Garden Journal columnist Ron Kujawski began gardening at an early age on his family's onion farm in upstate New York. Although now retired, he spent most of his career teaching at the UMass Extension Service.


How do you know if your tomatoes are under attack by a hornworm? Here are a few clues, pictured from left: No. 1. There are dark pellets on the ground beneath a tomato plant. No. 2. Defoliated branches on a tomato plant. No. 3. A large green caterpillar gnawing on tomato fruit.  

Let’s play Clue….no, not the board game. This is Garden Clue.

Clue No. 1: Dark green to black-colored pellets on the ground and on some leaves of tomato.

Clue No. 2: Tips of tomato stems are defoliated.

Clue No. 3: Green tomatoes are deeply gnawed.

If those clues haven’t yielded a guess, here’s one more clue:

Clue No. 4: A large green caterpillar, about 4 inches in length with white stripes along its sides and a spike or horn at the tail end, is occupying the damaged tomato plants.

Most veteran gardeners, who are growing tomatoes, will know that the answer in this game of Garden Clue is: hornworm. However, many may not know that there are two species of hornworms which may be found dining on their tomatoes. One is the tomato hornworm, better known to entomologists as Manduca quinquemaculata. The other is the tobacco hornworm or Manduca sexta.

I doubt that there are any vegetable gardeners in the Berkshires who are growing tobacco. Yet, the hornworms I found on a couple of my tomato plants this past weekend were tobacco hornworms. This hornworm can be distinguished from the tomato hornworm by its reddish horn as opposed to the dark blue or black horn of the tomato hornworm. Also, the tobacco hornworm has seven diagonal stripes along its sides while the tomato hornworm has eight V-shaped markings. The adults of these caterpillars are large moths with wing span as much as 5 inches. They flap their wings at a very rapid rate and are often mistaken for hummingbirds, which is why they are commonly called hummingbird moths.

“What is a tobacco hornworm doing on tomatoes?,” you may ask. Well, it is not surprising since tomato and tobacco are both in the nightshade family of plants, along with pepper, eggplant and potato. However, it’s not just vegetables that are in the nightshade family. Petunia, million bells, and, of course, flowering tobacco (Nicotiana spp.) are also in the same family.

Early in their caterpillar stage, hornworms may be controlled by applying the biological pesticide Bt (Bacillus thuringiensis kurstaki). However, this would have less effect once the hornworms reach more than an inch or two in size. At that point the best thing to do is to hand-pick the culprits. I suppose those as long as 4 inches could become family pets … uh, forget that.

There is one other control and that involves a naturally occurring parasitic wasp. This wasp lays its eggs inside the caterpillars. The resulting larvae of the wasp feed within the body of a hornworm and eventually emerge to form white cylindrical cocoons on the surface of the caterpillar. It is best to leave these parasitized hornworms alone and let Mother Nature assume control of hornworms at that point.

And so ends this round of Garden Clue.

Here are some clues for gardening tasks this that you do not want to leave alone:

  • Lift a pear to a horizontal position and give it a gentle twist. If it readily comes free, it is ripe. It’s best not to leave pears on the tree till they soften or lose all their green color. Left too long on the tree, the inside of pears become mushy, a technical term for overripe.
  • Apply water to the soil and not over the top of plants, whether they are growing in containers or in gardens. Give the heat and humidity of late, foliar diseases are quite common, especially if plant leaves remain wet for any length of time. Applying some mulch around plants even this late in the season will help reduce watering needs.
  • Store onions in a cool, dry, and well-ventilated place. However, do not store them near potatoes since onions give off ethylene which causes the potatoes to sprout.
  • Select a grass seed mixture containing a good proportion of perennial ryegrass if planning to renovate a lawn area subject to much activity. Many ryegrass varieties are not only attractive in lawns, but are easy and quick to establish and many have excellent wear tolerance when compared to bluegrass and fescue varieties.
  • Move houseplants kept outdoors this summer to a very shady location before moving them inside at the end of the month. This will acclimate them to reduced light indoors.
  • Prepare your supply of winter potting soil now. Mix together some garden loam with coarse sand and peat moss or compost. The exact proportions will vary depending upon the texture of your garden soil, but you can start by mixing equal parts of each component. The finished product should feel light and airy.
  • Make it an annual practice to root cuttings from coleus now growing outdoors in containers or in flower beds. They make attractive houseplants for winter if given a sunny window. Besides, these plants always look better when they are young. Come to think of it, so did I.
  • Add native and pollinator friendly plants to your home landscape. A good source for such plants is the Berkshire Conservation District’s online native plant sale: Income from the sale supports the conservation programs of the district. September will be an ideal time for planting native perennials, shrubs, and trees.