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What’s eating my corn

Few things say summer like the sweet flavour of homegrown corn. But then comes along a few corn earworms ( Helicoverpa zea ) to ruin things. The corn earworm is not a worm, but the caterpillar (i.e. immature larval form) of the corn earworm moth.

Few things say summer like the sweet flavour of homegrown corn. But then comes along a few corn earworms (Helicoverpa zea) to ruin things.

The corn earworm is not a worm, but the caterpillar (i.e. immature larval form) of the corn earworm moth. The mottled brown to tan moths (wing span of 35 – 45 millimetres) arrive from the United States and Mexico in late June through August, carried on south winds. Female moths lay their eggs singly on new leaves and silks. The eggs hatch within a few days. Colouration of the caterpillars is variable ranging from brown, green, pink and yellow to mostly black with alternating light and dark horizontal stripes along their back. Heads are usually orange or light brown. Two similar species, the European corn borer and fall armyworm, may be present at the same time.

The severity of damage depends on when the moths arrive. The moths themselves cause no damage, feeding on nectar. Rather, it is their offspring that cause all the trouble. If they arrive early, before the corn plants have started silking, caterpillars feed on young leaves and damage is usually limited to isolated plants. If in the silking stage, caterpillars tunnel into the ears, feeding on the silk and developing kernels. After the caterpillars go through a number of developmental stages (instars), they drop to the ground to pupate. There is only one generation per year in Canada (although multiple immigration events can occur depending on wind patterns) and all life stages are killed by fall frost.

While corn is their preferred host, corn earworms can be found feeding on tomato (hence one of its other common names, tomato fruitworm), artichoke, asparagus, cabbage, cucumber, eggplant, lettuce, pea, pepper, potato, pumpkin, spinach, squash and melon. They can also be found on a number of field crops like alfalfa, wheat, oat and sunflower and wide range of weeds.

It is difficult to impossible to control corn earworms once they have entered the protective environment of the corn ear. For best control, take a multi-pronged approach. First, recognize there are natural enemies, including a parasitic wasp, helping you fight this pest. Applying pesticides to control the earworm will diminish the wasps’ impact. Second, plant corn early or plant early-maturing varieties to avoid attack by late arriving moths. Third, as a last resort, apply a registered insecticide (carbaryl and permethrin). For effective control, multiple applications are required starting at early silk stage to kill the caterpillars before they have a chance to enter the ears. Follow label instructions.

Usually, you will find only one or two earworms per ear with damage limited to the top third of the ear. If you can get past the ‘yuck-factor,’ simply cut off the damaged section. The rest is perfectly fine to eat.

— This column is provided courtesy of the Saskatchewan Perennial Society (SPS) (www.saskperennial.ca; hortscene@yahoo.com). Check out our Bulletin Board or Calendar for upcoming garden information sessions, workshops and tours: Sept. 13, SPS members-only fall bulb sale and plant exchange (new memberships can be purchased at the door), Saskatoon Forestry Farm Park and Zoo; Sept. 19, Labour & Learn, SFFPZ; Sept. 19, Gardening 101 – Part III: Fall, University of Saskatchewan.