Marin Master Gardeners
University of California
Marin Master Gardeners

Marin IJ Articles

The case of the curious pest

May 31, 2009
D.F. Braun
You've looked outside after the last rain and realize with horror that some "thing" is eating your newly planted purple petunias. Don't run for your handy pesticide that will kill just about any bug - what of all the surrounding plants? They look healthy. What to do?
It's time for your Sherlock Holmes kit of pest identification to come to the rescue. Naming the culprit is a must for determining the safest and most promising way of seeking extermination.
Are you equipped? Do you have a magnifying glass? A "Mac's Field Guide," which is a pictorial list of both good and bad bugs? Traps: Do you have cut up pieces of old hoses, empty toilet paper rolls, empty grapefruit halves, inverted plastic cups of any kind with a small opening cut at the bottom? These items placed next to the infected plant at dusk and investigated very early in the morning sometimes reveal your thief in hiding.
And what about nematodes? How do you determine the good guys from the bad? Let's start with them. Do you know your garden is full of nematodes? Most of them are important, but virtually invisible. They're members of your soil's natural community of life forms. Nematodes (aka eelworms) are microscopic worms that wiggle through your soil. Many species prey on insect larvae and even other nematodes. You can buy the good guys at garden centers and use them to control pests.
How can you determine if a plant has fallen prey to an evil nematode? Here are some clues, Sherlock: Do plants seem thirsty despite watering? Is midday wilting severe? If you dig up a plant do you see knobby growth at the roots or tips forked like a snake's tongue? Have all the deep roots disappeared, leaving only a tangle of surface roots?
Although these clues may help, identification and management of this problem is difficult. One of the best ways to avoid nematodes is to use vegetable varieties and fruit tree rootstocks that are resistant to nematode damage. The symbols "VFN" identifies plant varieties that are resistant to nematodes. (The V stands for verticillium wilt and the F stands for fusarium wilt, fungal diseases that you don't want either.) Other methods of nematode control approved by the UC Cooperative Extension include fallowing, crop rotation and solarization. Growing nematode-suppressing plants, such as French marigolds, in a solid planting for an entire season may be helpful. For more information about any of these methods you can contact the help desk at the UCCE office, 499-4204.
Let's investigate the insect invaders. Most bugs that pester your garden can't spread from one type of plant to another; they must have a very special host, which should simplify your task. Move your annual plantings around each year forcing pests to venture afar, but often they won't move and will perish.
Aphids love the supple new growth on plants, so don't use heavy doses of fertilizer, which further stimulate lush growth. Encouraging the "good bugs" such as lady beetles, soldier beetles and lacewings will help control aphid populations. A blast of water will knock them off the plant, or a spot application of an insecticidal soap will work.
Thrips can be a challenge to the home gardener. Beneficial insects, such as green lacewings and minute pirate bugs, are predators on thrips. In small gardens, a forceful spray of water will remove them. Heavy applications of nitrogen fertilizers actually promote higher populations of the pests.
Whiteflies are another insect "bad guy." Fortunately, they are often lunch for beneficial insects such as lacewings, pirate bugs and parasitic wasps. Insecticidal soaps can provide control, but coverage of the underside of the leaf is essential. Broad spectrum, persistent insecticides are not recommended because of the toxic effect to beneficial insects.
Lastly, apply good sense, Sherlock. At some point you may come to the realization that there are some plants that will not do well in your garden, or require too much of your care and time. Are your neighbors afflicted with the same problems? If their gardens are in close proximity to your own it's important to share information concerning common plantings and pests - even if your neighbor is one of the latter.
The University of California Marin Master Gardeners are sponsored by UC Cooperative Extension. For questions about gardening, plant pests or diseases, call 499-4204 from 9 a.m. to noon, and 1 to 4 p.m. weekdays, or bring in samples or pictures to 1682 Novato Blvd., Suite 150B, Novato.

Top of page

Webmaster Email: banielsen@ucanr.edu