Less use of pesticides urged

MANILA, Philippines - Onion farmers are urged to stop the wanton spraying of insecticides on their crops to protect predators or friendly insects that help them control the onion leaf miner, scientifically known as Liriomyza trifolii.

Dong Arida, a pest management expert of the Philippine Rice Research Institute (PhilRice), said no amount of insecticidal spray can control the insect because its larvae, the most destructive stage, is well entrenched inside onion leaves and 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 three days and the problem begins.

As soon as the eggs hatch, the larvae start feeding inside the leaf, leaving behind them what appears like a mine tunnel, 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.

Leaf miner damage in onion has been observed in Nueva Ecija 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. 

In a two-year study, Arida observed that the number of oviposition punctures was correlated with the number of larvae in sprayed plants due to lower incidence of larval parasitism. 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, an IPM book published by PhilRice recommends the following: 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. – Sosimo Ma. Pablico and Hannah HM M. Biag