Hornworm can defoliate plant in short time

Q: I am finding a large worm varying from 2 to 5 inches in length on my grapevines. It is about a half inch in diameter. The body color varies — green, pink, gray 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, semicurved 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.

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 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 defoliate 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.

Q: We have a peach tree that we harvest in June. When is the best time to prune?

A: Any major pruning should be done after leaf drop in the late fall or during the winter months before leafing out in the spring in February. Some minor pruning, removing some new growth to open up the canopy a bit for better light penetration, can occur during the summer months but this has to be limited to new growth only.

Q: I wrote to you a few weeks ago regarding splitting pomegranates. Our homeowners association switched from grass to desert landscaping. The drip systems were increased to include new plants and older bushes and a new system was added around our older trees.

Our water bill for that period was much higher than normal. Is it possible that overwatering in the heat caused the seeds to grow faster, splitting the fruits? I had a good crop last year.

A: The splitting of any fruits, whether pomegranate or tomato, is nearly always associated with irrigation or rain. The most commonly held belief is that it is because of irregular irrigation patterns: overly dry periods followed by an irrigation.

It is thought that when the plant has a lack of water the “skin” of the fruit begins to become inflexible and the plant matures the fruit earlier than normal in “hopes” that it can reproduce by hastening seed development and maturity.

Then when an abundance of water is present around the roots, the plant takes up this water and it is transported to the fruit. The fruit, now with an inflexible “skin,” can no longer expand like it could when it was immature and splits.

Another belief is that rainwater is absorbed through the “skin” of some soft fruit, like cherries or tomato, when it is nearly mature and this absorption of water causes the fruit to split.

Regardless of the reason, fruit splitting is reduced with more regular watering at the appropriate times to prevent the soil from becoming overly dry and the use of surface mulches, particularly wood mulch.

Q: We have had these trees in our backyard for 15 years. All of a sudden, one has a bare spot. What might this be? I enclosed a picture.

A: From your picture the plant looks like Japanese or Texas privet. Privets are notorious for looking pretty bad, leaf drop and twig dieback when they are not kept moist. They usually do nicely in lawns or when surrounded by other plants with a similar water requirement.

Typically they do not do well in rock mulch and if the soil becomes too dry. So I usually recommend that the rock mulch environment is enhanced with wood mulch or that other plants are included around the base of the plant that require more frequent watering.

Q: I have a peach tree loaded with peaches about the size of walnuts. The branches hang way down low, and the peaches have a red color to them and are hard as a rock. What should I do? Is it too early to trim branches? I think cold weather hurt the tree.

A: It is a bit hard to get a handle on the problem without knowing more about which peach tree variety it is and when it normally has its fruit ripen. It is normal for peach fruits to be “hard as rocks” up until a week or so of a normal harvest time.

They may also develop color long before they are ready to harvest. However, judging from the size you mention (walnuts) and color development, it sounds like the fruits were not thinned earlier (removing many small fruits soon after they developed after flowering so that fruit spacing was about 4 inches apart) or the tree was not watered adequately or both.

Not watering with enough water or frequently enough will also result in an open canopy (lots of space between leaves so you can easily see through it) and small fruit.

Unless you are in a very cold climate or you had a very late hard freeze (hard freeze after flowering) I would not guess you had cold damage.

Q: My grapevines are about 20 years old and produced grapes last year. I added 4 inches of mulch dirt and feed with fruit and nut fertilizer in spring. There were no grapes this year and the leaves look mottled.

A: From your pictures the leaves look pretty normal with a little bit of wind damage. I suspect that the grapes were either not pruned correctly so that fruit was produced from last year’s growth (spur or cane pruning) or there might not be enough light.

You will need at least six hours of full sun to produce much. Less than this will result in very weak fruit production or no fruit production. If light is a problem it will only get worse as the tree gets larger.

Lack of light usually results in leaves that are larger than normal and thin, frail leaves easily damaged in wind. If growth from the previous year was totally removed during pruning you will have no fruit production as well.

Bob Morris is a horticulture expert living in Las Vegas and professor emeritus for the University of Nevada. Visit his blog at xtremehorticulture.blogspot.com. Send questions to Extremehort@aol.com.