If you see skeletonized leaves and large black droppings on your tomato plants and/or potato plants, look closely for the large green caterpillars most likely doing the damage. The tomato hornworm (Manduca quinquemaculata), and the tobacco hornworm (Manduca sexta) are both voracious eaters that can defoliate these plants in days and then, with their favorite green food gone, will also set about chomping holes in your tomato fruit.
The tomato hornworm has a black sided dark green "horn". The tobacco hornworm, pictured here, has a red “horn”. The white striping on their sides is also different. Tomato hornworms sport V's. The more dashing tobacco hornworm’s stripes are white diagonal slashes topped with little black dots.
It really doesn’t matter which hornworm is attacking your garden. Either kind is very destructive to the plants they eat.
Both of these caterpillars are quite large, three-and-a-half to four inches long in their last instar stage. Even at this size, they can be hard to spot since their bright green coloring is a good cloak on the stems and undersides of the leaves.
The best way to deal with them is to pick them off the plants. If you are as squeamish as I am, be sure to wear gloves. They have considerable holding power when you try to get them off a glove, so tug, and drop them into a handy nearby pail of soapy water.
Even if you think you have eradicated these big caterpillars from your garden, don’t let down your guard. There can be 2 generations a year, so watch those plants for any signs of munching. After the caterpillars mature, they drop to the ground, burrow in, and pupate. If the summers are long enough, another generation of moths will be born to lay eggs on the garden vegetables. If not, they will winter over in the garden soil.
The best way to cut down on next year’s problems is to dig up, roto-till, or plow the ground in the fall so that the over-wintering larvae are brought to the surface where they either will freeze or be eaten by predators. According to the University of Minnesota Extension Service, you can wipe out nearly 90% of the overwintering population just by digging and turning the soil in the fall.
Several “good” bugs out there kill these hornworm caterpillars. Wasps are the best natural predators. One is a small braconid wasp, Cotesia congregatus. The female braconid attaches eggs all along the caterpillar backs of the hornworms. The wasp larvae look like white spheres. Don’t kill or remove the caterpillars wearing white teardrops. The wasp larvae feed on the caterpillar, killing it, then emerge and look for more hornworm bodies to use as dinner. Allow these natural predators to live and carry out their hornworm eradication mission.
Even knowing the trouble they can cause in the garden, the adult hornworm moths are fun to watch in the garden. The tomato hornworm adult is the five-spotted hawkmoth. The tobacco hornworm adult is the Carolina sphinx moth. Both are called “hummingbird moths” because they resemble hummingbird shapes, albeit much smaller, and can be seen hovering and sipping nectar from flowers in the late afternoon garden.
Tomatoes are the hornworm caterpillars’ food of choice but they will also attack potatoes, eggplants, and peppers. Nearby weeds that are also food sources for hornworm caterpillars include nettles, nightshades, and jimsonweed.
The “horns” on these hornworms are just for show, and occur atop their backsides. The eating machine is on the other end.
It might take you a few minutes to spot the culprits but you will definitely see the missing leaves and large, by insect standards, black excrement on your plants quickly enough. Study the tops and bottoms of the leaves and stems. Sort of like hunting mushrooms on the woodsy floor. Once you see them, you’ll wonder how you could have missed them.
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