Protecting greenhouse tomatoes
Parasitic wasps don't sting and they're hardly larger than the whiteflies
they attack. Yet with careful monitoring both exotic and domestic species
are becoming successful biological control agents on greenhouse tomatoes
in the U.S. Southwest and in Europe.
Oscar Minkenberg, an entomologist in The University of Arizona College
of Agriculture, and his staff raised nearly 30 million wasps last year
for inundative release in commercial greenhouses and research facilities.
currently producing three strains of wasps from Israel, the United Arab
Emirates, and Pakistan," he says. "They were collected in the field
over there by collaborators in the USDA/ARS. At the UA we developed
a unique, mass-rearing technology that enables us to produce large numbers
of the wasps for release here in Arizona, and in other states and countries."
Why use a mini-wasp? Most of the chemical sprays that kill whiteflies
also kill the bumblebees that pollinate the tomatoes in greenhouses.
Growers have been searching for ways to handle their insect problems
without using pesticides.
Whiteflies damage plants by piercing leaf surfaces, sucking vital fluids
and potentially transmitting viruses. They exude a sticky substance
called honeydew that turns leaf surfaces black, cutting off the light
the plant needs for photosynthesis. In Arizona the Bemisia species,
or silverleaf (sweetpotato) whitefly has caused crop losses in cotton,
melons and other vegetables, and has attacked greenhouse vegetables
and flowers as well.
Parasitic wasps attack whiteflies by laying their eggs on or under
the whitefly nymphs. As the wasp larvae feed, they destroy the whiteflies
and emerge as adult wasps to begin the life cycle again. They do not
injure the plant or other beneficial insects. The trick is to keep the
balance between predator and prey steady enough to ensure the wasps
a constant minimum supply of food while minimizing crop damage.
Arizona cotton growers, the Arizona Cotton Research and Protection
Council and USDA/APHIS support biological control, according to Minkenberg,
and they have funded mass rearings of parasitic wasps for release in
"This is a unique collaboration between the UA and Arizona cotton
growers and USDA/APHIS out of Phoenix," he says. Although Minkenberg
has been releasing parasitic wasps in open field situations, he admits
that the controlled environment in greenhouses has yielded more consistent
"Greenhouses are the mainstay," Minkenberg says. "It's where we see
the successes right now." Species used in Arizona for biological control
include Eretmocerus californicus (new name, E. eremicus),
a native of the Southwest, and Encarsia formosa, the wasp commercially
"In the Southwest, Eretmocerus does a lot better because it is suited
to our high temperatures and lower relative humidity," Minkenberg says.
He started raising and releasing Eretmocerus in 1992, then got involved
with commercial biocontrol companies who have taken over mass rearings
of the native species. Both Koppert in Ann Arbor, Mich., and Novartis
(previously Ciba-Bunting North America) in Oxnard, Calif. have assumed
the mass rearing of Eretmocerus eremicus once handled by the
Three commercial growers discuss their results
Commercial greenhouses in the Southwest have had varying degrees of
success with the wasp, according to interviews with managers at three
of them: Willcox Greenhouses and Bonita Nurseries in Cochise County,
Ariz., and Colorado Greenhouses outside Denver, Colo.
"The best part about using biologicals is that you don't have to use
chemicals," says Piet-hein van Baar, a manager at Willcox Greenhouses,
where eight acres of beefsteak and cluster (cherry) tomatoes are cultivated
in greenhouses. Van Baar has met with the UA's Minkenberg for advising
"The biggest problem with chemicals is that when you spray, the chemical
doesn't work anymore after a while, because the insects build up resistance,"
van Baar says. "And the public doesn't want pesticides. So we started
with the biologicals in August 1996. It's difficult: you have to get
a balance between the beneficials and the pest. Sometimes you have to
finish with a chemical."
The situation is complex because other insects besides whitefly can
plague greenhouse tomatoes, and growers often juggle a mixture of insecticides
and biological controls targeted to each pest.
