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Ask the Master Gardener: Rid garden of hornworms through tilling or natural predators

By: Trish Grenfell Placer County Master Gardener
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Question: Upon returning from my vacation, I found one of my tomato plants decimated ? no leaves, just the fruit and the leaf skeletons remaining. My neighbor advised that he had picked off several hornworms, but the damage was already done. What is a hornworm and how can I keep them off my other tomato plants? Answer: The quick and dirty answer is to do what your neighbor did: Watch for missing leaves or fruit with munching scars, locate the culprit(s) which can be difficult since they camouflage so well, and hand pick or snip off with shears. This means patrolling your plants on a regular basis. However, there are other strategies. The tomato hornworm (Manduca quinquemaculata) is a caterpillar, the larval stage of the sphinx moth. The moths overwinter in the soil as the pupae (cocoons), so tilling by hand or discing will destroy most of the active larvae building cocoons or the cocoons themselves. If the pupae (cocoons) aren?t destroyed, the moths will emerge in late spring and begin the cycle of laying their eggs, which hatch in four to five days as a hornworm. The eggs, which will be round and a greenish-white color, will be laid on the underside of leaves. If you see these next year, remove and dispose of the leaf. The plants provide a good food supply and in four weeks the caterpillars are full-size. At this stage of the cycle, the caterpillar makes its way into the soil to pupate (build cocoons) and the cycle continues. There are several important naturally occurring parasites that help control hornworms in tomatoes. Hornworm eggs are attacked by Trichogramma parasites and the larvae by Hyposoter exiguae. Trichogramma released for control of tomato fruitworm will also attack hornworm eggs. Should you find a hornworm with white egg sacs covering its back, leave it. The worm is dying. These are the egg sacs of the parasitic wasp Braconid and you want the eggs to hatch. These are natural predators that not only will defend your garden from this hornworm, but many other garden pests. Conserve natural enemies by not treating with disruptive pesticides. Have gardening questions? Call the Master Gardener hotline at (530) 889-7388.