Imported Wasps Work Well as Biological
Tarnished plant bug, Lygus lineolaris, is a serious pest of alfalfa
being grown for seed.
Three major pests of alfalfa and other crops may soon be on the run from
a pair of parasitic wasps from Europe.
The wasps are cousins: Peristenus digoneutis and P. conradi.
Their targets: three plant bugs.
The alfalfa plant bug, Adelphocoris lineolatus Goeze; tarnished plant
bug, Lygus lineolaris Palisot; and lygus bug, L. hesperus Knight,
are important pests of crops grown for seed in the western United States. Each
year, they cause tens of millions of dollars in losses and control costs. They
suck the sap from flowers, young fruits, and seeds. The alfalfa plant bug is an
immigrant pest, but the two Lygus species are native to North America.
Vegetable and fruit seed crops suffer similar losses from the two
Lygus species, says entomologist William H. Day, who is with the
Agricultural Research Service.
"Nymphs and adults of each species attack the plants, and native parasites
can't adequately suppress any of them."
Although all three plant bugs occasionally reach high levels in the 24
million acres of alfalfa grown for forage, they usually do little damage to
this crop grown in the 48 contiguous states.
However, the two Lygus species pose a special problem: When alfalfa
is cut for hay, they fly off to infest and damage fruit and vegetable crops
such as strawberries, peaches, apples, and beans growing nearby.
Estimating plant-bug damage is difficult. "Visible injury by sucking
insects like these plant bugs is often confused with other causes or
overlooked," Day says. "Furthermore, their feeding on crops grown for
seed can both lower yields and reduce germination of seed that does
Day and three other ARS entomologists conduct laboratory and field tests on
parasites and predators of problem insects at ARS' Beneficial Insects Research
Laboratory in Newark, Delaware. The lab's mission is to import beneficial
insects into the United States for establishment in areas where insect
pestsespecially those of foreign originare abundant.
Day's current interest is wasps, especially P. digoneutis, a
quarter-inch-long parasite of the two native Lygus plant bugs, and P.
conradi, which attacks the alfalfa plant bug.
"A female Peristenus wasp stings a young plant bug nymph, laying
a tiny egg in it," he says. "A few days later, a wasp larva hatches
and begins to eat the nymph."
According to Day, "Using biological controls is better than chemicals
for controlling pests in northeastern alfalfa. Chemical insecticides add to
crop production costs and sometimes cause environmental problems. They can also
kill parasites previously established by our lab that now control three other
pests of alfalfathe alfalfa weevil, pea aphid, and alfalfa blotch
Beginning about 1978, entomologists at ARS' European Biological Control
Laboratory, now located in Montpellier, France, collected Peristenus
wasps in Europe and shipped them to Day in Delaware. Day released them from
1979 to 1983 in northern New Jersey and determined that they were successfully
established there in 1984.
He also led an interagency team to track where the parasites have become
established and spread. Its members included Day and ARS biological technician
Joseph M. Tropp, along with Robert J. Chianese of the New Jersey Department of
Agriculture, Trenton; Ronald F. Romig of West Chester State University, West
Chester, Pennsylvania; Roy G. Van Driesche of the University of Massachusetts,
Amherst; and Allen T. Eaton of the University of New Hampshire, Durham.
The team surveyed several hundred fields, collecting samples of plant bugs
from which parasites were later reared for identification.
A quarter-inch-long parasitic wasp, Peristenus digoneutis, prepares to
lay an egg in a tarnished plant bug nymph.
Day found that P. digoneutis had spread into New York by 1989. Since
then, the team has found this parasite in five new states (Pennsylvania,
Massachusetts, New Hampshire, Vermont, and Connecticut) and in a total of 36
countiesover 45,000 square miles.
"The probable range is likely larger," Day says. "We detected
P. digoneutis in 12 new counties we surveyed in 1995 and in 11
additional counties in 1996."
Most of its dispersion has been to the Northeast, he says. That's largely
because the wasps travel with the prevailing summer winds from the Southwest.
Plus, they apparently can't cope with the warmer climate to the south of New
"The parasite has only moved about 30 miles south in 12 years,"
notes Day, though genetic selection may eventually allow it to move farther
south. But in the near future, its control of tarnished plant bugs will likely
be limited to the northern United States and southern Canada, he says.
As for P. conradi, Day first discovered it had become established and
was attacking the alfalfa plant bug in 1988. P. conradi has spread less
extensively than P. digoneutis, partly because it reproduces only once a
year, while P. digoneutis breeds twice.
"P. conradi has spread into New York from its initial
establishment points in northern Delaware and central New Jersey. Joe and I
found it in 9 counties, but it's probably present in others," says Day,
"because our surveys for this parasite have been limited."
When Day and Tropp sampled northeastern alfalfa grass fields for P.
digoneutis and P. conradi wasps, they commonly found seven species
of plant bugs and three native wasp parasites. The latter were ineffective
controls for both the tarnished and alfalfa plant bugs, prompting the overseas
search for new parasite species.
They Know What They Like
Day says, "Our host range data also show that the two introduced
parasites significantly attack only the target pests, so they are unlikely to
reduce nontarget or nonpest species. That's good news for biocontrol efforts
and ensures that ecological balances are not disturbed."
The alfalfa plant bug, Adelphocoris lineolatus, is a non-native plant
Day's research turned up something unusual. He has long suspected that the
tarnished plant bug, a native U.S. insect, could be controlled by a foreign
parasite. Traditionally, researchers turn to a foreign biocontrol to control a
"But our experience with P. digoneutis indicates that
certain foreign biocontrol agents have untapped potential to go after some of
our troublesome native insect pests," he says.
Day believes that although introduced beneficials are carefully screened
before they are imported and released, to ensure they will not attack native
beneficial insects, "there has been insufficient research to determine if
foreign biocontrol insects might control some native insect pests."
Day's research on the wasps' dispersion and geographic limits provides
information needed to establish them in other regions of the United States.
"With what we now know about the spread and effectiveness of P.
digoneutis, it appears that it has the potential to control the two
Lygus bugs on alfalfa forage and seed crops over wide areas of the
northern United States," says Day.
Research now under way at several locations will determine if these
classical biological control results can be repeated on seed alfalfa in the
Northwest and on strawberries and other fruits and vegetables in the Northeast.
By Hank Becker
Day is at the USDA-ARS Beneficial Insects Research
Laboratory, 501 South Chapel St., Newark, DE 19713; phone (302) 731-7330