"We use parasitic wasps such as Diglyphus to attack the leafminers,
Trichogramma wasps for the pinworms, Eretmocerus eremicus and
Encarsia formosa for the whitefly," van Baar notes. "This year
wasn't a very good year for the wasps. We got some other pest bugs and
had to use a chemical [which disrupted the biological control]. You
always look at how many wasps there are, and you keep looking to see
what you need and whether it's working or not working."
Based on this weekly monitoring, the grower knows how to schedule
the wasp releases. Most of the parasitic wasps sold in the United States
come from the Netherlands or England, where growers began using Encarsia
formosa and other beneficial insects in greenhouses more than twenty-five
years ago. Company consultants visit the U.S. periodically to check
on the success of these wasp populations.
The wasp pupae arrive in small boxes which are placed on the plants
so the wasps can emerge and colonize the greenhouse. A new batch of
wasps is released weekly throughout the growing season.
Not only do the wasps control the whitefly, they also allow the plants
to grow stronger in the absence of insect damage. When plants are not
weakened by insects they can build their own natural defenses. Van Baar
has found that this year's crop is so strong that it is providing resistance
of its own right within the plant.
"What we see is that the crop is so strong now and the roots take up
so much water that the pressure is high inside the plant. The stem is
getting wet as the plant's own toxic substances ooze out of it; these
toxins kill whiteflies as well," he says.
According to Jan de Kok, of Bonita Nurseries in Willcox, Ariz., both
species of parasitic wasps are "working quite well." They just added
Eretmocerus this year; their forty acres of beefsteak greenhouse tomatoes
have been under biological control for the last two years.
"I am really surprised. We have not had to spray chemicals at all,"
de Kok says. "We started directly with biological control. In this location,
we are separated from the other greenhouse businesses here. The balance
was immediately there, and we didn't have a problem, even from the beginning."
Biological control can cost more than conventional pesticides when
several different biocontrol agents are used for a variety of pests.
The public doesn't always support the correspondingly higher priced
product at the grocery store.
"Most growers want to go along with biological control because you
get too much resistance with pesticides," de Kok says. "It's disappointing
that the public sees the value in it but will not support it monetarily.
People will not pay one cent more for pesticide-free tomatoes."
Colorado Greenhouses pest management advisor Frank Stonaker heard about
Minkenberg's work a few years ago and contacted him regarding the wasps.
"Oscar provided us with our first Eretmocerus last year for a time,"
Stonaker says. "Now we use the European suppliers. We'd like to use
locally developed products, but they're not available yet. We do have
the greenhouse whitefly situation pretty well understood."
Stonaker uses both Encarsia formosa and Eretmocerus on
70 acres of beefsteak and cluster tomatoes. He has been using the Encarsia
for about three and a half years, but began to rely more heavily on
Eretmocerus beginning last year.
"We're still working out which is best," Stonaker admitts."We're still
learning, and from time to time we still use pesticides. Probably sixty
to seventy percent of our efforts are real successful with parasites.
The other forty percent we're still working out. We don't have good
biocontrol agents for certain pests yet, such as the potato psyllid
and the western flower thrips.
"There are not a lot of field proven situations," Stonaker adds. "We're
writing the book and that's real frustrating sometimes. You assume you
can plug into somebody else's research, but the intense light here in
the Southwest, for example, throws off the introduction levels used
Still, Stonaker is optimistic that biocontrol will work out for Colorado
Greenhouses. "We use bumblebees to pollinate and we need pretty clean
greenhouses as far as pesticides go," he says. "Biocontrol is about
25% cheaper than using pesticides. And the benefits are huge, because
every time you spray a chemical you damage a plant. We are also concerned
about environmental pollution. Our goal is to be under 100 percent biocontrol
within two years."
Article written by Susan McGinley, ECAT, College of
This is part of the 1997 Arizona Experiment Station Research Report
This document is located at http://ag.arizona.edu/pubs/general/resrpt1997/parasitic_wasps.html
Return to Index for 1997 report
Department of Entomology
(Note: Oscar Minkenberg is no longer at The University of Arizona.)