

To utilize wood as a harborage and food, beetle adults and larvae have evolved elaborate morphological and physiological features, and specific behavioral patterns. Adult body shape is generally cylindrical, short, rounded, and sometimes with a spiny prothorax. This body form is geometrically efficient for boring, and may explain the similarity between adults of different wood-boring species. Ridges, spikes, and horns on the head, prothorax, and elytra often occur at sites where they do not impede boring or are located on the prothorax where they probably serve as an anchor or hold-fast to assist the mandibles. Larval forms are soft-bodied and capable of stretching, contracting, and pressing against the walls offeed-ing tunnels. Movement in narrow tunnels and galleries is assisted by setae, spines arranged on swollen segments, and legs ofvarious length and form. Mandibles ofwood-infesting beetles are usually modified to cut and tear at the substrate, or to plane it smooth with sharp surfaces. Species that feed on dry (8-12% moisture) structural wood have short broad mandibles, and species that feed on live trees, recently felled green wood, or wood with a high moisture content have long and thin mandibles. Various features of the digestive system of some species seem specifically suited to eating wood. Larvae of Anobium punctatum have a grinding gizzard in their gut, which is linked to their feeding on wood. The gizzard is a spherical enlargement of the foregut that has a dense layer of spines and teeth-like structures. It functions to grind and mix the food in preparation for digestion. Mouthparts work in conjunction with the gizzard to provide a continuous supply of uniform-size wood particles. Those too small or too large fall away and are swept to the back by the front legs of the larva. A gizzard is absent from anobiids that do not utilize cellulose as a source offood.

Pest status of beetles in the urban environment is based primarily on the feeding habits of the larval stage, and the overwintering behavior of the adults of a few species. Few species transmit pathogenic organisms, but some, such as dermestids or blister beetles, cause skin irritations. About 600 beetle species in 34 families are associated with stored grain or food materials made from grains. However, stored fruits and vegetables are also attacked. Some beetles are associated with stored food because they feed on the mold and fungi growing on these substrates, or they are predators of insects or arthropods that are the primary invaders. The original habitats of food-infesting beetles were probably dry, sheltered sites where plant and animal debris collected. Animals building nests or storing food in sheltered places created some of these sites. Likely habitats include bird nests built close to human dwellings, bat roosts, and the dens of rats and mice. Species that now attack grain seeds, such as wheat, maize, and other cereals, may not have fed on the seeds of their ancestral grasses. These seeds would have been very small and not likely infested by the larvae of these beetles. Development of large varieties of primitive grains would have provided beetle larvae sufficient food to complete development. Grain borers probably evolved from attacking the structural features of storage bins to the seeds. They probably fed on larger wild seeds such as acorns, which were often scattered, and moved from these to grain, which was stored in quantity and protected.

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