I found four of these caterpillars on my tomato plants. What are the white things on their backs?
Your photo shows a wonderful example of the checks and balances of nature. These tomato hornworms were parasitized by a beneficial wasp. What you see are wasp cocoons. First the wasp laid its eggs on the hornworm. They hatched, entered the hornworm to feed and now are in the cocoons turning into adults.
FOR THE RECORD - Correction
The July 29 Garden Q&A had an incorrect answer about parasitized hornworms on tomato plants. Hornworms covered with small white cocoons should be left alone so that the wasp parasitoids inside the cocoons can safely complete their lifecycle.
The Sun regrets the error.
These hornworms will not cause any further damage. The perfect camouflage of tomato hornworms often confounds gardeners who can't find what's eating tomato leaves but see brown, BB-sized droppings. Handpick any hornworms you find and destroy them or escort them to another area.
Gooseneck loosestrife has taken over my flower garden. Why wasn't it listed as invasive? I would never have planted it!
Invasive-plant lists refer to foreign plants so aggressive in natural areas that they damage them and wreak havoc with the ecosystem. Often called "invasives" for short, they are also a nuisance in the home landscape, but that's not what makes them a threat to our country. This is a relatively new specialized use of the term "invasive." To prevent confusion, it helps to label ordinary "pains in the neck," such as gooseneck loosestrife, as "rampant growers."
Tackle your gooseneck loosestrife by cutting it back and spraying the re-emerging growth with glyphosate systemic herbicide.
Hot, dry days and warm nights can prevent bean, tomato and pepper plants from setting fruit. Cool these plants with a spray of water during the day to keep them productive.
Are Asian tiger mosquitoes driving you crazy? Eliminate breeding sites by dumping out any standing water on your property.
Jon Traunfeld, regional specialist, and Ellen Nibali, horticulture consultant, work at Maryland Cooperative Extension's Home and Garden Information Center. Call the center's "hotline" (Monday-Friday, 8 a.m.-1 p.m.) at 800-342-2507 or e-mail plant and pest questions through the Send a Question feature at hgic.umd.edu.