Flying in the Face of Grapefruit
Wind tunnel bioassays are
used to investigate why Mexican
fruit flies prefer certain
fruits for egg-laying and
leave others untouched. Here,
entomologists Ivich Frazer
(left) and David Robacker
measure how attractive an
orange is to Mexican fruit
flies that are ready to
Keeping Mexican fruit flies out
of grapefruit orchards in South Texas is a tough jobdespite new findings
that when fruit flies hatch, they don't even like grapefruit.
This pest annually infests citrus in Texas, Mexico, and Central America and
threatens California and Florida. Female fruit flies lay eggs in the fruit.
When the larvae hatch, they feed on the pulp, ruining the fruit for human
Ten-year losses from the Mexican fruit fly in South Texas and northern
Mexico alone have been estimated at almost $7 billion from export sanctions,
lost markets, treatment costs, and reduced crop yields.
That Mexican fruit flies are not naturally attracted to grapefruit was a surprising fact uncovered by ARS entomologists David Robacker and Ivich Fraser as they investigated what factors attract the insect to an egg-laying site.
Working at the Kika de la Garza
Subtropical Agricultural Research Center in Weslaco, Texas, Robacker is
studying how the insect receives basic information about and reacts to its
environment. Eventually, this information will help him develop better
monitoring and control methods.
In one of their first experiments, Robacker and Fraser measured how
attractive a grapefruit was to wild-strain egg-laying Mexican fruit flies
compared to a yellow control ball of similar size. They expected to confirm
that grapefruit is highly attractive to the Mexican fruit fly, probably because
a grapefruit gives off semiochemicalsa type of chemical signature that
insects identify by smell.
"But we couldn't get the Mexican fruit fly to come to grapefruit any
more than to the yellow control ballnot even a little. That was kind of
surprising when most of the literature lists grapefruit as a favorite choice
for egg laying," Robacker says. "And if the flies don't like
grapefruit, why are they always such a problem in grapefruit orchards?"
Robacker and Fraser also looked at the response of laboratory-raised fruit
flies rather than wild-strain fruit flies. Laboratory-raised flies did respond
to grapefruit, but still at very low rates, says Robacker. He surmises that
being raised in laboratory colonies may have turned the flies into opportunists
that react to generalized fruit stimuli.
"We also looked at whether hunger for sugar might increase the flies'
tendency to lay eggs on grapefruit," Robacker says. "When flies
deposit their eggs, they puncture the surface of the fruit and often feed on
the juice that drips out."
But the scientists found that while twice as many starved as satiated
Mexican fruit flies landed on grapefruit, there was no difference in their
propensity to lay eggs there.
"While this did not explain how the fly makes an egg-laying selection,
it looks like we may have identified a behavior that the Mexican fruit fly uses
to cut down on its risk of being eaten by predators: It combines egg laying and
feeding flights," Robacker says.
But Robacker was more interested in discovering why the Mexican fruit fly
annually lays eggs in South Texas grapefruit orchards if grapefruit are not
At Fraser's suggestion, Robacker placed grapefruit into cages with the flies
for several days before the experiments. Once exposed to grapefruit, females
were attracted to it 400 percent more than were flies without exposure. In
addition, grapefruit-naive Mexican fruit flies were more likely than
fruit-experienced flies to lay eggs on the wall of the test chamber.
"Once the fruit flies learned what grapefruit was, they went to it
right away in the egg-laying test, even when we kept them away from grapefruit
for several days before the test. The results were dramatic, but there's no
evidence that the preference can be passed on to the next
generationunless they too receive the learning exposure to
grapefruit," Robacker says.
Bolstering the "need to learn" concept is that grapefruit are not
native to the Rio Grande Valley; they were introduced there commercially in the
early 1900s. The native host of Mexican fruit flies is the yellow chapote, a
small, yellow-green fruit that grows in mountain valleys in northern Mexico.
But Robacker and Fraser found that fruit flies have to learn to use chapote
just as they do grapefruit.
What this suggests to Robacker is that wild-strain, adult Mexican flies are
likely to choose whatever is close to them when they emerge from their
protective shells, called puparia. Those that emerge in grapefruit orchards
learn grapefruit as a food source because it is handy. They are then likely to
combine feeding and egg laying.
Such behavior, Robacker points out, reinforces the positive impact of
management techniques that call for growers to remove all grapefruit remaining
in trees and on the ground after the harvest is complete. Without the fruit,
wild Mexican fruit flies in the area will not have the opportunity to learn
about it as a food or egg-laying resource and will leave the orchard to learn
to use some other fruitor die trying.By J. Kim Kaplan,
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 Robacker and
Ivich Fraser are in the Crop
Quality and Fruit Insect Research Unit, Kika de la Garza Subtropical
Agricultural Research Center, 2413 E. Hwy. 83, Bldg. 200, Weslaco, TX 78596;
phone (956) 447-6320, fax (956) 447-6345.
"Flying in the Face of Grapefruit" was published in the June 2002 issue of Agricultural Research magazine.