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Corn: True Armyworm
Excerpt from Agronomy Guide for Field Crops (Chapter 3) Order OMAFRA Publication 811: Agronomy Guide for Field CropsTable of Contents
DescriptionTrue armyworms, full grown, are 4-cm (1 1/2-in.) dull-green to brown larvae that can be easily confused with other caterpillars, including variegated cutworm and fall armyworm. Two distinguishing features that true armyworm possess are white-bordered stripes running laterally along the body and dark diagonal bands at the top of each abdominal proleg. The head is yellow-brown with a network of dark brown lines creating a pattern. The adult sand-coloured moth has distinctive white spots on the centre of each forewing.
Plate 18. True armyworm larvae have black bands at the top of each proleg and the lighter brown head. | Top of Page | Life HistoryIn early spring, the moths prefer to lay their eggs in grassy vegetation. Larvae hatch from the eggs and feed at night or on overcast days for approximately a month. There are two generations per year, but it is the first generation that tends to do the most damage in corn in Ontario. DamageTrue armyworms feed at night. In conventional-till cornfields, damage usually occurs first in the border rows, whereas infestations may develop throughout no-till corn following small grains or grass. True armyworm also frequently invades corn fields from neighbouring cereal fields. Feeding begins on the margins of leaves and eventually the plant is stripped of leaves, with only the stalk and leaf midribs left. As long as the growing point of the plant is not damaged, the corn plant will be able to recover from moderate feeding. Scouting TechniqueThe best time to scout for true armyworm is early evening, just before sunset. Examine 20 plants from five locations in the field (100 plants total). During the day, you may find the larvae in the whorl, leaf axil, amongst the crop debris on the soil surface or under soil clods. Also, you may find brown frass, often mistaken for eggs, in the whorl or on the soil near the plant. Record the size and number of larvae. | Top of Page | Action ThresholdAs long as the growing point of the plant is not damaged, the corn plant is usually able to recover from moderate feeding. With early season feeding, insecticide may be warranted in seedling corn if there are two or more larvae per seedling and feeding damage exceeds 10%. For corn past the whorl stage, if 50% of the plants are showing damage and are infested with larvae smaller than 2.5 cm (1 in.), then insecticide treatment may be warranted. When scouting, check the backs of armyworms for eggs. These small, oval, yellowish eggs are usually located just behind the head of the larva. These are eggs of a parasitic fly. The eggs will hatch, and the maggots will kill the armyworm larvae. Management StrategiesIf the larvae are almost full grown (about 4 cm (1 1/2 in.) long), there is no benefit in applying insecticide since most of the feeding damage has already been done. Insecticides will only be effective on larvae that are smaller than 4 cm (1 1/2 in.). Refer to OMAFRA Publication 812, Field Crop Protection Guide, for insecticide recommendations. Treatment may be confined to infested areas. The parasites and other beneficial organisms usually keep armyworms from reaching damaging levels. Avoid treating with insecticides when large numbers of parasitized larvae are present. Because grassy, weed-infested corn is attractive to armyworm moths for egg-laying, eliminate weeds from the corn crop and field borders. Late-season grass control may not be a good option since this will cause the feeding larva to migrate from the grassy weeds that are now dead to the crop itself. (Order OMAFRA Publication 812) | Top of Page | Updates on Corn: True Armyworm| Top of Page | Related links...| Top of Page | For more information:Toll Free: 1-877-424-1300 Local: (519) 826-4047 E-mail: ag.info.omafra@ontario.ca |
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