Trichogramma spp.
Trichogramma wasps- Rate
- 50k–100k /acre weekly (1–2 /ft² in gardens)
- Temp
- 18–29 °C best; weaker at 15 °C
- Best for
- Egg releases timed to the moth flight
Pest
European corn borer
Ostrinia nubilalis
The short answer
A European moth, first found in North America near Boston in 1917, that has spread west to the Rockies and south to the Gulf Coast. Its caterpillar feeds in the corn whorl and tassel, then bores into the stalk and ear. In peppers it tunnels in under the fruit cap. It overwinters as a full-grown larva inside old stalks. Widespread planting of Bt field corn has greatly reduced its numbers, but sweet corn, peppers and snap beans can still be damaged. Every control depends on timing. Egg parasitoids and sprays work only before the larvae get inside the plant.
hero macro · larva in a split corn stalk
Late-instar Ostrinia nubilalis in a tunnel inside a split sweet corn stalk. Photo: pending.
Identification
The larva is light brown or pinkish-grey on the back, with round dark spots on each segment, a brown to black head and a yellowish-brown plate behind it. It grows from about 1/16 inch at hatch to about 3/4 inch (20 mm), and UMass gives up to 1 inch when full grown. Unlike the corn earworm, which is striped and feeds into the ear from the silk end, the corn borer is spotted, bores into stalks and tassel stems, and can enter the ear through the side of the husk as well as the tip.
The moths are small, with a wingspan of 3/4 to 1¼ inches (20–34 mm). Females are pale yellow to light brown, and males darker, both with dark zigzag lines and pale yellowish patches across the wings. Eggs are laid in flat clusters on leaf undersides: about 15 to 20 per cluster according to UF/IFAS, 15 to 30 according to UKY. Each egg is about 1 mm long and creamy white, and the eggs overlap like fish scales, darkening to tan as they age.
Easily confused with
- Corn earwormAlso a pestStriped rather than spotted, rough to the touch from tiny spines, and it feeds down into the ear from the silk end rather than boring into the stalk.
- Pepper maggotAlso a pestIn peppers, the maggot is completely white. A European corn borer larva has a dark head capsule.
Damage symptoms
In corn, young larvae feed on the tassel, the whorl and the leaf sheath, then bore into the stalk and the ear. Broken tassels follow, and heavily tunnelled field corn lodges. One or two larvae in a stalk are tolerable, but commercial growers accept none in a sweet corn ear. In peppers, larvae enter the fruit under the cap, and small fruit drop early. Water gets into the holes and the fruit rot.
- Young larvae in the whorl and on the tassel, mining the leaf midribs
- Broken tassels and frass at small entry holes; heavily tunnelled field corn stalks lodge, and pepper stems can break
- Tunnels in the side of the ear or the cob
- Pepper fruit with a hole under the cap, rotting inside
- Small pepper fruit dropping early
Lifecycle & degree days
Full-grown larvae overwinter in a tunnel in old corn stalks or other thick stems, and pupate in spring. The number of generations runs from one in northern New England and Minnesota to two in southern and central New England, two to three in Kentucky, and three to four in Virginia and further south. UMass times each stage by growing degree-days from a 50 °F base: the first moth flight starts at about 374, the first eggs at 450, and the second flight at about 1,400. The second generation does most of the damage to sweet corn ears and pepper fruit.
- Overwinters
- Full-grown larva in old stalks
- Generations / yr
- 1 (northern New England, Minnesota) to 3–4 (Virginia and south)
- First flight
- 374 GDD, base 50 °F; first eggs 450; peak flight 631 (UMass)
- Second flight
- 1,400 GDD; second-generation hatch about 1,550 (UMass)
- Larval development
- About 50 days in the field, six instars (UF/IFAS)
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Host crops
It attacks almost any robust herbaceous plant with a stem thick enough to enter; UMass counts more than 200 hosts. Among vegetables, UF/IFAS singles out snap and lima beans, pepper and potato. Corn borers also turn up in dahlias, zinnias, hollyhocks and other thick-stemmed flowers. Hot peppers are the most resistant pepper cultivars, and most green bell peppers are susceptible.
