Entomopathogenic nematodes are very small worm-like organisms
[actually parasitic roundworms] that live to kill other insects.
The nematodes do not act alone; rather they serve as a delivery
service for bacteria that kill the insect. A good way to think
of this relationship is to picture a missile with an explosive
warhead; the nematode is the "rocket and guidance system"
which delivers the bacterium ("warhead").
How do they work?
The "infective juvenile" [usually the 3rd instar]
stage locates a host insect, usually an immature form (larva or
pupa) in or on the soil, enters the insect through the mouth,
anus, or breathing tube. Once inside, the nematode releases the
bacteria from its gut, and the bacteria starts growing, ultimately
killing the insect. Meanwhile the nematode is feeding on both
the multiplying bacteria and the insect. The nematode is also
growing, developing into an adult, ultimately going through one
or more generations (usually 2-3 life cycles per season) inside
the carcass of the insect. As the bacteria and nematodes use the
nutrients in the insect host, more infective juveniles (ijs) are
produced which load up on the bacteria then emerge from the now
long-dead insect and continue the cycle.
Two important entomopathogenic nematode genus are: Steinernema
spp. the "ambushers" and the Heterorhabditis
spp. the "hunters". If targeting vine weevils, root
weevils, white grub or black weevils between late June and mid-September
treat with Heterorhabditus spp. If you wanted to treat other pests
earlier in the season as well as the above, a Steinernema and Heterorhabditus blend is the way to go.
A few organic farms on Vancouver Island are applying beneficial
nematodes each spring as a preventative action. Some are cranberry
growers, potatoes, vegetables and berry fruit growers. They are
happy with the results, finding pest damage lessened to a degree
where it is more acceptable.
There have also been cherry
fruit fly and codling
moth studies done in Yakima, WA by Dr. L. A. Lacey and others.
Copies of these papers are available by request*. The research
found that
cherry fruit fly larva could be controlled to a certain degree
with applications of Steinernema spp., between mid-June to
mid-August as the larva drop from the tree.
Field trials for codling moth were done as a spray directly
on the logs on in the bins where the diapausing (resting) larvae
were located. They found on most trials the emergence was reduced
from 31.2% to 83.4% and as high as 93.5% with one strain Heterorhabditus
spp. High temperatures and low humidity have a negative impact
on efficacy making the best season for control late February to
late April. Temperatures would have to be about 12-15º Celsius
for daytime application, although there is a new Steinernema strain that works well in cooler climates and is commercially
available.
A short list of some pests controlled by nematodes:
Beans & Legumes - army worm, cucumber beetle, cutworm, lesser cornstalk borer,
white grubs
Berries - Banded fruit weevil, cranberry girdler, crown
borers, cutworms, raspberry beetle, root weevils, strawberry moths,
white grubs
Corn - Beet armyworm, black cutworm, corn rootworm, corn
earworm, fall armyworm
Fruit Trees - European apple sawfly, codling moth, peach
tree borer, stem borer, white grub
More about parasitic
nematodes
*Donna Kimball, Technical Consultant, The
Bug Factory.
Email: donnak@thebugfactory.ca.
Phone: 250-468-7912 Fax: 250-468-9484
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