Tracking the pests

Pesticide resistant flies are observed

Published: Sunday 15 July 2001

Silverleaf whitefly: Persists< tiny silverleaf whiteflies are resisting the latest crop of pesticides in fields of melons and peppers in the lower Rio Grande Valley, Texas, us . Tong-Xian Liu, researcher at the Texas a&m University, in the us , and his assistants, searching for signs of the tiny silverleaf whiteflies say there are signs that the species is evolving. "Whatever you use, sooner or later it will become resistant," says Liu.

The tiny whiteflies weaken crops by feeding on plant sap, producing waste that encourages fungus to grow, blocking photosynthesis. They also carry plant diseases. Liu's project intends to check whether the flies are developing resistance to 'Admire', an insecticide used widely in the us .

Knowing if and when an insect is becoming resistant can help researchers tell growers to change insecticides, believes Liu. In similar trials performed on cotton and melon aphids, the researchers found it took 12 generations for the flies to develop resistance. "Knowing this, we were able to advise growers to alternate their treatments of Admire with other insecticides to avoid a build-up of resistance," he says.

The problem, Liu says, is that the three favoured pesticides for melons have similar chemical makeup, so resistance to one could mean resistance to all.

The whiteflies have been known to humankind for the last 100 years. But it was in 1986 that they became a menace. The whiteflies devastated the Florida poinsettia industry, in the us, and began spreading throughout the southern states of the country. Researchers think pesticides used against other insects might have been killing off the whiteflies' predators. By 1992 and 1993, the insects appeared as clouds over the cotton fields in the region. Between 1986 and 1992, they caused loss of about us $1 billion worth of agricultural income.

Since then, an integrated method involving new chemicals, natural predators, and trap crops has brought the insects under control, but Liu says, keeping them that way will demand meticulous monitoring. Once collected, the flies are treated in his labs with various concentrations of insecticide. The generation born to those that survive also are treated with the same concentration and the process is repeated until a pattern of resistance is determined, if one exists. Since each generation has a life span of approximately three to four weeks, Liu says it takes time to test generation after generation.

"We are now testing the third generation of whiteflies in this test, so we have a while to go before we can make any kind of determination," he says.

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