Study Shows Insecticide Has Little Control Effect on Onion Leafminer
Wearing a grim face, Andres Savella looks around his 1,000-square meter onion field. Disheartened, he tells himself that there’s no solution to his problem. A certain kind of insect feeds inside the onion leaves, and no amount of insecticide can kill it.
The problem of this 62-year-old farmer is the onion leafminer, a yellowish insect with a black spot on its back is suspected to have been accidentally introduced in the Philippines through imported cut flowers, according to Dong Arida, supervising research specialist at the Philippine Rice Research Institute (PhilRice). 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 leafminer as Liriomyza trifolii (Burgess).
The larvae of the insect eat the inner portion of the leaves, and by the time the mines are noticed, the leafminer larvae are already well protected by the leaf cuticle from insecticidal spray.
According to the book Integrated Pest Management in Rice- Vegetable Cropping Systems, which was published by PhilRice, the adult female leaf miner lays or oviposits its eggs inside the leaf by puncturing or piercing the leaf tissue with its ovipositor. The adult lays its eggs singly inside the leaf, which hatch in three days. There are as many eggs laid in a leaf as there are oviposition punctures on it. The adult is yellowish with a black spot on the back portion of the body.
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 because the mine tunnels hinder the movement of nutrients inside the leaves, and 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.
Savella’s problem is not an isolated case. 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 leafminer 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 leafininer 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 leafminers 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 leafminer damage with increased insecticide application.
IMPACT OF INSECTICIDE APPLICATION ON THE PEST
This situation encouraged Arida to conduct a study for two years in Bongabon to determine the impact of weekly insecticide application on the onion leaf miner. The study recorded the population density of the larvae and adults, oviposition punctures, number of mines, and effect of insecticides on natural enemies. It was part of the 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 installed sticky yellow board traps to estimate leafminer abundance in his test area. Leafminer adults caught by the traps were recorded weekly from transplanting to harvest. He also collected onion leaves with leafminer larvae and reared these in plastic jars to determine the percentage of parasitism. Likewise, he recorded the number of generalist predators starting at two weeks after transplanting until harvest.
Results of the study showed that in plants sprayed with insecticide, oviposition punctures were at their highest at 39 and 67 days after transplanting. In the unsprayed plants, oviposition punctures were highest at 67 days after transplanting. On the other hand, the number of larvae per plant in the unsprayed plants went down after a peak at 46 days after transplanting.
Arida observed that the number of oviposition punctures was correlated with the number of larvae in sprayed plants. This was not the case in the 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.
The highest incidence of parasitism in unsprayed plants was 23 percent at 39 days after transplanting, while it was only 6.3 percent in the sprayed plants.
The highest number of damaged leaves was recorded at 46 and 67 days after transplanting in both sprayed and unsprayed plants.
Except for the incidence of larval parasitism, there was no significant difference between the sprayed and unsprayed plants in oviposition. punctures, number of larvae, and damaged leaves. It is possible that wanton application of insecticide during the previous years has almost wiped out the predators of onion leafminer.
Results of the study also showed that the sticky yellow board traps caught more leafminer flies as the crop neared maturity. What’s more, the number of flies caught in the sprayed and unsprayed plants did not differ significantly.
“This shows that weekly application of insecticide did not reduce the adult fly population as flies from adjacent fields moved into the study area,” Arida said. He pointed out that instead of helping the farmer, insecticidal spraying killed the generalist predators, which should be responsible in keeping the leafminer population down. There were more predators in the unsprayed plants than in the sprayed plants.
Overall, the study showed that insecticide spray has little control effect on leafminer larvae and adults.
To control the onion leafminer, do the following management practices recommended in the IPM book mentioned earlier:
• Apply biological control. The onion leaf miner has several natural enemies or predators. Conserve these predators by judicious use of chemical insecticides.
• Install yellow board sticky traps for monitoring and mass trapping of leafminer adults.
• Clean up and burn infested plant remains after harvest to substantially reduce leafminer populations in subsequent generations.
• Apply insecticide as a last resort. Take note that the onion leafminer is reported to be highly resistant to organophosphate, carbamate, and pyrethroid insecticides.