HPIPM:Biological Pathogens
Author: Whitney S. Cranshaw
Insect Pathogens
Although infrequently observed, insects and mites often suffer from infection by pathogens, many of which produce lethal diseases. Certain kinds of fungi, bacteria, protozoa, or viruses all can kill insects and many are important in their management.
Viruses
Viruses are most commonly found among the caterpillars and sawflies. One particularly gruesome group of these viruses (nuclear polyhedrosis viruses/NPV) cause "wilt disease". Caterpillars or sawflies infected by these viruses are killed rapidly, their virus-filled bodies hanging limply by their prolegs. At the slightest touch, the insects rupture, spilling the virus particles onto leaves below them to infect other insects. Wilt diseases are important biological controls of several caterpillars including cabbage looper, gypsy moth, and Douglas-fir tussock moth. Other types of viruses, such as one that affects codling moth, act considerably more slowly. These other viruses may produce only modest external symptoms such as a chalky color of the infected insect and/or a general listlessness.
Similarly, infection by bacteria usually show little external evidence. Many bacteria attack the digestive system of the insects, which cause the insects to develop slowly. Some bacteria reproduce in the blood and cause blood poisoning. Bacillus thuringiensis is the best known bacterium which produces insect disease, as it is commonly used as a microbial insecticide. Several strains exist, each of which affect only certain types of insects that ingest the bacteria or the toxic protein crystal that it produces. Currently, the most commonly available strains affect either caterpillars (kurstaki, aizawi strains), leaf beetles (tenebrionis/san diego strain) or larvae of mosquitoes, fungus gnats and related flies (israelensis strain).
Fungi
Fungi 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, when conditions are right, become covered with a white, light green or pink fuzz - the fruiting bodies and spores of the fungus. One of the most commonly encountered fungus diseases of insects is Entomophthora muscae, which infects various root maggot flies. Infected flies die stuck to the tops of plants and at other high points around the garden. Beauveria bassiana is a very common disease of many insects and is also currently marketed for insect control in yards, gardens and greenhouses. Some commercial development has also occurred with Metarhizium anisopliae, which infects many kinds of insects that live in soil.
Protozoa
Protozoa 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. European corn borer and grasshoppers are among the groups of insects that are common hosts of protozoa. Nosema locustae preparations can infect developing grasshoppers and are sometimes used for grasshopper control.
Nematodes
Many kinds of nematodes develop as parasites of insects. One diverse group is the mermithid nematodes that develop as internal parasites. They may develop slowly in their host causing debilitating disease that ultimately is lethal. Mermithid nematodes are common in many beetles, mosquito larva, and some grasshoppers. The largest of the mermithid nematodes one might encounter in a garden is Mermis nigrescens, a parasite of grasshoppers that may be over three inches long.
Another group of large worm-like parasites that affect insects are the horsehair worms. These are placed in a separate phylum from the nematodes, the Nematomorphs, and are thicker and darker than Mermis nigrescens, the common nematode parasite of grasshoppers. Horsehair worms have a wider host range and can affect many insects - various crickets and beetles perhaps most commonly. Several species in the genus Gordius may come to the attention of gardeners, particularly near water sources. Eggs of horsehair worms are laid in water and infection of insects occurs when they ingest the young larvae while drinking. After the horsehair worm has completed development within the insect host, infected insects ultimately move to water where the mature parasites emerge. Their occurrence in animal watering troughs is the basis for the common name horsehair worm.
