The short version: The caterpillar boring a neat hole into green tomatoes, or chewing the tip of a sweet corn cob, is most likely Helicoverpa: corn earworm (Helicoverpa armigera, also called tomato grub or cotton bollworm) or its relative, native budworm (Helicoverpa punctigera). Check the top of each plant twice a week from first flower and first silk, squash the tiny eggs and young caterpillars, keep moths off with insect mesh where you can, and if you need a spray, use Bacillus thuringiensis on small caterpillars and follow the label.
You pick a tomato that looks perfect from above, turn it over and find a round, clean hole ringed with dark crumbs. Or you peel back the husk on your first sweet corn and meet a striped caterpillar eating the tip.
Both are usually the work of the same pair of moths, and the caterpillars are easiest to beat before they tunnel in. That takes a few minutes of looking, not a shelf of sprays.
Two moths and a handful of names
- Corn earworm, Helicoverpa armigera. Business Queensland calls it corn earworm, South Australia's primary industries department calls it tomato grub, and New South Wales uses cotton bollworm. It eats grasses as well as broadleaved plants: sweet corn, tomatoes, capsicum, chilli, eggplant, beans and lettuce.
- Native budworm, Helicoverpa punctigera. An Australian species that breeds on inland wildflowers after winter rain and flies coastward on spring winds. It prefers broadleaved plants such as beans, peas and sunflowers, though it is recorded on cereal heads, so corn earworm is the usual Helicoverpa in a corn cob. Fall armyworm, established across the north and east, also attacks cobs; look for a pale inverted Y on its head and dark raised spots near the tail.
The two look alike as eggs and young caterpillars, so treat them as one pest. Both were once called Heliothis.
How to recognise it
- Eggs: single pearly white ribbed domes, about half the size of a pinhead, on growing tips, new leaves, buds, young fruit and corn silks. They darken before hatching, about three days after laying in warm weather.
- Caterpillars: pale with a dark head when newly hatched, growing to about 40 mm, green, yellow, pink, brown or nearly black, striped along the body, with sparse stiff hairs.
- Damage: a neat entry hole in a green tomato or capsicum, often with droppings and rot inside; chewed buds; holed bean pods; a caterpillar at the tip of a cob.
Armyworms are smoother and plumper; see our armyworm guide for the lookalikes.
Why October matters
A fully grown caterpillar pupates up to 10 cm down in the soil. From southern Queensland south, corn earworm spends winter as a resting pupa and the moths emerge from about September to October, while native budworm moths ride in from inland on the warm winds ahead of spring cold fronts.
In summer the cycle takes four to seven weeks and a female lays around a thousand eggs, so numbers usually peak from midsummer into autumn. Caterpillars stop feeding below about 12°C.
Check plants twice a week
Business Queensland advises checking twice weekly from early flowering. Start at the first flower buds, and on corn a few weeks before the tassels appear, then focus on the silks. Look over the top third of each plant, where most eggs are laid. Shake bushy plants over a white tray, and cut open holed fruit: the caterpillar is often still inside.
Young caterpillars often graze leaves or silks for a few days, then move into buds, pods, cobs and fruit, mostly by about 8 mm long, where sprays struggle to reach.
Control, in the order that works
- Squash and pick. Rub out eggs between finger and thumb and drop caterpillars into soapy water.
- Keep the moths off. Cover tomatoes, capsicums and beans with fine insect exclusion mesh on hoops before the first flowers open, and weigh the edges down. The moths fly mainly at night, so if a crop needs visiting insects to set fruit, lift the mesh on flowering mornings and close it by dusk.
- Close the cob tip. Corn is wind pollinated, so skip the mesh. Once the silks have browned, some gardeners close the husk tip with a rubber band or clothes peg to make it harder for hatchlings to crawl down to the kernels.
