|
Principal Insect Pathogens
Just like humans and other vertebrates, insects are susceptible
to many disease-causing organisms known as pathogens. Thousands
of species of bacteria, fungi, viruses, protozoa and nematodes can
sicken or kill insects. Even if the insects survive, the pathogens’
“sub-lethal” effects can keep their victims from feeding
or reproducing.
Bacteria. Most bacteria infect specific insect
orders. Some naturally occurring insect-pathogenic bacteria have
been isolated and mass-produced for commercial use. One of these,
Bacillus thuringiensis or Bt, is the world’s most
widely applied biological control agent. It exerts its toxicity
only after plant-eating insects actually consume it. A highly dense
protein crystal, the Bt toxin kills victims by first paralyzing
their mid-gut, then their entire bodies. Like most other bacterial
pathogens, Bt is specific to certain insect orders. Its short residual
period also makes it an ideal candidate for pest management in fruits
and vegetables.
Fungi. Although an estimated 700-plus species
of fungi can infect insects, fewer than 20 have been developed for
insect management. Most insect-pathogenic fungi need cool, moist
environments to germinate. Compared to most other insect pathogens,
they have an extensive host range. Beauveria bassiana,
for example, can help manage beetles, ants, termites, true bugs,
grasshoppers, mosquitoes and mites as well as other arthropod pests.
It unleashes a toxin that weakens its host’s immune system,
then overwhelms its dead host’s intestinal bacteria with an
antibiotic. The tell-tale sign of B. bassiana’s carnage
is its victim’s “white bloom” of fungal spores.
Fungi can invade their insect host through natural openings in
its cuticle. Thus, hosts need not consume pathogens but only come
into direct contact with them. Although some fungi can take up to
several weeks to kill their hosts, most infected insects die within
three to seven days.
Viruses. Most viruses that attack insects belong
to a group called nuclear polyhedrosis viruses or NPVs. Their victims
are usually young larvae of butterflies and moths, which become
infected by eating NPV particles and typically die within several
weeks. Some infected larvae hang limply from the tops of crop canopies,
prompting the common name “caterpillar wilt” or “tree
top” disease.
Prevailing environmental factors heavily influence the insect-killing
efficiencies of viruses. For example, they are adversely affected
by sunlight, while the relatively slow speed at which they kill
has also hindered their widespread acceptance for biocontrol.
Nematodes. Nearly 40 known families of nematodes
parasitize and consume insects and other arthropods. Some are hunter-cruisers
while others are ambushers. The most beneficial of these “entomopathogenic”
nematodes belong to the Heterorhabditidae and Steinernematidae families.
Both families are “obligate” parasites: their survival
depends on their hosts and on the symbiotic relationships the nematodes
have evolved with disease-causing Xenorhabdus and Photorhabdus bacteria.
Parasitic nematodes transport bacteria inside their host, penetrating
the host via the mouth, anus, spiracles or cuticle. Once inside,
the nematodes release the bacteria, which quickly multiply and kill
the host. In turn, the nematode uses the bacteria and insect cadaver
for food and shelter, maturing, mating and reproducing inside it.
Infective-stage juvenile nematodes eventually emerge from the cadaver
and seek out another host.
Because they are highly mobile and can locate and destroy new victims
in just a few days, entomopathogenic nematodes make outstanding
candidates for all kinds of biological control. Some are applied
to soils to successfully manage the underground life stages of insect
pests.
Previous Section
| Top | Next Section
|
