By MM Umali
January 2005
University of The Philippines Los Baños
UPLB HORIZON, Volume 7, No. 1

They come, they eat, they multiply, and they fly - this easily sums up the life of the Asiatic Corn Borer (Ostrinia furnacalis) or ACB. To farmers, however, coping with the damage left behind by the most serious corn pest is never easy. In fact, it would take money, sweat and another planting season to recover what they lost. That is, if they know what is best to do when they see the insect on their next crop.

Thanks to scientists at the National Crop Protection Center (NCPC), farmers and corn planters can effectively control ACB using technologies and control techniques the former have developed. These technologies are simple and cost-effective and are readily available.

A hungry bug

The Asian Corn Borer, according to Dr. Pio A. Javier, Research Associate Professor II of NCPC, can be found everywhere in the country. "Once you plant corn, you'd be sure to find ACB. It is most widespread, however, in major corn-growing areas like Regions II (Cagayan Valley), X and XI, especially in places where corn is planted year-round."

Dr. Javier, who specializes in economic entomology and biological control of pests, has been studying ACB since 1976. He believes that ACB is not a native insect. "Based on its history, ACB was probably introduced in the country. Unfortunately there is no definite literature which explains how ACB got here." He adds that the first recorded occurrence of ACB was in 1906 and the moth has been assigned different scientific names such as Pyrausta vastatrix, Pyrausta salientalis, Pyrausta damaolis, and Pyrausta nubilalis before Ostrinia furnacalis (Guenee) became the valid name for the Philippine species.

ACB occupies a wide geographic range that includes several temperate regions across the tropics and Australia. "This moth is a strong flier and can reach up to 1 mile from where it came," says Dr. Javier. ACB starts to infest corn plants 20 to 25 days after planting. "Within this period ACB will start to lay eggs, usually underneath the leaves. Sometimes the eggs are laid on the upper surface of the leaves near the midrib. These eggs will hatch within three days."

After two hours, the newly hatched larvae will start to feed, causing pinhead-sized holes on the leaves. The young larvae will molt, grow bigger, feed again, then drill larger holes the size of a match head on the leaves. In their third instar (ACB goes through six instars or larval stages), the insect will eat more, this time boring on the leaves holes the size of shots from a gun. When the corn plants bear tassels (flowers), the larvae will go up and infest the tassel, which clump and eventually break.

According to Dr. Javier, ACB larvae are most damaging when they are on their late fourth instar. "This is when the larvae bore inside the corn stalk and eat their way through. Once inside the stalk, they are difficult to control. The transport of nutrients is disrupted and the stalk eventually breaks. This is what happens when we see an entire field of fallen corn plants that were hit as if by a storm."

Asked if there would be anything left to salvage after ACB damaged a cornfield, Dr. Javier says it would depend on the degree of infestation. "The estimated yield loss due to ACB is 20% to 80%. If the population of the corn borer was high, then the damage or loss will be high, then the damage or loss will be high because aside from attacking the tassel and stalk, most of the ACB will damage the ears. As such there is a big, if not total loss."

The importance of being early and watchful

Do not plant late if you don't want your corn to be feasted on by ACB, Dr. Javier advises. "You have to plant before or during the planting season. ACB prefers younger plants. If you plant late, then the newly built population of ACB from other fields will transfer to your field and devour your corn."

Dr. Javier says that it is also important to monitor the number of ACB in the field. He strongly warns against going gung-ho and spraying any available insecticide upon seeing a corn borer. "After 20-25 days of planting, visit your cornfield and monitor the ACBs. If you see 3-4 ACB egg masses per 100 plants, that is when you start control measures." These control measures include releasing natural ACB predators in the field and, if needed, applying selective insecticides.

ACB has a number of natural enemies which can control its population and damage to plants. Theses include Trichogramma (Trichogramma evanescens), earwig (Euborellia annulata), flower bug (Orius tantillus), lady beetle, and others.

"Trichogramma is an egg parasite of ACB. It is an effective biocontrol agent because it attacks corn borer eggs right away, leaving them no chance to feed on leaves," says Dr. Javier. Trichogramma feeds on the fluid found in corn borer eggs. "If you see 3 to 4 ACB egg masses per 100 plants, release 50 to 70 cards of Trichogramma per hectare. It is important to release Trichogramma within 20 to 25 days after planting, when ACB eggs are newly laid and still full of fluid which the egg parasite can feed on."

According to him, it is easy to see if a mass of corn borer eggs has been parasitized by Trichogramma. Parasitized eggs are pale dark and depressed while those that are not, are shiny dark in color. Further, if the egg mass did not hatch within four days after it was laid, then it might be parasitized. If it hatched within four days, then Trichogramma was not able to parasitize the eggs. Trichogramma has a life cycle of 7 to 8 days so if the ACB eggs were parasitized, Trichogramma adults and not corn borer larvae will come out of the eggs. These adults will look for another egg mass to feed on.

After a week, if you noticed that less than 30% of the ACB egg masses were parasitized, release another batch of Trichogramma. Dr. Javier says that this should be done two to three times a week. But if more than 30% of the egg masses have been parasitized, additional release of Trichogramma may not be necessary.

Aside from Trichogramma , you can also release earwigs and Orius. These insects, according to Dr. Javier, are general predators of ACB, meaning that they attack not only the eggs but also the young larvae and pupae (earwigs only). Except for Orius, they are easy to rear. To ensure that Orius will be present in the cornfield, plant the spiny amaranth (Aramanthus spinosus) around the periphery of the cornfield as the weed serves as natural enemies of ACB.

There are times when not all ACB will be attacked by natural enemies. While you monitor the egg masses, says Dr. Javier, observe also the damage on the leaves. "If more than 30% (30 out of 100) of the plants being monitored have leaf damages (either pin, match, and shot holes), spray selective or apply granular insecticides. Selective insecticides include those made up of Bacillus thuringiensis (Bt) formulation, which are specific for the larvae, or granular insecticides. Do not use any available insecticide, especially those that are being sprayed since it will also kill the beneficial insects." He emphasizes, however, that there is no need to use insecticides if there are healthy populations of Trichogramma, earwigs, Orius and lady beetles in the field.

To reduce the ACB population, you can also remove the tassels (flowers) of 75% of all corn plants. This technique, called detassling, allows you to dispose of the larvae that attack the tassels. As such, they would not multiply and their number will be significantly reduced. "Detassling controls up to 50% of the ACB larvae that goes into the tassel. Just make sure to do this when at least 2 cm of the stem has emerged or prior to pollen shedding. Also, properly dispose of the tassels you removed either by burning them away from the field or feeding them to livestock. This way ACB will not have a chance to go back to the plants," says Dr. Javier. He adds that if more than 30% of plants with intact tassels have symptoms of corn borer larvae damage, you can spray selective insecticides.

All of the above methods are simple, effective, and cost-efficient ways to control ACB. What's more, they are environment-friendly.