Tomato fruitworm, Helicoverpa zea
Order: Lepidoptera
Family: Noctuidae
Description
Adults are medium-sized moths with a 2.5-3.3 cm (1-1.3 inch) wingspan. Sometimes they can have a slight green hue but are typically pale tan or medium brown with irregular markings on the forewings that usually include a dark spot in the center. The tips of each forewing are lined with a lightly colored inner band and a darker colored outer band. Females lay spherical eggs that are ribbed with coarse striations. The eggs are initially creamy white but turn red to brown 24 hours before the larvae hatch. The eggs are very small, about 0.5 mm (0.02 inches).
Young larvae are creamy white with black tubercles and a black head capsule. Later instars are about 25.5 mm (1 inch) long and are green, brown, pink, yellow, or a combination of these colors. They also have alternating dark and light stripes running lengthwise along the top of the thorax and abdomen, and brown or black heads with rows of dark bristles or bumps along the body that vary in length, depending on the instar. Pupae are brown or dark red.
Quick Facts
- The tomato fruitworm, also referred to as the corn earworm or bollworm, is a major pest of corn in Colorado. Other susceptible vegetables include asparagus, cabbage, cantaloupe, collards, cowpea, cucumber, eggplant, lettuce, lima bean, melon, okra, pea, pepper, potato, pumpkin, snap bean, spinach, squash, sweet potato, tomato, and watermelon.
- Tomato fruitworm is the common name assigned to this species when occurring in tomato, while corn earworm and bollworm are the common names when occurring in corn and cotton, respectively.
- Adults are moths that can travel up to 300 miles in one night. In northern Colorado, tomato fruitworm populations do not survive the winter and are reestablished annually when moths migrate north from suitable overwintering sites.
- Monitoring for tomato fruitworm moths can be done with pheromone traps and is recommended to help growers determine when control measures are necessary.
Caterpillar of tomato fruitworm boring into an overripe tomato. Image credit: Whitney Cranshaw, Colorado State University, Bugwood.org
A single egg of tomato fruitworm. Image credit: Whitney Cranshaw, Colorado State University, Bugwood.org
Mature larva of tomato fruitworm on tomato foliage. Image credit: Alton N. Sparks, Jr., University of Georgia, Bugwood.org
Pupa of tomato fruitworm. Image credit: Whitney Cranshaw, Colorado State University, Bugwood.org
Adult tomato fruitworm resting on a tomato plant. Note the centrally located dark spot and two bands at the tip of each forewing. The dark spot may be more pronounced on some individuals. Image credit: Whitney Cranshaw, Colorado State University, Bugwood.org
Life history and habits
This species has a very broad host range that includes tomato, corn, and cotton. Tomato fruitworm is the common name when occurring in tomato, while corn earworm and bollworm are the common names for this pest in corn and cotton, respectively. Tomato fruitworm overwinters in southern Colorado as a pupa in soil, though the weather in some areas may reduce survival of overwintering pupae. The temperatures in northern Colorado are not suitable for winter survival and are repopulated each growing season after moths emerge and migrate north from overwintering sites. The moths are nocturnal and can often be found resting beneath foliage during the day.
Flowering tomato plants are highly attractive egg laying sites. The moths lay eggs singly on the leaves, flowers, fruits, and stems of tomato plants in the summer, and each female moth can lay as many as 25 eggs in one day. Moths tend to lay eggs on the three terminal leaflets of peripheral leaves in the upper region of the canopy, but eggs can also be deposited on the upper surface of leaves during cloudy and cool weather. Caterpillars enter fruits that are between 1.9-5 cm (0.75-2 inches) in diameter by boring into the stem side. An individual caterpillar can emerge from one tomato and enter another and will complete its development inside of the fruits. There are two generations each year, possibly more depending on temperature.
Injury
Caterpillars tunnel into the fruit of tomatoes, peppers, and eggplant which reduces their quality and makes them less marketable. In tomatoes, caterpillar feeding leads to premature ripening and a watery internal cavity that contains frass and cast skins. In addition, entry wounds can lead to secondary infections of bacteria or fungi. A single larva can feed on multiple tomatoes. Young caterpillars will enter ripe fruit late in the season and, due to their small size, may be problematic in tomatoes designated for canning. Feeding injuries can lead to unmarketable tomatoes designated for fresh market produce. When fruits are not available, caterpillars will instead feed on leaves, stems, and flowers, producing holes in the plant structures which can lead to distorted leaves. This can decrease product yield and quality.
