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Tomato plants enter tough season

Posted: July 14, 2012 - 11:04pm

The No. 1 vegetable grown in the United States is the tomato, but gardeners have many problems with them.

Hot weather can cause tomatoes to stop setting fruit. When the daytime temperature is higher than 95 degrees and the nighttime temperature is 85 or higher, it kills the pollen. We have had these conditions several weeks now.

Another problem for tomato growers is insects, including caterpillars.

The the tomato hornworm (Manduca quinquemaculata) and tobacco hornworms t feed primarily on solanaceous plants (those in the potato family). They include tobacco, tomato, eggplant, pepper and some weedy plants. Tobacco and tomato plants are preferred.

Hornworms have a hornlike structure projecting from their tail that gives them their name. On the tobacco hornworm, the projection is usually curved and red. Tomato hornworm larvae have eight V-shaped markings on each side; the horn is straight and black. Both species are about 3 to 3½ inches long.

Usually, gardeners don’t know they have a problem until the damage from the caterpillars shows up. Hornworms strip the leaves from tomato vines. If a heavy infestation develops, these caterpillars also feed on developing fruit. They feed on the surface of the fruit, leaving large, open scars. Fruit damage, however, is much less common than loss of leaves. Hornworm damage usually begins to occur in midsummer and continues throughout the remainder of the growing season.

Hornworms overwinter in the soil as pupae. Moths of this overwintering generation begin to emerge in early June and can continue to emerge as late as August. Hornworm moths frequently can be seen hovering over plants at dusk. At night, they lay eggs on the underside of leaves.

Hornworms emerge in about four days, depending on temperature. After feeding for three weeks, hornworms burrow into the soil to pupate. In summer, the pupal period lasts three weeks, after which a new generation of moths emerges.

There are some natural control options for hornworms. The first is a wasp. Brachonid wasps deposit eggs into the hornworms; the larvae feed inside, and then pupate on the backs of the hornworms. These pupal cases are seen as white projections on the back of the hornworm. Infected hornworms don’t feed, so leave it on the plant for the next generation of beneficial wasps to hatch.

There is an organic control option called Bt that will work. Also, planting basil around the tomatoes will help repel the moth of hornworms. The last option is to hand-pick the caterpillars.

The second caterpillar to cause problems is the tomato fruitworm (Helicoverpa zea). This caterpillar feeds on tomato, corn and cotton and is also called the corn earworm or the cotton bollworm. It also attacks soybeans, peppers, tobacco, beans, okra and eggplant.

The adult tomato fruitworm is a moth. It lays eggs singly, usually on the lower sides of leaves close to the flower or fruits. The eggs are creamy white when laid but develop a reddish-brown band just prior to hatching. The caterpillars are yellowish-white with a brown head. The color of older caterpillars varies from greenish-yellow to brown or even black with paler stripes running lengthwise on the body. The caterpillars will grow to about 1½ inches long.

Fruitworms feed on tomato leaves and fruit, but they may also bore into stalks or midribs. When fruit is present, larvae enter it soon after hatching. They prefer green fruit and will enter it usually at the stem end, causing the fruit to rot. The larvae are cannibalistic, so there is rarely more than one larva per fruit. The caterpillars usually complete their development in a single fruit, but when fruits are small they may feed in several.

The best control for tomato fruitworms is to use a product that contains Bt.

Charles Phillips is a retired Columbia County Extension Service agent and operates Hort Consulting. Reach him at cphillipshort@comcast.net or at (706) 836-2152.

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