Use sex urge to kill onion pests

Onion farmers can capitalize on the sex urge of the moths of two major insect pests of onion — common cutworm and armyworm — to kill them.

Philippine Rice Research Institute researchers have found that at night when cutworm and armyworm moths are actively feeding on the leaves of onion crops, the female moths emit a chemical called sex pheromone that attracts the male insects for mating. Even at a far distance, the male moths could smell the sex pheromone and out of instinct, would fly immediately to the location of the chemical in search of a mate.

This characteristic behavior of the insects could be tapped by onion farmers by using sex pheromone traps to be able to determine the most appropriate time to spray insecticides and at the same time, catch a considerable number of the moths.

Dong Arida, PhilRice insect pest management expert, said onion farmers can use sex pheromones to be able to apply control measures at the most appropriate time. In general, sex pheromones could be used for three purposes.

First, they are used for detecting the presence of a particular species and as a tool for effective timing of control measures. Second, they could be used to catch a big portion of the male moths so that reproduction is reduced. Third, sex pheromones could be used to disrupt communication between sexes during mating. This is done by applying pheromone in large amounts in different locations to permeate the air with chemicals so that the males are confused on the exact location of the females.

This technique would enable farmers to save 70 to 90 percent of their insecticide cost without reducing yield, PhilRice insect control specialists said.

The sex pheromone traps monitor the presence of cutworm and armyworm populations that are feeding on the leaves of onion crops. The traps are effective only at night when the moths are active. This is because the insects stay under the leaves or burrow in soil cracks during the daytime.

The moths must be prevented from laying eggs, as the larvae or worms are very destructive. They bore holes in the leaves, causing the upper part of the plant to dry up and fall off.

When the number of moths caught in the traps start to build up, that is already when farmers should spray insecticide.

In actual practice, however, farmers spray insecticides as soon as they see worms or damaged onion leaves in their fields. They believe that spraying more often prevents insect damage.

In Nueva Ecija alone, farmers spray 22 times on the average in one cropping season, spending much on insecticide and labor.

Farmers should know that onion plants can compensate for the early damage done by cutworms and armyworms until 30 days after planting and, hence, insecticide application is not required during the first month.

With sex pheromone traps, onion farmers can detect when to spray insecticide. Instead of spraying 22 times, they can only spray as low as three times and still produce big onion bulbs. Thus, they are able to reduce their insecticide cost by at least 70 percent, as well as minimize health and environmental hazards.

PhilRice insect control specialists led by Arida have demonstrated the advantages of using sex pheromone traps in Talavera, Sto. Domingo and Bongabon, Nueva Ecija by installing them in some fields one month before transplanting.

The traps can catch the male and female moths even in the absence of onion plants. This would indicate the initial population of the insects before the onion crop is planted.

Arida said the moths caught in the traps must be counted three times a week to show the periods when the insect population is at its peak. For best results, insecticide spraying should be done only at five to seven days after a peak is observed in the trap catch. Onion plants sprayed with insecticide based on the peak of trap catch yielded higher than those sprayed weekly. — S. Ma. Pablico