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The use of insect parasitic nematodes and other biological
control agents to manage insect pests has grown in popularity. This is
primarily due to the changing problems associated with pest control. For
example, many pests have developed resistance to certain pesticides, new
pests have arisen to replace those successfully controlled, the effectiveness
of natural control agents (predators, parasites and pathogens) has been
reduced by pesticide use, pesticides are no longer inexpensive to use,
and there is increased concern about pesticide safety and environmental
quality. These beneficial organisms can be an important component of an
integrated pest management (IPM) program for ornamental crops and turf
grass sites.
What are beneficial nematodes?
Nematodes are morphologically, genetically and ecologically diverse organisms
occupying more varied habitats than any other animal group except arthropods.
These naturally occurring organisms are microscopic, unsegmented round
worms that live in the soil and, depending on the species, infect plants
and animals. The two nematode families Steinernematidae and Heterorhabditidae,
contain the insect parasitic nematode species. The most commonly used
beneficial nematodes are Steinernema carpocapsae, S. feltiae, S. glaseri,
Heterorhabditisheliothidis and H bacteriophora. Nematodes
that are endoparasites of insects attack a wide variety of agricultural
pests.
The life cycle of beneficial nematodes consists of eggs, four larval stages
and the adults. The third larval stage is the infective form of the nematode
(IT). They search out susceptible hosts, primarily insect larvae, by detecting
excretory products, carbon dioxide and temperature changes. Juvenile nematodes
enter the insect host through the mouth, anus or breathing holes (spiracles).
Heterorhabditid nematodes can also pierce through the insect’s body
wall. The juvenile form of the nematode carries Xenorhabdus sp.
bacteria in their pharynx and intestine. Once the bacteria are introduced
into the insect host, death of the host usually occurs in 24 to 48 hours.
As the bacteria enzymatically breaks down the internal structure of the
insect, the Steinernematids develop into adult males and females which
mate within the insect's body cavity. Heterorhabditids produce young through
hermaphroditic females. This form of nematode has the sexual organs of
both sexes. As the nematodes grow, they feed on the insect tissue that
has been broken down by the bacteria. Once their development has reached
the third juvenile stage, the nematodes exit the remains of the insect
body.
Why are these organisms beneficial?
Parasitic nematodes are beneficial for six reasons. First, they have such
a wide host range that they can be used successfully on numerous insect
pests. The nematodes' nonspecific development, which does not rely on
specific host nutrients, allows them to infect a large number of insect
species.
Second, nematodes kill their insect hosts within 48 hours. As mentioned
earlier, this is due to enzymes produced by the Xenorhabdus bacteria.
Third, nematodes can be grown on artificial media. This allows for commercial
production which makes them a more available product.
Fourth, the infective stage is durable. The nematodes can stay viable
for months when stored at the proper temperature. Usually three months
at a room temperature of 60o to 80o F and six months when refrigerated
at 37o to 50o F. They can also tolerate being mixed with various insecticides,
herbicides and fertilizers. Check nematode product label for compatibility.
Also, the infective juveniles can live for some time without nourishment
as they search for a host.
Fifth, there is no evidence of natural or acquired resistance to the Xenorhabdus
bacteria. Though there is no insect immunity to the bacteria, some insects,
particularly beneficial insects, are possibly less parasitized because
nematodes are less likely to encounter beneficials which are often very
active and escape nematode penetration by quickly moving away.
Finally, there is no evidence that parasitic nematodes or their symbiotic
bacteria can develop in vertebrates. This makes nematode use for insect
pest control safe and environmentally friendly. The United States Environmental
Protection Agency (EPA) has ruled that nematodes are exempt from registration
because they occur naturally and require no genetic modification by man.
What are the target insects?
Experiments have shown that beneficial nematodes can reduce the populations
of a variety of ornamental and turf pests. Control has been reported for
the larvae of black vine weevil; strawberry root weevil; the clearwing
borer Synanthedon culiciformis in alder and S. resplendensin sycamore;
peach tree borer; dogwood borer; and banded ash borer with Heterorhabditis
bacteriophora. White grubs (Japanese beetle, oriental beetles, chafer
beetles, June beetle and Ataenius) are also controlled by this nematode.
Turf larval pests controlled include surface pests such as cutworms, sod
webworms and dog and cat flea larvae; and the soil inhabiting pests billbugs
and crane fly can be controlled with Steinernema carpocapsea nematodes.
Nematodes can be used on these and other pests as long as proper application
procedures are used and the environmental conditions are favorable.
