More On Corn Defoliators (Part 1)
Let’s continue our discussion on pests that devour leaf tissues. As a refresher, we consider these pests as significant since they could effectively reduce the plant’s ability to produce more food via photosynthesis and lower the corn’s defenses from stress and diseases.
I promised last month that we will write on three more corn defoliators. I really thought it was important to have these three included so our farmers can distinguish them from the major ones like the Asiatic corn borer (ACB). As we all know, Bt corn is specific only to specific Lepidopterans (family of moths and butterflies) such as the ACB, the true armyworm, and pink stemborer (Sessamia inferens). This has been observed and confirmed in the field by Pioneer agronomists. However, we don’t have a firm conclusion for the rest such as common cutworm, black cutworm (described in last month’s article), corn earworm, and semi-looper. In fact, Bt corn growers should expect them in their field every now and then since they are non-target pests.
COMMON CUTWORM (Spodoptera litura Fabricius)
In young seedlings, the common cutworm is one of the most common pests. Oftentimes, farmer ignore them because they are sensitive to chemical control, damage is assumed to be minimal, and they are usually gone by the time the plant reaches mid-whorl. But let’s not discount the possibility of the common cutworm to emerge as a major pest.
Description and life cycle. Eggs of the common cutworm adult are round, pearl-white, laid in mass on leaves or objects on the ground and covered with yellowish-brown hairs. One female can lay up to 5 egg masses with an average of 300 eggs per egg mass. Incubation period is 3-5 days. Young larva is greenish with 2 black spots about 1/3 the length from the head. Full grown larva is dark green with bright yellow dorsal line and lateral stripes with black spots. Larval period, the destructive stage, is 20-30 days. Pupa is reddish brown, and 1.6 cm long. Adult is, 20 cm-25 cm long with forewings that are purplish-brown with numerous lines and spots. Hind wings are whitish with narrow band along the outer margins.
Damage symptoms. Seedlings are often cut off at ground level. Cutworm larva can be found in the soil (up to depth of about 5 cm) near the plant base. They always curl-up when disturbed. Cutworms feed only at night, so generally, they are not found on plants or on the soil surface during the day. Young larvae eat the soft leaves of the plants, scraping the leaf tissues and leaving irregular grayish white patches on the leaves. Older larvae may cut the stem and leaves including veins and midribs. Young plants may be completely defoliated by older larva.
Monitoring. Look for larvae on lower surface of leaves or in the plants whorl. Older larvae may chide on the soil clods near the base of the plant. Inspect and record the damage on 20 consecutive plants in 5 areas of the field. Control if 3%-5% of the plants show damage and 2 or more larvae are found per 100 plants.
MANAGEMENT AND CULTURAL PRACTICES
1. Removal of weeds in and around fields will reduce egg-laying sites and will help in the prevention of cutworm infestation. Do this at least 2-3 weeks before planting to reduce the incidence of cutworm larvae transferring to newly planted crops.
2. Plow and harrow fields before planting. This will destroy eggs and expose larvae to chicken, ants, birds and other predators.
3. In irrigated areas, flooding is recommended to prevent cutworm population.
4. Collect egg masses and crush them.
5. Make small trenches around the field and put some cut grasses for shade; collect hiding larvae in the early morning.
6. Chemical control (use ofpyrethroids, methomyl, chlorfyrifos, triazophos insecticides). Precaution: Before you select and apply an insecticide, review the manufacturer’s label for information on the safe use of the material.