The Living Dead: What lurks inside these insect cadavers?
Technician Kathy Halat rolls
in a coating.
It sounds like something out of a
horror or science fiction movie: scientists working in laboratories to plant
thousands of worms into a carcass of another insect. But that's what a team of
ARS researchers is doing.
Why are they doing this?
Because the hidden worms are beneficial nematodes that will emerge to attack
pest insects of orchards. The cadavers are placed in the field, and the
nematodes come out and eat the unwanted insects.
Nematodes are roundworms. Some can be pests, but others are helpful. One group of beneficial nematodes is called entomopathogenic, that is, they cause disease and reproduce in insect pests, but do not harm humans, other animals, or the environment. These nematodes are used as natural alternatives to chemical pesticides. The usual method of releasing the nematodes into a desired environment is through a liquid spray solution. But it is often more expensive than traditional insecticides, and reliability may vary.
Coated Carcass Carrier
The idea of using dead insects to transport nematodes has been discussed in
the past; it allows many more nematodes to be reared at a lower cost than other
methods. But it was not feasible because the cadavers are fragile, and they
break apart easily when handled and stored.
Infected carcasses can rupture or stick together during transport or when being applied at the site, according to entomologist David I. Shapiro-Ilan at the Southeastern Fruit and Tree Nut Research Laboratory, Byron, Georgia. A method was needed to protect insect remains from deterioration.
Inside one of these plump wax
So Shapiro-Ilan and Edwin E. Lewis, a professor of entomology at Virginia Polytechnic Institute and State University, Blacksburg, Virginia, teamed up with entomologist Robert W. Behle, of the ARS National Center for Agricultural Utilization Research, Peoria, Illinois, and Michael R. McGuire, who heads the Western Integrated Cropping Systems Research Unit, Shafter, California. The research team devised a way to successfully coat insect cadavers with sticking agents and powders. The coating makes it easier to store and handle the carcasses and also improves tolerance to environmental extremes.
The scientists experimented with several dipping agents before developing
the patented formulaa specific combination of starch and clay. During
early research, they used cadavers of Tenebrio molitor, commonly known
as the yellow mealworm, and Galleria mellonella, or greater wax moth.
The nematode-infected hosts are first dipped in a liquid mixture and then
rolled in a powder. It sticks to the cadaver and has no significant harmful
effect on nematode reproduction.
When the carcass surface becomes
moist again, the powder dissolves. The coating biodegrades rapidly, because it
is made from environmentally friendly materials.
"Beneficial nematode progeny escape their deceased hosts after they are
placed in orchard or greenhouse soil," Shapiro-Ilan says. "The
nematodes then protect crops such as citrus, pecans, cranberries, greenhouse
ornamentals, and mushrooms from soil-dwelling pests like citrus root and black
What seems complicated at first is really just nature taking its course. The
scientists are reinforcing a natural chain of events in order to undermine
several insect pests. This form of biological cosntrol reduces reliance on
The scientists used a nematode species called Heterorhabditis
bacteriophora in their research, but they say they can easily transfer
their findings to other species. H. bacteriophora carry a bacterium of
the genus Photorhabdus. The bacteria live in the nematode in a symbiotic
relationshipthat is, in exchange for carrying them, the bacteria provide
nutrients to the nematode.
Left to right: A live, noninfected wax
The nematodes infect while an insect is still alive. Once inside, the bacteria leave the nematode. Working together once more, they are able to kill their insect host after about 48 hours. The nematodes and bacteria then feed off the bug and reproduce inside it.
The nematodes multiply in the dead insect for one to three generations until
bursting out of it. The number of nematodes in a single insectdepending
on the insect speciesranges from 10,000 to 500,000. Although a single
juvenile nematode is barely visible to the human eye, clusters can be seen
pouring from the cadaver in what Shapiro says resembles an explosion. The
nematodes then find other insect hosts and start the cycle anew.
Shapiro-Ilan and Lewis wanted to know if using insect cadavers to introduce
the nematodes into a location was more effective than the spray method.
Previously, Shapiro-Ilan and Itamar Glazer, a nematologist at the Volcani
Center in Bet Dagan, Israel, found juvenile nematodes dispersed further when
they were applied through cadavers than with water sprays, but they didn't know
why. Shapiro-Ilan and Lewis found that when nematodes were introduced while in
cadavers, the infection rate of juvenile nematodes in insects later increased.
There seems to be a chemical cue from the cadaver that affects the nematodes'
Although the exact compound hasn't been identified yet, its existence was
proven when an extract from a cadaver was also able to increase infection rates
in the laboratory. The nematodes associate the compound with their natural
hosts. In further research, they found that some of the cues appear to be
"It seems to be nature's way of assisting the nematodes in their quests," Shapiro-Ilan says. "The compound, in effect, tells them to move forward." They found that small amounts of nitrogen were released from the insect early after nematode infection, which attracted more nematodes. However, greater amounts of nitrogen were released later, and this repelled other nematodes. "We suspect that the purpose is to attract mates when they are needed, but also to signal later that there is no longer any room in a particular host."
A New Biological Control Method for the Market?
Shapiro-Ilan expects the cost of entomopathogenic production in insects to
decrease with this new innovation. He says it is a more efficient production
method than methods that result in a liquid spray solution, because it has
fewer steps. It also offers better control of insect pests because the
nematodes are more fit; that is, they disperse and infect better.
A small start-up company in Perry, Georgia, H&T Alternative Controls,
LLC, signed a cooperative research and development agreement with the
laboratory to mass-rear the nematodes and produce infected cadavers. H&T,
the Byron laboratory, and Virginia Tech are sharing grant funds from USDA's
Small Business Innovation Research Grant Program to develop this new
technology. According to co-owner Louis Tedders, they have been very happy with
the success they are having.
Shapiro-Ilan says field tests are continuing and should confirm the method's
effectiveness. He says some modification of the formula and its application
technique will be necessary before wide-scale use of the procedure.By
Agricultural Research Service Information Staff.
This research is part of Crop Protection and Quarantine, an ARS National
Program (#304) described on the World Wide Web at
David Shapiro-Ilan is with
the USDA-ARS Southeastern Fruit
and Tree Nut Research Laboratory, 21 Dunbar Rd., Byron, GA 31008; phone
(478) 956-6444, fax (478) 956-2929.
"The Living Dead: What lurks inside these insect cadavers?" was published in the May 2002 issue of Agricultural Research magazine.