If fighting drought and heat over the past summer wasn’t uncomfortable enough, the dry weather conditions packed an extra punch for New York State: an influx of aphids, thrips, and spider mites. These pests, which thrive under warm, dehydrated conditions, live on plant sap, and have been causing large problems for plant growers.
Prof. Brian Nault, entomology, at Cornell’s New York State Agricultural Experiment Station in Geneva, explained the correlation between hot and dry conditions and aphid, thrip, and spider mite populations. “Some population growths will grow exponentially in certain points of the summer,” he said.
Under these weather conditions, bugs launch assualt on defenseless greenery: “When hot and dry, plants are less able to produce defensive chemicals in their leaves,” said Patricia Curran, the horticulture manager of Cornell Cooperative Extension of Tompkins County.
The insects are soft-bodied creatures with piercing mouth parts, and are able to cause damage simply by virtue of quantity.
Aphids can also cause damage with their excrements, honeydew, which causes mold to grow on their victims. The most devastating weapon of aphids, however, is their ability to transmit viruses and according to Nault, the damage done this year has been the worst its been over the last four years.
Since 2001, Nault and other extension agents have been collecting samples from all over western New York to study infected green bean, also known as snap bean, fields. From their studies, Nault and his colleagues have discovered several different types of viruses.
The clover yellow vein virus and the bean yellow mosaic virus are two examples but according to Phillip Griffiths, Nault’s colleague and fellow assistant professor of entomology at Cornell University’s New York State Agricultural Experiment Station in Geneva, the cucumber mosaic virus is the most important.
Nault said “nearly 100 percent of green bean plants in certain fields have been infected [by the cucumber mosaic virus].”
The cucumber mosaic virus causes blistering and downward puckering of the leaves. The virus also causes yield loss, pulp distortion, and internal pulp necrosis.
Soybean aphids have also been especially potent this year.
“The numbers in soybean fields have been extremely high. There have been cases where there were over a 1000 on one plant,” Nault said.
Thrips have also had damaging effects on certain plant populations. Thrips, like aphids, are very small but unlike aphids, they possess a mandible, which they use to scrap surfaces and suck the contents of the leaves.
Onion thrips, in particular, have been “extremely horrific this year-some onion growers have never seen onion thrip infestations this high,” Nault said.
The only way to stop thrips is with insecticides. However, onion thrips, along with many other types of thrips, are resistant to insecticides available to growers, rendering their attempts fruitless. The attack of the insects raises the question of what a grower is to do.
For now, “we are in the process of evaluating new insecticides,” Nault said.
Nault and vegetable entomologists from other universities around the country have been working on a federally funded project called the IR-4 program.
We “are evaluating promising new insecticides for thrip control and we were successful in identifying several,”Nault said.
Though these new insecticidesare not yet on the market, Nault said that “working with the IR-4 program, we can accelerate the registration of these products for growers.”
Aphid control is particularly complicated.
“The problem is [aphids] don’t life cycle on snap bean plants,” Griffiths said.
Aphids first get viruses from sources such as alfalfa and other legume crops and then transmit the viruses to snap beans.
Another problem is the rate at which aphids transmit viruses.
“Aphids transmit viruses within seconds. Insecticides can’t work fast enough,”Nault said. “There is nothing we can do. Ultimately, the answer is developing a snap bean variety that’s resistant to the viruses responsible for yield loss. At this point, there is none.”
Griffiths is in the process of developing a snap bean variety that is resistant to the cucmber mosaic virus but progress has been slow.
Griffiths has looked at over 500 different types of common beans but none has shown resistance. He has found resistance on a related species, however, called scalloped runna beans. Griffiths and his colleagues are now working to move the resistance from the scalloped runna beans to the snap beans.
Griffiths concluded by saying “we hope to have plant materials to test in field trials in 2006 or 2007. At the moment, work has been mostly greenhouse based with virus inoculations, selection, and crossing to select snap bean type plants.”
Archived article by Virgnia Nam
Sun Staff Writer