A reader had a complaint regarding tiny green caterpillars in her geranium plants. The answer is to get rid of the white moth that lays an egg in each geranium bud. The caterpillar egg hatches out in a few days and the tiny worm eats the inside of the bud. Hence, there’s no flower or a damaged one.
The secret to my hedge rows of beautiful full-blooming geraniums is a black wasp that buzzes in and out of the bushes and leaves them thoroughly devoid of eggs and caterpillars. The wasps come to my garden uninvited. Where they come from, I do not know. They just solved a bad situation.
Margaret Tucher, Davis
You are fortunate that a beneficial insect – a parasitic wasp – is limiting pests on your geraniums, said UC master gardener Lorraine Van Kekerix.
The wasps break the life cycle of a common moth that lays eggs in several common flowering plants including petunias and geraniums, one of the moth’s preferred host plants. The adult moth does not eat the plant.
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The moth’s wings are about 1 1/2 inches across; the color ranges from light green to brownish with lighter colored bars across the wings. It’s not the familiar white cabbage moth but another pest, the geranium or tobacco budworm moth.
When the moth eggs hatch, geranium budworm larvae emerge. The larvae eat the plants and do the damage. Specifically, geranium budworms eat the developing flower buds so the buds do not open. Severely affected plants may not produce flowers at all.
The geranium budworms eat flower petals as well as the buds. If the infestation is particularly large, they may eat leaves as well. While this pest prefers geraniums, petunias and tobacco (including flowering tobacco), it will also attack other flowers and plants.
What you describe in your rows of geraniums is a beneficial insect, a parasitic wasp. A parasite feeds on a host organism. Most parasites are smaller than the host and often are the larval stages of an insect. Specialized flies and wasps are the most common types of parasitic insects, and there are several types of parasitic wasps that can attack geranium budworms.
Most of these wasps are tiny and do not sting people. Parasitic wasps lay eggs in or on the geranium budworm. When the wasp larvae emerge, they develop by feeding on and killing the worm. Parasitic wasps can lay hundreds of eggs a day.
A beneficial insect is part of the natural cycle of checks and balances when it destroys or reduces a rapid increase in the pest population. We benefit as we no longer need to deal with the pest. There are many ways to protect and increase the population of these naturally occurring beneficial insects in our gardens.
Start by reducing use of broad-spectrum pesticides (that kill a wide range of insects) in the garden. Broad-spectrum pesticides often kill the beneficial insects in higher proportions than the pests. Many pesticide residues persist in the garden, and those residues can reduce the reproduction of these beneficial insects or kill them long after the pesticide was originally applied.
If a pesticide is needed, spare the beneficials by choosing a less persistent pesticide or one that kills only specific pests. For example, Bacillus thuringensis affects only caterpillars including geranium budworms, hornworms and cabbage worms.
To maintain a population of beneficial insects, design your garden to provide the food and habitat they need. These insects need nectar, pollen and shelter throughout the growing season so the population is large enough to control the pests.
Gardens with a wide variety of plants that bloom at different times throughout the seasons can provide these good guys the food and shelter they need at all life stages.
For more information on beneficial insects, visit the University of California’s Integrated Pest Management website and obtain Pest Note 74140, “Biological Control and Natural Enemies.” You can find it at www.ipm.ucdavis.edu.
Or you may receive this pest note by mail. Send a stamped, business-size, self-addressed envelope to: Pest Note 74140, UC Cooperative Extension, 4145 Branch Center Road, Sacramento, CA 95827.
The IPM website also features a picture gallery of natural enemies, which includes beneficial insects.
The gallery is very useful in identifying beneficial insects. It’s likely you’ll recognize several that are already helping to control the pests in your garden.
Questions are answered by master gardeners at the UC Cooperative Extension services in Sacramento and Placer counties. Send questions to Garden Detective, P.O. Box 15779, Sacramento, CA 95852. Send email to firstname.lastname@example.org. Put “Garden Detective” in the subject field and include your postal address. To contact UC Extension directly, call:
Sacramento: (916) 875-6913; 9 a.m.-noon and 1-4 p.m. Monday-Thursday
Amador: (209) 223-6838;
10 a.m.-noon Monday through Thursday; email email@example.com
Butte: (530) 538-7201;
8 a.m.-noon and 1-5 p.m. weekdays
Colusa: (530) 458-0570; 9 a.m.-noon and 1-4 p.m. Tuesdays; website: cecolusa.ucanr.edu
El Dorado: (530) 621-5512;
9 a.m.-noon Tuesday-Friday
Placer: (530) 889-7388;
9 a.m.-noon on Tuesdays, Wednesday and Thursdays or leave a message and calls will be returned; website: http://pcmg.ucanr.org/Got_Questions
Nevada: (530) 273-0919;
9 a.m.-noon Tuesdays through Thursday or leave a message
Shasta, Tehama, Trinity: (530) 225-4605
Solano: (707) 784-1322; leave a message and calls will be returned
Sutter, Yuba: (530) 822-7515; 9 a.m.-noon Mondays and Tuesdays and 1-4 p.m. Thursdays
Yolo: (530) 666-8737;
9-11 a.m. Tuesdays and Fridays, or leave a message and calls will be returned
To read past Garden Detectives, go to sacbee.com/gardendetective