It is important to select the proper nematode species when trying to control
a particular pest. S. glaseri has increased mobility in the soil and can
target all of the various white grubs. S. feltiae is most effective in
the habitats occupied by dipteran pests like fungus gnats. The heterorhabditid
nematodes prefer a moister soil and tend to go deeper into the soil profile
(3" to 6"). Nematodes should be applied at the first sign that
a pest population is causing damage. If nematodes are definitely going
to be used during the growing season and can be stored for up to six months
under proper conditions, it is best to order them ahead of time so that
they are in stock before the damaging stages of particular pests arrive.
Reapplying nematodes depends on the success of the first nematodes released.
Their survivorship and success are based on environmental conditions and
soil type, increases in original pest population, and percentage of living
nematodes actually released during the first application. Nematodes should
be reapplied on seven-day intervals if damage continues.
Problems associated with nematode use.
Though nematodes can be an effective and safe pest management options,
there are limitations to their use. The first is related to their manufacture
and storage. It is difficult to synchronize the development of infective
juveniles under laboratory conditions. Also, the nematodes must be shipped
in the proper media and stored at the correct temperature. Thus, it is
a good practice to check the percent viability of a package of nematodes
before applying them. This can be done by placing a small amount of nematode-containing
material in water and observing the live nematodes under a microscope
or hand lens.
The other factors to consider relate to their actual usage. In order to
ensure maximum effectiveness, it is crucial to apply them at the optimum
environmental conditions needed for their survival. Therefore, it is best
to irrigate the target site, both before and after application, because
they need moist conditions to prevent desiccation and aid with movement
to find hosts. Also, the best results are obtained when the relative humidity
is high, ambient temperature is neither extremely hot or cold, soil temperature
is between 55o and 90oF, soil is moist and direct sunlight is minimal.
All of these factors help prevent the nematodes from drying out and increase
their survival.
Nematodes are sold to various ornamental/lawn care companies and mail-order
pest management supply companies. They then sell the nematodes to consumers
using various product names. In most cases, the cost of using nematodes
on large areas, at the 1 billion/acre rate, would be higher than conventional
insecticides, but treatment for small-scale problems should not be prohibitive
For pesticide recommendations call the UConn Home
and Garden Education Center at 877-486-6271.
References:
Bedding, R.A. and L.A. Miller. 1981. Use of a Nematode,
Heterorhabditis heliothidis to Control Black Vine Weevil, Otiorhynchus
sulcatus, in Potted Plants.Ann.Appl.Biol. 99:211-216.
Davidson, J.A., S.A. Gill, and M.J. Raupp. 1992. Controlling Clearwing
Moths with Entomopathogenic Nematodes: The Dogwood Borer Case Study. J.
of Arboriculture. 18(2):81-84.
Georgis, R. and G.O. Poinar. 1989. Field Effectiveness of Entomophilic
Nematodes Neoaplectana andHeterorhabditis. Pages 213-224, In A.R. Leslie
and R.L. Metcalf (eds.). Integrated Pest Management for Turfgrass and
Ornamentals.United States Environmental Protection Agency, Washington,
DC.
Gill, S., J.A. Davidson, and M.J. Raupp. 1992. Control of Peachtree Borer
Using Entomopathogenic Nematodes. J. of Arboriculture.18(4):184-187.
Kaya, H.K. 1985.Entomogenous Nematodes for Insect Control in IPM Systems.
Pages 283-303, In M.A. Hoy and D.C. Herzog (eds.).Biological Control in
Agricultural IPM Systems,New York: Academic Press.
Kaya, H.K. and L.R. Brown. 1986.Field Application of Entomogenous Nematodes
for Biological Control of Clear-Wing Moth Borers in Alder and Sycamore
Trees. J. of Arboriculture. 12(6):150-154.
Owen, N.P., M.J.Raupp, C.S. Sadof, and B.C. Bull. 1991. Influence of Entomophagus
Nematodes and Irrigation on Black Vine Weevil in Euonymus fortunei (Turcz.)
Hard. Mazz.Beds.J.Environ.Hort.9(3):109-112.
Poinar, G.O. 1986. Entomophagous Nematodes. Pages 95-121, In H.Franz(ed.).Biological
Plant and Health Protection, Fortschritte der Zoologie, Bd.32.G.Fischer
Verlog, Stuttgart, New York. Reprint.
Rutherford, T.A., D. Trotter, and J.M. Webster. 1987. The Potential of
Heterorhabditid Nematodes as Control Agents of Root Weevils. The Canadian
Ent. 119:67-73.
Shetlar, D.J. 1989. Entomogenous Nematodes for Control of Turfgrass Insects
with Notes on Other Biological Control Agents.Pages 225-253, In A.R. Leslie
and R.L. Metcalf (eds.) Integrated Pest Management for Turfgrasses and
Ornamentals. United States Environmental Protection Agency, Washington,
DC.
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