Q. I am finding a large worm varying from 2 – 5 inches in length on my grape vines. It is roughly 1/2 inch in diameter. The body color varies, green, pink, grey, and brown worms have been removed and destroyed. Each worm has white diagonal stripes over the length of the segmented body. What is it, and how do I avoid it in the future?
A. You will notice a stiff, semi-curved spine sticking up from its rump as well. This is the hornworm, this spine contributing to its name. The hornworm is commonly found on tomato so it is usually called the tomato hornworm but also damages peppers and eggplants as well.
|Not my picture and not from southern Nevada bud demonstrates the "spine"|
This is a larva or immature form of the hummingbird or sphinx moth which is a very beautiful large moth. The moth began laying eggs sometime in around March and April so the hornworms have been damaging garden plants since around April or May.
The hornworm is a voracious eater and can defloiate a plant making it leafless in a very short time. Because of their green color they are hard to see on garden plants even though they are large. On a windless day you can watch your garden for a few minutes and see the plant or leaves move because of their size and weight. This way you can identify their locations.
You can also look for defoliation, entire leaves missing from plants, and that is also a very good sign they are present. Their excrement is large so seeing that is another way to identify their presence. If you have a blacklight that you used for "seeing" bark scorpions at night you can use this at night as well. Hornworms "fluoresce" or shine a different color at night from plants and can usually be easily found this way.
Control is fairly easy by handpicking when you see them and hunt them down or use the same biological spray that you use for grape leaf skeletonizer or tomato fruitworm; Bt or Spinosad.
Both Bt and Spinosad can be used by organic gardeners safely. Spinosad is a little rougher on bees than Bt so it is always best to spray any pesticide at dusk or early dawn when bees have not started to forage. Always follow label directions for use.