Insecticides: Think Before You Spray - April 8, 2015
Jeff Schalau, Agent, Agriculture & Natural Resources
University of Arizona Cooperative Extension, Yavapai County

Aphids and thrips are out in full force this spring following our warmer than normal winter. Many residents’ first reaction to a plant feeding insect is to apply a broad spectrum insecticide. In doing so, there are often unintended, negative consequences to non-target insects. These non-target insects include bees (and other pollinators) as well as predatory, parasitic, and benign insects.

Applications of broad spectrum insecticides such as malathion, carbaryl, and pyrethroids often have a greater negative effect on pollinators and predators than the pest species being targeted. If you see aphids, in a week or two, you should also see ladybird beetles (i.e. ladybugs). These ladybird beetles are mating and laying eggs which hatch to produce predatory larvae that feed on aphids. Similarly, thrips have natural enemies which include predatory thrips, green lacewings, minute pirate bugs, predatory mites, and parasitic wasps. Your patience will allow those natural enemies to build up their numbers and naturally reduce pest populations.

Pollinator protection is an important consideration when considering insecticide use. Nationwide, honeybees have been declining in numbers due to a combination of factors. The phenomenon is called Colony Collapse Disorder (CCD). Known factors include varroa mites, viruses, and gut pathogens, and stress induced by transportation to multiple locations across the country. In addition, environmental stressors are also being investigated for their role in CCD. These stressors include scarcity and lack of diversity of pollen and nectar, pollen and nectar with low nutritional value, limited access to clean water supplies, and accidental or incidental exposure to pesticides at lethal or sub-lethal levels.

While exact causes of CCD continue to be investigated, recent research has pointed to a class of pesticides called neonicitinoids, and specifically the active ingredient imidacloprid, as being a contributing factor in CCD. Imidacloprid is readily available in garden centers and nurseries. It is a systemic insecticide which is available in liquid and granular formulations. Imidacloprid labels include directions for use on non-bearing fruit and nut trees. Some researchers believe that imidicloprid labels have not given adequate consideration to the protection of honeybees and other pollinators. All of this to say, you might think twice before applying imidacloprid in situations where honeybees and other beneficial insects could be exposed to it.

So, how should we approach management of aphids and thrips? We should learn to tolerate some plant damage – aphids and thrips do cause some cosmetic damage, but they do not normally kill plants or totally destroy crops. If you are seeing lacewings, ladybird beetles, and other predatory insects, do not apply broad spectrum insecticides. High pressure water sprays can be used to mechanically dislodge these aphids and thrips if non-target insects are also present. Horticultural oils and soaps can be used when damage is more significant. These are best applied to non-flowering plants in the evening to further minimize risk to non-target insects, particularly pollinators.

Thrips are particularly fond of peaches and plums. You can monitor for thrips by “whipping” a branch tip onto a sheet of white paper and observing for small (1/20 inch) linear shaped insects. They are small, but their movement is visible upon careful observation. Spinosad (a fermentation product of a naturally occurring bacterium) is thought to be more effective against thrips than other pesticides. Even so, spinosad can be toxic to certain natural enemies and bees when sprayed and for about 1 day afterward; do not apply spinosad to plants that are flowering and consider late evening applications to protect beneficial and benign insects.

We can also plant pollinator friendly flowers and food plants for insect predators and parasites. Be aware that native ground and wood nesting solitary bees are also pollinators and these insects have a positive impact on the ecosystems and food plants we value. Finally, avoid using broad spectrum insecticides unless absolutely necessary. If an insecticide is warranted, use least toxic, target-specific insecticides and give consideration to pollinators and other beneficial and benign insects. As usual, I have included additional resources below.

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Additional Resources

Pests in Gardens and Landscapes: Aphids
University of California IPM

Pests in Gardens and Landscapes: Thrips
University of California IPM

Honey Bee Health and Colony Collapse Disorder
USDA Agricultural Research Service

Free Pollinator Friendly Planting Guides
Pollinator Partnership

Least Toxic and Organic Pesticides for Gardeners
Oregon State University Extension

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Arizona Cooperative Extension
Yavapai County
840 Rodeo Dr. #C
Prescott, AZ 86305
(928) 445-6590
Last Updated: April 6, 2015
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