Monitoring
Pheromone traps can be placed at the margins of fields for detecting adult activity. Plants can be checked for tomato fruitworm caterpillars and signs of feeding injury in late June or early July before populations reach peak densities. Tomato is very attractive to tomato fruitworm and is a good plant for initial scouting in fields and gardens. To detect eggs, the upper and lower surfaces of the first three leaflets in the upper and middle-upper plant regions can be inspected. It is recommended that the two leaves located below clusters of open or opening flowers be examined. In smaller production systems such as home gardens, a 5-minute search may be sufficient in which four or five plants are randomly inspected in different locations. In large scale production systems, it is recommended that two leaves be checked on 20-80 plants. To prevent significant yield loss, control is necessary before larvae enter fruit in large numbers. Many eggs can be parasitized by a wasp, Trichogramma spp. and other natural enemies, which should be taken into consideration when making treatment decisions. The presence of black eggs is indicative of parasitoid activity.
Cultural control
Tomato, pepper, and eggplant fields should not be planted near corn fields. Adults of tomato fruitworm will seek out alternative hosts for egg laying as corn silks turn brown. Plant debris, including weeds, can be plowed to reduce the availability of overwintering sites and remove infested fruits and pupae. Removing larvae by hand is a time consuming but effective management strategy.
Recent research has shown that certain corn varieties like Sugar Jean and Java can be used as effective trap crops. To be effective, the corn silk emergence should be synchronized with tomato flowering, such that fresh corn silks are available for ovipositing moths while tomato plants are flowering. Including a corn border has been shown to reduce the rate of infestation in tomato fields.
Biological control
Insect predators of tomato fruitworm larvae include lacewings, minute pirate bugs, and damsel bugs. Wasps in the genus Trichogramma are naturally occurring parasitoids of the eggs of tomato fruitworm. These wasps can be purchased and released in fields, a management tactic that requires specific timing and maintenance of wasp populations. This has shown limited success in neighboring states like Utah. During periods of tomato fruitworm oviposition in large scale production, inundative release of 100,000 parasitoids per acre can prevent significant yield loss.
Chemical control
In conventional and organic production, applications of Bacillus thuringiensis (Bt) or certain formulations of spinosads can be effective. Applications of Bt will be most effective after eggs hatch but before larvae enter fruit where they are protected from chemical controls. For vegetables other than corn, treating with insecticides is generally not recommended.
References
Alston, D., S. Olsen, J. Barnhill. 2011. Corn Earworm. Utah State University – Extension. Available https://extension.usu.edu/pests/research/corn-earworm
Figueiredo et al. 2020. Risk Assessment for Tomato Fruitworm in Processing Tomato Crop-Egg Location and Sequential Sampling. Insects. 12(13): 1-18. Available https://mdpi-res.com/insects/insects-12-00013/article_deploy/insects-12-00013-v3.pdf?version=1609834933
Kuhar, T., C. Philips, H. Doughty, A. Alford, E. Day. 2019. Corn Earworm on Vegetables. Virginia Cooperative Extension. Available https://www.pubs.ext.vt.edu/content/dam/pubs_ext_vt_edu/3103/3103-1537/ENTO-312.pdf
Rhino, B., I. Grechi, G. Marliac, M. Trebeau, C. Thibaut, and A. Ratnadass. 2014. Corn as trap crop to control Helicoverpa zea in tomato fields: importance of phenological synchronization and choice of cultivar. International Journal of Pest Management. 60(1): 73-81. Available https://web.p.ebscohost.com/ehost/pdfviewer/pdfviewer?vid=0&sid=397dcc84-fe80-4360-95ff-1aadda71507a%40redis
UC IPM. 2013. Tomato Fruitworm. University of California Department of Agriculture and Natural Resources. Available https://ipm.ucanr.edu/agriculture/tomato/tomato-fruitworm/#DEGDAY
University of California. (n.d.). Corn earworm (Tomato fruitworm) – Helicoverpa zea. University of California – Statewide Integrated Pest Management Program. Available https://ipm.ucanr.edu/PMG/GARDEN/VEGES/PESTS/cornearworm.html#:~:text=Destroy%20culls%20and%20plants%20immediately,and%20before%20caterpillars%20enter%20fruit.