Reports of the deaths of hundreds of thousands of honeybees in Fresno County orchards last February, possibly caused by pesticide poisoning, should concern home gardeners. Honeybee and native bee populations have been in serious decline for over a decade. Several causes are suspected in their decline including mite infestations, decrease and lack of diversity in pollen and nectar sources, loss of habitat, and pesticide use. Home gardeners, each in our own small way, can help native and honeybee populations recover by planting a wide variety of pollen sources that bloom successively throughout the season, by providing habitats (bee houses, undisturbed dirt areas (many bee species build underground nests in undisturbed soil), sources of clean slowly trickling water, and by seriously curtailing or eliminating the use of pesticides that are in toxic to bees in our gardens.
Here’s a short list of pesticides commonly used by home gardeners that may well be on your garden shed shelves. Some have low toxicity; some are rated as being highly toxic to bees. The ratings given are drawn from the University of California’s IPM (Integrated Pest Management) website, www.ipm.ucanr.edu/TOOLS/PNAI, which rates all pesticides on a scale of I to IV (Roman numerals) with I being the most toxic and IV the least. These ratings or a variant warning of toxicity to bees are not always printed on pesticide labels. Labels should prominently list the active ingredients, but sometimes the ingredients are in very fine print.
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Bacillus thuringiensis or Bt (products include Garden Safe Worm and Caterpillar Killer, Caterpillar Clobber): applied to kill all forms of caterpillars, it is rated IV or low in toxicity. The UC IPM sites states Bt “can be applied at any time with reasonable safety to bees”. The term ‘reasonable’ should be noted since it indicates a slight chance of toxic effects.
Insecticidal soap (inc. Bayer’s Advanced Natria Insecticidal Soap, Safer Brand Insecticidal Soap): rated IV or low toxicity. The soap kills targeted insects only on contact and has little or no residual effect. Again, UC IPM suggests that insecticidal soaps can be sprayed at any time with “reasonable” safety to bees.
Neem oil: no bee warnings on most labels. It is rated a type III or slightly toxic pesticide which should not be sprayed on blooming flowers. UC IPM also cautions that neem oil should be applied when bees are not foraging-at night, in the early evening or in the early morning hours.
Imidacloprid (the active ingredient in many systemic insecticides including many Bayer products): Imidacloprid, whether sprayed or used as a soil drench, is rated I or highly toxic to bees. The UC IPM site advises against spraying blooming plants with imidacloprid and cautions against applying soil drenches of imidacloprid as a systemic insecticide prior to bloom since the pesticide can move into nectar.
Spinosad (inc. Monterey Insect Spray, Orchard Rose and Flower Insect Spray): a naturally (not chemically) derived pesticide which is rated as I or highly toxic to bees on the UC scale. The UC IPM website advises that spinosad should be sprayed only in late evening, at night, or in the early morning when bee are not active. Other sources rate spinosad as highly toxic to bees when wet with low toxicity when dry.
Carbaryl (the active ingredients in the Sevin line of insecticides): it is rated as I, or highly toxic to bees. The UC IPM website notes that carbaryl baits are less toxic to bees than sprays.
Send Elinor Teague plant questions at firstname.lastname@example.org.