Corn earworm
Helicoverpa zea
also called Tomato fruitworm, Cotton bollworm
One caterpillar with two names: it eats down the tip of a sweetcorn ear and bores into tomatoes. A few drops of oil in each silk channel keeps it out of the corn.
How to know it
A caterpillar up to 40 mm long, varying wildly in color from pale green to pink to near-black, always with fine pale stripes running the length of the body and a tan head. Look for it head-down in the tip of a corn ear with a heap of wet, sawdust-colored frass behind it, or half-in and half-out of a green tomato.
What you'll see
- Chewed, brown-stained kernels in the top few centimeters of a sweetcorn ear, with damp frass packed around them.
- A neat round hole at the shoulder of a green tomato, and rot spreading inward from it.
- Silks cut off short and browning early, before pollination is finished.
- Ragged windows chewed in whorl leaves on young corn before the ears form.
- Pepper and okra fruit hollowed out with the caterpillar still inside.
Where it comes from
It cannot survive a hard winter over most of the continent, so each year it flies north in waves from the south and arrives just as corn silks. Moths lay single eggs directly on fresh silk; the caterpillar chews down the silk channel into the ear, feeds for two or three weeks, then drops to pupate in the soil. Several overlapping generations in a long season.
Timing is everything: an ear that silks during a moth flight is hit and one that silks a fortnight later may not be. Late plantings in the south and warm, humid nights bring the heaviest pressure.
Dealing with it
- Put five drops of vegetable oil into the silk channel of each ear a few days after the silks brown — it smothers the young caterpillar and does not hurt the corn.
- Slip a rubber band or a clothes-peg over the tip of each husk once pollination is done.
- Spray Bt (Bacillus thuringiensis) onto fresh silks every few days during the flight; it only works on small caterpillars.
- Cut the damaged tip off the ear in the kitchen — the rest is perfectly good and this is a cosmetic loss more often than a real one.
- Pick and bin bored tomatoes and peppers rather than leaving them to finish the caterpillar off.
Keeping it out
- Sow corn early so it silks ahead of the main moth flight in your area.
- Choose sweetcorn cultivars with long, tight husk extensions past the ear tip.
- Cultivate the soil lightly after harvest to expose the overwintering pupae to birds and frost.
- Keep flowering umbels nearby for the trichogramma wasps that parasitize the eggs.
- Do not plant a late succession of corn beside a maturing one; the second block inherits the first block moths.
What it goes for
Sweetcorn first, then tomatoes, peppers, okra and beans.
Who hunts it for you
- BatsChiroptera A single bat takes hundreds of moths and mosquitoes in a night, and the moths are the parents of your cutworms, corn earworms and hornworms. Put up a bat box on a warm south wall, leave a night-scented corner unlit, and never spray after dark. Grow to recruit it: Evening Primrose,Honeysuckle,Nicotiana,Summer Jasmine
- Garden birdsPasseriformes Both things are true: birds strip your currants and a brood of blue tits takes several thousand caterpillars out of one fruit tree in a fortnight. Net what you must and welcome them everywhere else — a nest box near the apples is worth more than a spray. Grow to recruit it: Echinacea,Elderberry,English holly,Goldenrod,Rudbeckia,Sunflower
- Parasitic waspsBraconidae & Trichogramma Tiny, stingless wasps that lay eggs in or on caterpillars, aphids, and moth eggs — the white cocoons riding a hornworm are their calling card, so leave that caterpillar be. Umbel flowers with shallow nectar keep the adults fuelled. Grow to recruit it: Cilantro,Dill,Fennel,Lovage,Parsley,Sweet alyssum
- Tachinid flyTachinidae Bristly, house-fly-lookalike parasitoids that lay eggs on caterpillars, beetle grubs, and earwigs — the maggot consumes the host from within. Adults sip nectar from shallow umbels and daisies. Grow to recruit it: Cilantro,Dill,Feverfew,Parsley,Sweet alyssum
See also
The Cottage Garden Companion — the gardener's bestiary at cottagegardencompanion.com