Trap Crops Prove Irresistible to
Diamondback moth larvae feed on a cabbage leaf.
Cabbage, broccoli, collards, kale and other cole crops are an
all-you-can-eat salad bar for diamondback moths, a pest named for the
diamond-shaped markings embellishing its wings. Moth larvae, which chew on
plant leaves, take a big bite out of cabbage and other crops worldwide, costing
billions of dollars in control costs and losses.
Pesticide spraying can be costly, ranging from about $10 to $21 an acre for
each applicationdepending on which pesticides are usedand typically
costing growers $80 to $168 per acre, or more, each season to produce a crop.
To the farmer's dismay, diamondback moths are becoming resistant to almost
everything, including Bacillus thuringiensis (Bt)-based insecticides
that are widely used to kill certain pests while preserving beneficial insects.
Now entomologist Everett R. Mitchell is taking another approach to spoiling
the moth's meal. He says giving the pest a heaping serving of another
vegetablecollard greensspoils its appetite for cabbage. The moths
can't resist the collards when planted completely around the edge of cabbage
fields, a strategy called trap cropping.
"Invading diamondback moths stop and deposit their eggs on the
collards, rather than on adjacent cabbage plants," says Mitchell.
"Diamondback populations continue to recycle in collards as long as plants
remain green and continue to grow." Mitchell heads the Insect Behavior and
Biocontrol Research Unit, which is part of ARS' Center for Medical, Agricultural, and
Veterinary Entomology in Gainesville, Florida.
Mitchell recently conducted experiments on nearby farms in northeast Florida
that showed that the moths prefer to feed on highly fertilized collard plants.
He tested this approach for more than 2 years. In all cases, he says, there was
minimal cabbage damage from diamondback moth larvae. The quantity and quality
of cabbage produced equaled that from conventionally sprayed fields.
This simple, low-tech, cost-effective pest control method also reduced
pesticide use. "Cabbage fields surrounded by collards required 75 to 100
percent fewer sprays to control diamondback moths than fields treated
conventionally with pesticides. That's a huge savings for farmers," notes
He also says Diadegma insulare, a naturally occurring parasitoid that
attacks diamondbacks, builds in numbers in the collards and helps keep
diamondback populations in check. The tiny D. insulare wasp stings the
larvae, preventing them from developing into adults and laying more eggs. Once
stung, a larva becomes sluggish and stops feeding within a few hours. The wasp
doesn't attack other insects or humans.
"We established that there needs to be a threshold of 0.3 moth larva
per plant before a farmer has to apply pesticides," Mitchell says.
"We found that even though moth larval populations built up in collards
planted around field margins, populations in cabbage generally remained well
below the threshold." By Tara Weaver, Agricultural Research
Service Information Staff.
This research is part of Crop and Commodity Pest Biology, Control, and
Quarantine, an ARS National Program that can be viewed at
Everett R. Mitchell
is in the USDA-ARS
and Biocontrol Research Unit, 1600 S.W. 23rd Dr., Gainesville, FL 32604;
phone (352) 374-5710, fax (352) 374-5804.
"Trap Crops Prove Irresistible to Diamondbacks " was
published in the March 1999 issue of
Agricultural Research magazine.