4 min read
A hornworm on a plant.

Scouting and monitoring your garden for pests at least two to three times per week can greatly reduce the damage that is acquired from insects. It is also helpful to use a 10X hand lens to help identify small insects and insect eggs. Keeping records will help identify their peak activity each season and can help you predict when peak activity might occur in future years. If you have a field of tomatoes, sample the field in a pattern that covers the entire field in a zigzag pattern. It is best to check plants along the field edges separately. Then walk into the field to begin the zigzag pattern.

The most common caterpillar (aka, worm) pests of tomatoes include fruitworms, armyworms and hornworms. Other insect pests include aphids, whiteflies, leaf-footed bugs and stink bugs. Tomato fruitworms, armyworms and hornworms may be controlled by sprays of Bacillus thuringiensis (Dipel, Thuricide, along with others). This naturally occuring bacteria only kills caterpillars. Always read the labels to know what insects a pesticide will control, and if it is safe for vegetables. The most effective control time is just after eggs hatch and caterpillars are still small. Insecticidal soaps are effective for control of aphids and whiteflies if sprays are directed to the upper and lower leaf surfaces. Insecticidal soaps only kill stinkbugs when newly hatched.

From my own personal experience, caterpillars (aka, worms) on your newly set tomatoes can be very depressing. Just as we’re ready to eat these on a sandwich…so are the worms. Below are the worms most commonly found in your garden on your tomatoes. Refer to these pictures when you scout and use the appropriate control measures to ensure that you get to reap your own tomato harvest.

Tomato Fruitworms/Bollworms

Tomato Fruitworms feed on more than 100 plants including the tomato, but prefers corn. Newly hatched larvae usually enter tomato fruit at the stem end when the fruit is small and green. They bore deeply into the fruit and during development they may emerge from one fruit and enter another. Their feeding results in a watery, internal cavity filled with feces; damaged fruit will ripen prematurely and be susceptible to secondary invasion by fungi and bacteria.

Beet Armyworm

The beet armyworm is primarily a foliage feeder, but they will also attack fruit, usually creating single or closely grouped round or irregularly shaped holes. Feeding damage is usually superficial, and larvae only occasionally develop inside the fruit. Unfortunately, other organisms can then enter the feeding-damaged areas and can rot the fruit.

Yellowstriped Armyworms

Yellowstriped armyworms feed on a variety of crops including tomatoes, alfalfa, bean, beet, cabbage, clover, corn, cotton, cucumber, grape, grass, morning glory, onion, pea, peach, peanut, sweet potato, tobacco, turnip, wheat, and watermelon. The insect usually overwinters as a pupa in the soil. Egg masses are deposited on foliage, trees, or buildings. There are three to five generations per year. Yellowstriped armyworms feed during the day.

Depending on the year, environmental conditions, and proximity of adjacent host crops, infestations of these insects on tomatoes may range from very light to extremely severe. The tomato fruitworm (aka, corn earworm and bollworm), and the beet armyworm both overwinter in Alabama; so early season infestations of these species may occur. However, later season tomato plantings are often damaged more severely because fruitworm and armyworm populations generally increase as the season progresses. It is common that the most severe fruitworm damage in tomatoes occurs after dry-down or harvest of adjacent corn, and heavy beet armyworm damage in tomatoes occurs after population buildup on cotton. An effective strategy for managing the fruitworm and armyworm is to monitor fields regularly for signs of insects or damage and to apply an insecticide only when necessary.

Field trials in Alabama tomatoes demonstrated that using insecticides in conjunction with a monitoring program saved an average of $44 to $65 per acre in insecticide costs alone; when compared with a program applying insecticides on a weekly basis. In addition to applying insecticides only when needed, the fruitworm/armyworm scouting program enables growers to apply sprays at the optimal time, when the worms are young and most susceptible to insecticides.

Sampling for Tomato Fruitworms and Armyworms

Because beet armyworm is a foliage feeder, infestations may begin early–before the flower and fruit stage. Therefore, it is prudent to check young plants regularly for beet armyworm egg masses or small larvae. The presence of beet armyworm larvae can also be detected by shaking foliage over a shake cloth. The critical period for tomato fruitworm and fruitworm egg sampling begins at flower or, at the latest, when there are a significant number of green fruit at least 1 inch in diameter. Fruitworms are usually not a concern before flowering unless high numbers are present. Beet armyworm egg masses are covered by fuzzy white scales and are easy to spot. It is a good idea to check or shake some of the lower foliage for beet armyworm egg masses and larvae.

Fruit sampling serves as an extra precaution and a backup to the foliage sampling for worms and worm eggs. When scouting for insects and insect damage, check out the fruit. Tomatoes at least 1 inch in diameter are good candidates of insect attractants and damage. Check to see if any fruit have worm-feeding damage. Slice open damaged fruit to determine if damage is due to fruitworm (feeding deep inside fruit, feces often present) or armyworm (feeding usually confined to the surface). It is important to know which worm species is present to select the most effective insecticide needed.

Tomato Hornworm

Hornworm larvae usually have green bodies with seven diagonal white stripes on the side, or eight V-shaped markings, depending on the species. The name comes from the large horn on the posterior end of the body. Hornworms can grow longer than 4-1/2 inches in length. They strip the leaves and may cause feeding scars on fruit. Hornworms are often controlled naturally by parasites. Parasitized hornworms may be identified by the presence of many white cocoons attached to the upper body surface. The preferred management approach is to wait until fruit begins to mature before applying insecticides. Insecticides may be applied sooner if extensive foliar feeding is observed. Insecticides are not highly effective against late stage larvae.

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