Experts tackle readers’ garden questions.
Q: The fruit on my 5-year-old white nectarine tree was a bust last year. Thinking it was thrips, I sprayed the tree three times with Spinosad. Now I’m not sure I sprayed correctly. Should I spray the tree or the fruit? I have weed fabric and mulch under the tree, so there are no weeds for overwintering. Is there a treatment I can apply to prevent overwintering of the thrips? I’m ready to cut down the tree and plant something else.
Mary Ortiz, Orangevale
Master gardener Rachel Tooker: From your photos, it does appear that your nectarines are suffering from western
Thrips are tiny (less than 1/20 inch long), slender insects with fringed wings. They feed by puncturing their host and sucking out the cell contents. There are many thousand species of thrips and very few are considered pests. Some of them are actually beneficial predators that feed on mites and other insects.
Western flower thrips (Frankliniella occidentalis) fall into both the pest and beneficial categories. They feed on many herbaceous ornamentals (impatiens, petunias), vegetables (cucurbits, peppers), fruits (grape, strawberry) and some shrubs and trees (rose, stone fruit). But they also feed on mites.
Thrips prefer to feed in rapidly growing plant tissue. Feeding typically causes tiny scars on leaves and fruit, called stippling, and can stunt growth.
Fruit are vulnerable to damage at bloom, when thrips feed in the blossom on developing fruit, and again later in the season at fruit ripening. Damage that occurs at bloom appears as russeting (a brown, corky netlike patch on the fruit’s skin). Feeding damage that occurs at fruit ripening appears as silvering on the fruit surface. Because western flower thrips prefer to feed in protected sites, late-season damage commonly occurs where clustered fruits are touching.
Management of thrips requires an integrated approach using active monitoring, good cultural practices and protection of natural enemies, with use of least-toxic pesticides as a last resort. University of California Integrated Pest Management Pest Note on Thrips, No. 7429, is available online for free download, and provides a good summary of these techniques. Find it at http://www.ipm.ucdavis.edu/PMG/PESTNOTES/pn7429.html.
Monitor for thrips by beating branches or shaking foliage over a sheet of cardstock or paper. It helps to have a hand lens to identify the insects.
Because you have considerable russeting on your fruit, begin monitoring for thrips at bud and bloom.
Thrips often migrate to nectarine trees from drying weedy areas or grasslands, so in addition to mulching under the tree, pay attention to surrounding grassy areas and keep these at a greater distance, or keep the grassy areas healthy and vigorous to discourage the thrips from migrating into your trees.
To encourage beneficial insects, control dust and avoid using persistent pesticides that will attack beneficial insects just as easily as pests. Avoid excess use of nitrogen fertilizer; that will slow production of tender new shoots. After natural fruit drop in June, thin remaining immature nectarines to about 5 to 7 inches apart to ensure that fruit will not touch others as they mature.
If chemical control is necessary, it is important to time the application. When russeting is noticed on ripening fruit, it is already too late to control thrips – often they are long gone. When applying chemicals, improper timing of the applications, failure to treat the proper plant parts, and inadequate spray coverage are all common mistakes that can determine the success or failure of a spray program.
On plants with a history of severe, unacceptable damage, begin treatment early. In most cases, you can start with a narrow range oil spray, such as neem oil, when thrips are first observed. Alternately, Spinosad has been found to be somewhat more effective in treating thrips but it should be applied no more than twice a year. Apply early in the morning, and again one week later.
Given your history of damage, you may wish to start with Spinosad, but then supplement with narrow-range oils if you are continuing to observe an infestation.
Thrips can have several generations (up to eight or more) a year, and the life cycle can be complete as quickly as two weeks when the weather is warm. Repeat applications (usually five to 10 days apart, depending on temperature) are usually required because the sprays kill newly hatched thrips and recently emerged adults. Make sure to fully cover growing shoots, leaves, bark areas and buds. For most effective coverage it will help to keep your tree pruned to a height easily reached by your sprayer.
Rachel Tooker is a UC Cooperative Extension master gardener for Sacramento County.
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