The Other Insect Pests 2 : Corn Defoliators
Some months back, we wrote about another set of insects that is currently becoming a scourge to corn growers in Luzon: the corn plant hoppers and their cousins such as aphids and grass derbids. These sucking insects are somewhat of lesser importance before but for some reasons, they have become very important. The problem is that farmers and even agronomists are not familiar with these pests and the appropriate control strategies.
For the benefit of our corn growers in Luzon, a control strategy devised by Pioneer to control the planthoppers is included in this article. We hope this guide can be of help to both farmers and agricultural technicians.
This month we will talk about another set of corn pests with a different way of inflicting damage on corn: defoliation. As we all know, any reduction in the leaf area results in lower photosynthetic output (food for the plant) since there are lesser tissues to capture the sun’s energy. Defoliation can starve the plant and hence, it results in lower yields and predisposes the plant to diseases.
We are listing a few very important corn defoliators in this write-up. This is not a complete list but we believe these are the things that farmers should watch out for. They are pretty common across the country’s corn growing areas as most of them are caterpillars or larvae of moths and butterflies. We are sure that many can easily identify them.
Several types of caterpillars feed on the seedlings and may cut the stem of the corn plants. They are commonly referred to as cutworms and may reduce plant stand and yield if abundant. Field with heavy plant residues or early season weed growth are susceptible to cutworm attack.
Larvae are light gray to nearly black in color with an overall greasy appearance. When viewed through a hand lens, the skin has a rough pebbly texture. The head has two black stripes, and there is a pale band along the top of the body. A key characteristic is that on the top of each abdominal segment, there are two sets of paired spots which are unequal in size. Fully grown cutworm larvae are 1.5 to 2 inches long. Coloration will vary among species, but all tend -to be stout-bodied caterpillars with four sets of prolegs. They have the tendency to curl into a ball when disturbed.
CONDITIONS THAT FAVOR BLACK CUTWORM INFESTATION
Cutworms can occur in any cornfield. The following conditions appear to be conducive to serious infestations:
1. corn following an abundance of annual or perennial weeds;
2. fields that are planted to legume pastures;
3. fields on bottom land or in low portions of fields;
4. fields that are bordered by dense vegetation or pastures;
5. tillage practices that allow plant residues or weeds to remain in the field.
Damage symptoms. Depending on the species and growth stage of cutworm and weather factors, a variety of damage symptoms may be observed. Young cutworms feed on leaf tissues, cutting small holes. When black cutworms are about half grown, they become big enough to cut stalks or severely injure the plants by feeding at the base. A black cutworm can cut up to four 1-leaf stage corn plants or one 4-leaf stage corn plant before completing its development. Soil moisture conditions may also influence the type of damage. If the soil surface is dry and crusted, cutworms are more likely to feed on the stalk below the ground. Usually when corn reached the 6-leaf stage, the stem is too thick for cutworms to cut the plant.
Scouting for Cutworms. Scout fields once or twice a week beginning with plant emergence. All fields should be examined regardless of whether a planting-time soil insecticide was applied. Cutworm scouting can be ended once plants are at the 6-leaf stage since little additional cutting will occur. Observe plants at several locations in each field for evidence of leaf feeding or cutting of plants. Pay special attention to poorly drained areas and places where weeds were present before planting. If cut plants are found, examine the soil around damaged plants for cutworms. Cutworms feed at night and hide in the soil or under debris during the day. Scrape the soil around the base of damaged plants and look for a fat-bodied, grayish-brown to blackish colored worm curled into a C-shape.
Cutworm Control. The general recommendation for cutworm control involves use of post-emergence rescue treatment once economic levels of cutworm damage are detected. Early detection of the problem is essential because most of the cutting occurs within 7-10 days of plant emergence. Cutworms longer than 1 inch are likely to pupate before causing additional significant damage. Preventive treatments applied at or before planting have generally given erratic control, especially where cutworm numbers have been high. Also, most fields do not have a cutworm problem every year so preventive treatments are often unnecessary. Planting time treatments may be advisable if replanting is necessary due to cutworm damage and if cutworms are 1 inch or less in length. A variety of insecticides such as pyrethroids, methomyl, chlorfyrifos, and triazophos are effective against cutworm. Insecticides are effective if applied in a timely manner and directed at the soil around the base of the crop plants. The use of granular soil insecticides such as Carbofuran are also recommended. However, excessively dry conditions may limit the activity of granular insecticides.
