Infrequently observed, insects and mites often suffer from lethal disease. Periodically, epizootics resulting from infection by fungi, bacteria, protozoa, or viruses may sweep through an insect population.
Although the classes of organisms that cause human disease are the same for many other animals and plants (e.g., viruses, bacteria, fungi), it is important to keep in mind that insect diseases are very specific in their effects. Insect diseases do not infect mammals or birds, restricting their effects to the arthropods. Furthermore, most insect diseases are so specific in their effects that they only can infect a few insect species.
Are most commonly found among the caterpillars and sawflies. One particularly gruesome group of these viruses (nuclear polyhedrosis viruses/NPV) cause 'wilt disease'. Caterpillars infected by these viruses are killed rapidly, their virus filled bodies hanging limply by their hind legs. At the slightest touch, the insects rupture, spilling the virus particle on the leaves below them to infect other insects. One type of wilt disease is an important biological control of the Douglas-fir tussock moth. Other types of viruses cause less spectacular infections. Most are slower acting than the wilt viruses. External evidence of these viruses may be a chalky color of the insects and a general listlessness.
Although virus diseases of insects are widespread in nature, rarely have they been adapted for applied biological control. Much of this is due to problems in registering these as insecticides. Regulatory agencies have had difficulty in deciding how to insure the safety of such mysterious particles as viruses. Manufacturers have also been leery of developing viruses due to problems in production (they need live cells to develop) and because the selectivity of viruses allows them to only be used against a few insects.
have received more attention, due almost entirely to the successful adaptation of Bacillus thuringiensis. Several manufacturers have produced and marketed various strains of this famous bacteria. Bacillus thuringiensis, and most other bacterial diseases of insects, work by disrupting the 'gut' lining of susceptible insects, ultimately killing them by a type of blood poisoning. Infected insects usually shrivel and darken. Strains of this bacteria are effective against caterpillars (kurstaki, thuringiensis, aizawai strains), leaf beetles (tenebrionis/san diego strain) and larvae of certain flies such as mosquitoes and blackflies (israelensis strain).
produce some of the more spectacular diseases of insects. A wide variety of insects succumb to fungus disease around the yard and garden. Fungus killed insects and mites become stiff and often are tightly attached to a leaf or stem. When conditions are right they become covered with a white, light green or pink 'fuzz', the spores of the fungus. At least one fungus, Beauveria bassiana (e.g., Naturalis, BotaniGard), is currently marketed to control insects on ornamental plants.
tend to cause debilitating infections among insects. Effects are often subtle, such as reduced feeding, activity or reproduction. Immature stages are usually much more susceptible to protozoan infections and survival can be reduced. Spruce budworms and grasshoppers are among the groups of insects that are common hosts of protozoa.
The information herein is supplied with the understanding that no discrimination is intended and that listing of commercial products, necessary to this guide, implies no endorsement by the authors or the Extension Services of Nebraska, Colorado, Wyoming or Montana. Criticism of products or equipment not listed is neither implied nor intended. Due to constantly changing labels, laws and regulations, the Extension Services can assume no liability for the suggested use of chemicals contained herein. Pesticides must be applied legally complying with all label directions and precautions on the pesticide container and any supplemental labeling and rules of state and federal pesticide regulatory agencies. State rules and regulations and special pesticide use allowances may vary from state to state: contact your State Department of Agriculture for the rules, regulations and allowances applicable in your state and locality.