Some of Our biocontrol
organisms
Microphoto shows a single Steinernema infective juvenile, about 0.5mm
long from prof. Randy Gaugler, Rutgers
Steinernematid life cycle
Fig. 1 infective nematodes surround living pest insect. Note mass around head.
The lifecycle of Steinernematid nematodes proceeds as follows:
Infective juveniles (AU) surround pest insects (Fig. 1), infect through natural body
openings, mouth, anus, spiracles, and penetrate through to the body cavity, hemocoel.
There they shed their outer cuticle if it was still retained and begin to grow. A
symbiotic bacterium is released. The symbiont has numerous functions to help the nematode.
It produces antibiotics to hinder secondary infections and produces toxins to inactivate
insect immunity and kill the insect. The symbiont also produces growth factors and
helps digest the insect. Death usually occurs within 24 hours after infection.
Fig. 2 Adult nematodes shown within dead pest. The largest is about 3-4 mm.
The infectives quickly grow into adult males and females(Fig. 2). They mate, then eggs
are laid and hatch into first stage juveniles. At 68°F, 20°C, this occurs within
about three days.
In a few days more the juveniles develop into second then third stage juveniles. In
conditions with few nematodes and much food in the insect cadaver another reproductive
cycle begins. Under opposite conditions, little food, many nematodes the 3rd stage
juveniles develop an extra outer cuticle and become infectives.
Fig. 3 Infectives exit dead insect. Picture from G.O. Poinar
After about two weeks, the infectives are repelled by the spent cadaver, exit,
and begin searching for new pests insects to kill (Fig. 3).
Safety
As stated St and Heterorhabditid nematodes are safe for people, pets, and plants.
Vertebrates- humans, dogs, birds, etc. - are completely resistant. Earthworms and helpful
insects like bees, ladybugs, and lacewings are also resistant. Plants are completely
resistant; there is no phytotoxicity. Scanmask, Ecomask, Heteromask, and Bioroach
are all exempt from the EPA registration required for chemical pesticides (Federal
Register vol. 47, #106, 23928).
Vertebrate safety is mostly based upon them being killed at or above 98°F (37°C).
However, it is also due to their sensitivity to the vertebrate immune system. Their
symbiotic bacterium is likewise temperature and immune sensitive.
Many beneficial insects are protected by their choice of hot and/or dry environments.
Other beneficials like ladybugs have a natural high resistance, which is not currently
understood. Predaceous mites and Earthworms naturally prey upon beneficial nematodes as
well as upon the insects they kill. One side effect of applying beneficials is often
an increase in earthworms. Still the St nematode you apply might still persist at moderate
levels for a year before needing to be replaced.
Development of genetically based insect population resistance to nematodes has never
yet been observed. Since the pathogenicity of nematodes to insects is based upon multiple
modes of action, resistance is less likely to develop compared with chemicals with single
modes of action. |
