Harvest Time

Stop indiscriminate spraying to save onion predators


By Dr. Sosimo Ma. Pablico

ONION FARMERS may as well stop spraying insecticides indiscriminately on their crops to protect predators or friendly insects that help them control the onion leaf miner, scientifically known as Liriomyza trifolii.

This advice comes from Dong Arida, a pest management expert of the Philippine Rice Research Institute (PhilRice), who has been studying the control of the onion leaf miner for a number of years already. He said no amount of insecticidal spray can control the insect because its larvae, the most destructive stage, is well entrenched inside onion leaves and, hence, would be difficult to reach with the chemical.

The onion leaf miner larvae feed in the inner portion of the leaves, and by the time the damaged leaves are noticed, the larvae are already well protected by the leaf cuticle from insecticidal spray. The adult female leaf miner lays its eggs inside the leaf by puncturing the leaf tissue with its ovipositor. Laid singly inside the leaf, the eggs hatch in 3 days. And the problem starts.

As soon as the eggs hatch, the larvae start feeding inside the leaf, leaving behind them what appears like a mine tunnel within the leaf, which is whitish to pale yellow. “Mined” portions of the leaves eventually dry up. The mine tunnels hinder the movement of nutrients inside the leaves as well as reduce the manufacture of food through photosynthesis. The result is a reduction in yield.

Early season damage results in a delay in the maturity of onion plants, Arida said. Oftentimes, this ultimately results in yield loss.

The insect was first collected in 1997 from onion fields in Central Luzon by Dr. Sonja J. Scheffer of the Agriculture Research Service, US Department of Agriculture. Based on laboratory-reared flies that she collected, Scheffer identified the leaf miner as Liriomyza trifolii (Burgess).

In many parts of the country where onion is grown, farmers observe that the mines are affecting their yields. For instance, onion farmers in Nueva Ecija have observed leaf miner damage in onion during the last few years. Elsewhere this insect is slowly but surely becoming a menace unless farmers stop the indiscriminate spraying of insecticides.

Arida said heavy infestation of the leaf miner is usually the result of wanton insecticide applications that kill the insect’s natural enemies or predators. Unfortunately, as farmers observe more mines in their onion leaves, they also spray insecticides more frequently.

In Bongabon, Nueva Ecija where onion is planted in large areas during the dry months from December to May, it has been observed that farmers spray insecticides on their onion crop 22 times in a cropping period.

The control of leaf miners by insecticides alone is often difficult and temporary because the insect rapidly develops resistance to pesticides, Arida said. In some cases, he added, onion farmers even reported to have more leaf miner damage with increased insecticide application.

This situation encouraged Arida to conduct a study for two years in Bongabon to determine the impact of weekly insecticide spray application on the onion leaf miner. The study was part of an Integrated Pest Management Collaborative Research Support Program in a PhilRice partnership with the Virginia Polytechnic Institute and State University, which was supported by a grant from the US Agency for International Development (USAID).

Arida observed that the number of oviposition punctures was correlated with the number of larvae in sprayed plants, but not the case in unsprayed plants due to higher incidence of larval parasitism.

“Insecticide spraying had detrimental effect on the larval parasitoids of leaf miner as shown by lower level of parasitism in the sprayed plants,” Arida said. The average larval parasitism in sprayed plants during a cropping season was 1.5 percent, which was very much lower than the 7.1 percent in unsprayed plants.

Arida said weekly application of insecticide did not reduce the adult fly population, as flies from adjacent fields moved into the study area. Instead of helping the farmer, the insecticide spray killed the predators.

To control the onion leaf miner, three management practices are recommended in the IPM book mentioned earlier, as follows: apply biological control; install yellow board sticky traps to monitor and trap leaf miner adults; clean up and burn infested plant remains after harvest to substantially reduce leaf miner populations; and apply insecticide as a last resort. Take note that the onion leaf miner is reported to be highly resistant to organophosphate, carbamate, and pyrethroid insecticides. (With Hannah HM M. Biag)

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