Army worms got their name from their ability to march in droves, taking out any grassy vegetation along the way. Most at risk are cornfields that are in their path when going from one field to another. Once on a plant, they begin feeding along the margins of the leaves but as the larvae develop, plants can be entirely stripped. Only the midrib and stalk may be left. Corn under 8 inches in height that is attacked by armyworms will usually have the leaves eaten off entirely. With larger corn, midrib of the leaves will sometimes be left, but the center of the young stalk is so eaten out that it dies.
TRUE ARMYWORM (Mythimna separata)
This insect requires 41-65 days to complete a generation (egg to adult). Female moth prefer to lay their eggs in dense, grassy vegetation. True army worm larvae have two characteristics that distinguish them from other armyworm larvae: (1) a white-bordered orange line down-each side of the body; and (2) a large, single dark spot at the base of its fleshy, abdominal prolegs.
Damage Symptoms. The true and black are the primary armyworm pests on corn. True armyworm begins feeding on the lower leaves before progressing to the top of the plant. Young plants (less than 8 inches tall) can be completely defoliated, but older plants retain the midrib of the leaf even during heavy infestations. Feeding damage is most common after a cold, wet weather that slows the plant’s growth and reproduction of the true armyworm’s natural enemies. Infestations may also be more severe in late-planted, no-till fields.
Armyworm Scouting. Corn growers should scout for armyworm infestations when plants first emerge and continue until plants develop several leaves. Armyworm larvae are generally inactive during the day; therefore, fields should be scouted during the early morning or late afternoon hours. To determine if your field has an economic infestation of armyworm, randomly examine 20 plants at each of 5 locations per field. Take note on the location of infestations in the field, larval size, and where the larvae are feeding on the plant. If larvae are not visible but some feeding damage is evident, carefully unroll and inspect the whorl of damaged plants for the larvae.
Economic Threshold and Control Measures. For the true armyworm, treatment is recommended when 25 percent or more seedling corn plants are damaged and larvae are still present. The use of pyrethroids, methomyl, chlorfyrifos, and triazophos insecticides can control the pest. Just a caution: Before selecting and applying an insecticide, review the manufacturer’s label for information on the safe use of the material.
GRASSHOPPERS (Melanoplus spp.)
Grasshoppers are general feeders on grasses and weeds and often move to cultivated crops. Their population vary from year to year. Crop damage is likely to be greatest in years when dry weather accompanies high populations. Drought conditions reduce natural vegetation, forcing grasshoppers to move to cultivated crops.
Grasshoppers are brown to grayish-green jumping insects that are moderately long and have prominent heads and large compound eyes. The front pair of wings are narrow, leathery, and thickened. The hind pair are thin, broadly triangular, transparent, or sometimes brightly colored. Nymphs generally resemble adults except for size and the absence of fully developed wings. The grasshoppers of importance in corn are up to 44 mm long when fully grown.
Damage symptoms. Nymphs and adults will feed on corn in any plant growth stage but are usually not observed until- R1 to later stages. The outer rows of corn are usually the first attacked, but as the grasshopper reached the adult stage they move further into the field eating the leaves, 5ilks (may interfere with pollination), and ear tips. When grasshopper populations are high and damage is severe, they may only leave the leaf midribs, pruned ears, and barren stalks.
Sampling Method. Conduct early and late season grasshopper surveys in favored egg laying areas (i.e, grass pastures, weedy water-ways, fence-rows, small grain fields etc;) bordering cornfields. And if grasshoppers are noted around the fields, walk ‘into cornfields past the end-rows checking for grasshopper activity.
1. Trapping of hoppers into pits and manually collecting and killing them.
2. Use nets to trap the flyers.
3. Chemical application in identified roosting and breeding sites using either carbaryl, chlorfyrifos, and pyrethroids insecticides.
4. Build smokes around the cornfield to repel the insect.
5. In many parts of the country, these are collected and fried for food.
There are at least three more defoliators common in the cornfields of the country. Our list includes the common cutworm, the corn semi-looper, and the corn earworm. We will discuss them in the next issue.