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Missoula Valley Water Quality District

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Water Quality District - Least Toxic Options

Good Gardening

Helps Reduce Pesticide Use

-Helen Atthowe
Missoula County Extension Horticulturist
(Supported by the Missoula County IPM Committee, Missoula City Council and Montana EPA)


Here is an example of an integrated approach to a LEAST-TOXIC OPTION.


First, know about your pest. Aphids are very small, soft-bodied insects ranging in color from light green through pinkish to black. Aphids cluster on stems and/or undersides of leaves and produce a sticky "honeydew" which is fed upon by ants. Aphids often appear to be attached to the plant surface by their sucking mouthparts. Young aphids are wingless. Adults may have wings. There are many different kinds of aphids; most are specific to particular plants, but some attack many species of plants.

Second, evaluate whether or not your pest is at a high enough level to actually harm plants:

Monitoring: In early spring, check ten terminal shoots weekly; look especially at leaf undersides. If 25-50% of the terminals on young trees and shrubs are infested, some control may be required. Older, large trees can usually tolerate 50% or more terminal infestation before control is necessary. Check for predators. If predators are present at a ratio of 1 predator to 5 aphids, control is usually not required. If leaf curling is occurring already, treat immediately, especially on aphid-susceptible species like plum and green ash.

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Third, design a whole system management plan.

Cultural: Aphid outbreaks are encouraged by cool, wet spring weather and when pesticides used on other pests harm aphid predators. Excessive fertilizer, especially nitrogen, causes succulent growth, and encourages aphids. Avoid pruning that encourages early spring growth. On aphid-susceptible species, prune in late spring after aphids arrive and prune a little at a time. Stop pruning by July so you don't encourage a fall flush of growth.

Biological: Lacewings, gall midges (Aphidoletes), and ladybird beetles are effective aphid eaters. Researchers in Colorado found that lacewings are the best predators under hot conditions, and lady beetles and Aphidoletes do best under cool temperatures.

Aphid populations are also decreased by fungus disease and parasites. Diseased or parasitized aphids turn brown, puff up and stick to leaves.

Mechanical: Yellow sticky traps catch winged aphids, especially in indoor growing situations, if populations are not too high. Sticky traps may catch predators also! For vegetable gardens, using a foil mulch underneath plants can confuse flying aphids searching for hosts. When growing squashes or cucumbers, you may get somewhat the same effect by using silver-leafed varieties.

Physical: Washing with soapy water (use a dispenser attached to the hose) can reduce aphids if done repeatedly. Washing may also enhance conditions for fungus diseases that attack aphids. Prune out and destroy aphid-infested terminals.

Chemical: Insecticidal Soap is only effective on soft-bodied insects. It has a short activity period. Repeat applications are needed. Avoid application in direct sun or at temperatures >85F. Soap can burn foliage of tender plants. Wait 48 hours after application before releasing predators.

Two percent horticultural oil: (Sunspray or Volck Supreme Spray) should not be applied in direct sun and/or at temperatures >85-90F or <40F. It may burn leaves, especially of sensitive or drought-stressed plants. Wait 48 hours after application before releasing predators.

Botanical: Neem oil (Margosan-O, Bioneem, or Azatin) is best applied 2-3 times in succession, 7-10 days apart. Spray in the evening. I t has a short activity period, but is relatively safe for predators.

Pyrethrum/Pyrethrin works best if combined with insecticidal soap: add 1/2 to 1 Tbsp. per gallon. Spray in the evening. This botanical breaks down rapidly, especially in sunlight. Repeat applications and spot spray if possible. It will kill predators and parasites.


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