- Spray Bacillus thuringiensis on young caterpillars. Bt is a naturally occurring bacterium, sold as caterpillar sprays registered for home gardens. A caterpillar that eats it stops feeding within hours and dies within days. It works best on small caterpillars, so spray as eggs hatch, covering tips, buds, young fruit and corn silks as the label allows. Sunlight breaks it down and rain washes it off, so spray late in the day and repeat as the label directs.
Why the strong sprays backfire
Australian field populations of corn earworm resist synthetic pyrethroids and carbamates, and several other groups to a lesser degree. A broad-spectrum spray that misses the caterpillar inside the tomato still kills the predators that were eating its relatives.
Predatory bugs, native earwigs, lacewings and spiders eat the eggs and caterpillars, while tachinid flies and parasitoid wasps, including the tiny Trichogramma that develop inside the eggs, parasitise them. Business Queensland notes that parasites seldom give full control alone.
Clean up, dig over and plant early
- Clear finished crops promptly, because caterpillars carry on from one planting to the next.
- Fork the bed over to about 10 cm after harvest and, where corn earworm overwinters, again before the end of August, to break open pupal chambers.
- Pull host weeds such as sowthistle, fat hen and marshmallow.
- Plant early in your window. A crop that flowers in spring tends to miss the midsummer corn earworm peak, though not native budworm.
When to expect it, zone by zone
The zones are broad groupings. Between two, lean on local cues such as your last frost.
- Tropical (Darwin in the Northern Territory, Cairns and Townsville in Queensland): from central Queensland north, corn earworm stays active all year. Watch dry-season tomatoes, beans and capsicums.
- Subtropical (Brisbane and south-east Queensland, northern New South Wales): moths emerge in September and October, and numbers peak from midsummer into autumn.
- Arid and semi-arid (Alice Springs in the Northern Territory, Broken Hill in New South Wales, Mildura in Victoria, Kalgoorlie in Western Australia): native budworm breeds inland after good winter rain, so a wet winter can mean early moths on spring crops.
- Temperate (Sydney and the New South Wales coast, Melbourne in Victoria): the warmer New South Wales coast sees moths from spring; around Melbourne, corn earworm is mainly a summer pest.
- Mediterranean (Perth and southern Western Australia, Adelaide in South Australia): native budworm reaches Western Australian farming districts in late winter and spring; in South Australia corn earworm is mostly a summer problem.
- Cool and alpine (Tasmania, Canberra in the ACT, Ballarat in Victoria, Orange and Armidale in New South Wales, Stanthorpe in Queensland): cool nights hold the caterpillars back, and in Tasmania corn earworm is generally a summer pest.
Using any spray safely
Choose a product registered for home-garden use that names your crop and this pest on the label, and follow the label for rate, repeat interval, protective clothing and any withholding period. Store it away from children and pets. Commercial Bt is non-toxic to bees, people and pets, Business Queensland says, but it can also kill butterfly and other moth caterpillars feeding on sprayed plants, so spray the crop, not the whole garden. Business Queensland's page on corn earworm and native budworm links to the national register kept by the Australian Pesticides and Veterinary Medicines Authority.
Frequently asked questions
Is the hole in my tomato fruit fly or tomato grub?
A tomato grub leaves one neat entry hole, usually with a caterpillar and droppings inside. Fruit fly leaves tiny pinprick stings, soft patches and small white maggots.
Can I still eat a damaged tomato or cob?
Cut the chewed tip off a corn cob and cook the rest if the kernels are sound. Trim a damaged tomato if the flesh around the hole is firm and clean. Discard any cob or tomato with mould or rot.
Why is there usually one caterpillar per cob?
Corn earworm caterpillars will eat each other, so one tends to end up in each cob tip.
Will a pheromone trap stop them?
No. It lures male moths so farmers can see when moths are flying. It does not protect the plants.
A look twice a week gives you a good chance of catching the grubs before they reach the fruit.
Related growing guides
Browse our tomato seeds and corn seeds. We cannot send corn seed to Western Australia, Tasmania or the Northern Territory, and some tomato varieties cannot be sent to Western Australia, a few of them not to Tasmania either; each product page says so.