Monitoring & action thresholds
Action threshold
More than 15% of sweet corn plants infested
UMass treats sweet corn when more than 15% of plants have one or more larvae when emerging tassels are scouted. In pepper, UMass starts sprays one week after trap catches reach 7 moths a week, with fruit present, and stops a week after catches fall below 21 a week. These are commercial figures; we found no garden threshold.
Commercial growers catch moths in pheromone or blacklight traps. UMass uses two Scentry Heliothis net traps at least 50 feet apart, checked once or twice a week. Traps tell you when to look, not how many larvae you have, so in corn follow a catch by searching for egg masses and scouting the emerging tassels for young larvae. In peppers, UKY finds searching leaves for egg masses impractical and times sprays from trap catches instead. In a garden, watch for frass and feeding in the whorl before tasselling, and for holes under the pepper caps from midsummer.
Beneficial
Biological control
Egg parasitoids and predators work on the exposed stages, before the larvae tunnel in. Trichogramma ostriniae is the best-tested release. In pepper trials, egg parasitism averaged 48.7% in release plots against 1.9% without releases, and fruit damage 8.7% against 27.3%; the suppliers listed on the site's Trichogramma page sell other species, so check which one you are buying. In Québec processing sweet corn, ear damage after Trichogramma releases was 1.0%, the same as with insecticides and well below the 8.7% in untreated plots. UF/IFAS notes that native Trichogramma releases gave variable, moderate results. Wild predators, among them minute pirate bugs (Orius insidiosus), lacewings and lady beetles, often take 10–20% of the eggs. Imported parasitoids include a tachinid fly, Lydella thompsoni, which can kill up to 30% of second-generation borers in some areas, and a braconid wasp, Macrocentrus grandii. Bt kurstaki works on larvae before they bore in.
Bacillus thuringiensis kurstaki
Bt (kurstaki)- Rate
- Label rate — ½–4 tsp per gallon on home concentrates; varies by product
- Best for
- Whorl and tassel, before larvae bore in
Orius insidiosus
Minute pirate bug- Rate
- 1–2 /m² per release; up to 10 /m² on hot spots
- Temp
- Best above 20 °C; 18–28 °C
- Best for
- Wild egg predator
Cotesia congregata and other Braconidae
Braconid wasps- Kind
- Wild — attract it
- Best for
- Wild larval parasitoid (Macrocentrus)
Tachinidae
Tachinid flies- Kind
- Wild — attract it
- Best for
- Wild larval parasitoid (Lydella)
Cultural controls
The borer overwinters in old stalks, so what happens to the stalks after harvest sets up next year.
- Mow old corn stalks close to the ground and plough or dig them under in autumn or early spring. UF/IFAS says low mowing removes over 75% of larvae and works best combined with ploughing to 8 in (20 cm); disking is not enough
- Pull out old pepper plants and dahlia or zinnia stems that may hold overwintering larvae
- Choose hot peppers, or accept that most green bell peppers are susceptible
Chemical, last resort
Broad-spectrum sprays such as pyrethroids kill natural enemies, including the Trichogramma released against the eggs, and can set off aphid outbreaks. Larvae already under the pepper cap or inside the stalk are protected from sprays.Check any product before it goes on. Open the compatibility matrix →
- Bt kurstakiREI 4 h
Rating: the effect on this pest's natural enemies if a spray hits them or they feed on treated plants. REI is the re-entry interval on farm and greenhouse labels; home labels usually give none, so follow the label.
Sources
UF/IFAS EDIS EENY156, European Corn Borer, Ostrinia nubilalis (Capinera, rev. 11/2023)
UMass Extension Vegetable Program, European Corn Borer (updated 20 July 2017)
Univ. of Kentucky ENTFACT-301, Common Insects Attacking Peppers (Bessin, rev. 1/26)
Kuhar et al., J. Econ. Entomol. 97:1209 (2004), T. ostriniae in solanaceous crops
Gagnon et al., J. Econ. Entomol. 110:59 (2017), T. ostriniae in processing sweet corn
DiPel DF label, EPA Reg. 73049-39 (2022)
UC IPM, Relative Toxicities of Insecticides Used in Tomatoes to Natural Enemies (09/16)
Page status
Drafted from the sources below, then checked claim by claim against those sources on 23 September 2026.
Suggest a correction →European corn borer by crop
The same pest behaves differently on different hosts. Each page below gives the host-specific control, rate and threshold.