Tomato Fruitworm

Tomato fruitworm adults are medium-sized moths with a wingspan of about 1 to 1.3 inch (25–35 mm). They are pale tan to medium brown colored or sometimes have a slight greenish tinge. The front wings are variously marked and usually have an obscure dark spot in the center and a lighter band inside a dark band around the tip. The hind wings are drab white and have a dark gray band around their tip. A diffuse light spot is in the center of the dark band.

At hatching, tomato fruitworm larvae are creamy white caterpillars with a black head and conspicuous black tubercles and hairs. Larger larvae vary in color from yellowish green to nearly black and develop fine white lines along the body but retain the black spots at the base of bristlelike hairs. Older larvae also have patches of stubby spines on their body segments that are much shorter than the bristles and can be seen best with the use of a hand lens.

The tiny, spherical eggs are slightly flattened on top with coarse striations or ribs running from base to tip. They are easy to confuse with looper eggs, but looper eggs have finer striations. Fruitworm eggs are laid singly on both upper and lower surfaces of the leaves usually in the upper part of the plant. When first laid, eggs are creamy white, but develop a reddish brown ring after 24 hours and darken just before larvae hatch.

Tomato fruitworm is also called cotton bollworm and corn earworm.

When there is fruit present, the tomato fruitworm will complete its larval development inside fruit. Early stage larvae enter the stem end of fruit when it is between 0.75 to 2 inches in diameter. During development, caterpillars may emerge from one fruit and enter another. Their feeding results in a messy, watery, internal cavity filled with cast skins and feces. Damaged fruit will ripen prematurely. Late in the season, small larvae will also enter ripe fruit. Small larvae are difficult to detect and, thus, may be a problem in processing tomatoes for the canner. In fresh market tomatoes, any feeding results in unmarketable fruit that will need to be culled at harvest or in the packing shed.

Management of tomato fruitworm requires careful monitoring for eggs and small larvae. When control is needed, it is essential to treat before large numbers of larvae enter fruit, where they are protected from sprays. Trichogramma parasites and other natural enemies often destroy significant numbers of eggs, so it is important to check for parasitism and predation before making treatment decisions. Early-season processing tomatoes rarely need treatment. Late-season fields may be more seriously affected.

Naturally occurring beneficial insects are very important in the biological control of tomato fruitworm, especially in the Delta area and the Sacramento Valley. These include Trichogramma spp. egg parasites, the larval parasite Hyposoter exiguae, and predators such as bigeyed bug and minute pirate bug. Conserve these parasites and predators whenever possible and monitor their presence, as described in MONITORING AND TREATMENT DECISIONS.

A tomato fruitworm egg parasite, Trichogramma pretiosum, is available from many commercial insectaries. Inundative release of 100,000 parasites/acre during the period of fruitworm oviposition and when fruit are susceptible to fruitworm feeding can help prevent unacceptable levels. Monitor the success of Trichogramma pretiosum releases using the egg sampling technique (indicated by black, parasitized eggs) and use the table below to determine if pesticide treatments are needed. Be sure to monitor the releases to make certain that parasitism is occurring.

Biological control and sprays of Bacillus thuringiensis and the Entrust formulation of spinosad are acceptable for use on organically certified produce.

Damaging populations of tomato fruitworm rarely occur before August. Monitor adult activity in July using a Heliothis trap baited with a pheromone lure to determine when to sample for eggs, which are laid during the flight periods. When moths are being caught in the traps, begin sampling leaves for eggs.

Degree-days

If eggs are detected in samples taken during July, start accumulating degree-days using a lower threshold of 55°F and an upper threshold of 92°F to predict egg laying of the generation in August that attacks the fruit. It takes an average of 968 degree-days for tomato fruitworm to complete a generation.

Calculate degree-days for tomato fruitworm for your location using the tomato fruitworm pest model. To learn more about using degree-days to time insecticide applications, watch the degree-days video.

Fresh Market Tomatoes

Traps may be helpful to determine when a flight has begun. Conduct a 5-minute search of leaves for eggs. If eggs are found, a treatment may be warranted. Later in the season, sample both leaves and fruit when monitoring for caterpillars (tomato fruitworm, beet armyworm, etc.).

Processing Tomatoes

Start sampling for eggs when a significant number of green fruit are one inch in diameter. Sample for eggs by picking the leaf below the highest open flower on 30 plants selected at random throughout the field. If three or more healthy, white eggs are found in the 30-leaf sample, sample 30 more leaves (stop sampling if less than 3 eggs are found). If five or more eggs are found in the second 30-leaf sample, apply a treatment to coincide with hatching. Aim insecticide treatments at newly hatched larvae. Once larvae are in fruit they have already caused damage and are difficult to kill. When sampling for fruitworm, also look for fruitworm damage. Six to eight weeks before harvest, also monitor potato aphids in your sample and record results on a monitoring form (PDF).

Assess egg parasitism for processing tomatoes in the Sacramento Valley

In Sacramento Valley processing tomatoes, several species of parasitic wasps (Trichogramma spp.) can be found parasitizing tomato fruitworm eggs in late August and September at sufficient densities to control the pest. Most, but not all parasitized eggs will eventually turn black. Because there is a lag period, some white eggs in field samples may actually be parasitized but not recognizable as such; consequently a threshold may falsely appear to be exceeded. The following table provides adjusted treatment thresholds, using the number of black and white eggs present in samples of 30 tomato leaves, to compensate for not being able to distinguish eggs in the early stages of parasitism.

The letter "T" indicates the ratio at which treatment is recommended. If no black eggs are recorded, collect and observe white eggs for 48 hours and subtract those that turn black due to parasitism.

• Degree-day calculator
• Degree-day table
• Photos of natural enemies of fruitworms
• Tomato fruitworm monitoring form (PDF)
• Using degree-days to time insecticide applications