Saturday, June 19, 2010

A Summer Bummer: Aphids in the Garden by MJ Lechner

You have a cup of coffee in your hand as you stroll through your garden in the early spring morning, enjoying all the new sights and smells.  As you approach the roses, you notice something is not quite right- large clusters of white are covering the new shoots…  what the devil?  Oh no!  APHIDS!
What are they?
Aphids, also known as plant lice or greenflies, are small plant-eating insects, and members of the superfamily Aphidoidea. Aphids are soft-bodied insects that use their piercing sucking mouthparts to feed on plant sap. They usually occur in colonies on the undersides of tender ‘terminal growth’ (at the ends of the leaves). Heavily-infested leaves can wilt or turn yellow because of excessive sap removal. While the plant may look bad, aphid feeding generally will not seriously harm healthy, established trees and shrubs.

 Aphids produce large amounts of a sugary liquid waste called "honeydew". The honeydew that drops from these insects can spot the windows and finish of cars parked under infested trees. A fungus called sooty mold can grow on honeydew deposits that accumulate on leaves and branches, turning them black. The appearance of sooty mold on plants may be the first time that an aphid infestation is noticed. The drops can attract other insects such as ants, that will feed on the sticky deposits.

Esteemed Colorado State entomologist, Whitney Cranshaw, notes that “Aphid populations often spike in springs when there is a prolonged period of cool, wet weather.”  Some reasons for this include yummy new plant growth, promoted by rains and favorable temperatures, provides perfect plant conditions for aphids thrive.  Cool temperatures slow down their predators quite a bit more than the aphid, so the aphid populations are safe and their numbers quickly soar.

“Aphids can occur on a very wide variety of plants - indeed it is hard to find any plant species that does not support one or more of the 350+ species of aphids that occur in Colorado.   Oaks, lindens, walnuts, poplars, Norway maple, and most ‘stone fruits’(like peaches and apricots) are among the trees that often support large numbers of aphids.  Spirea, roses, and many flowers can be common aphid hosts in spring.  Even weeds may now have large numbers of aphids, such as Canada thistle. “ according to Cranshaw.

So what’s a gardener to do? 
Non chemical: Wash off aphids with a steady stream of water. Avoid heavy applications of nitrogen fertilizer which can encourage succulent plant growth. Natural predators such as lady beetles and aphis lions feed on aphids but may not always provide adequate control. Don’t be lured into buying the bags of ladybugs on the nursery shelves.  The mature lady bugs/beetles are not all that interested in aphids as their youngsters are.  They may be amusing for the kids, but they will soon fly off and you will be left with the aphids.

Chemical: Aphid controls include pyrethrins, horticultural oils and soaps, permethrin, acephate and imidacloprid.  Check the label of products before using to ensure applications can be made to your affected plants. Some of these suggested controls may not be applied to food crops.  Contact the Jefferson County Extension at 303-271-6620 for more information.

Aphid control is most important for new plantings, where excessive sap removal is more likely to affect general plant health. Established and otherwise healthy plants can tolerate moderate to heavy aphid infestations, although affected leaves may wilt and turn yellow and there may be some premature drop.
 Good cultural practices, such as watering and fertilization, will help to reduce stress by these insects. Problems with honeydew and sooty mold may develop but tend to be temporary and disappear after the aphids are gone.
 A few aphid species produce cupped or distorted leaves; these plants may lose some of their esthetic appeal for the season. Once the distortion occurs, the leaves will remain cupped and twisted until they fall off. Usually, the infestation is not noticed until the injury has occurred. Insecticide applications often are less effective because the aphids are protected in the gnarled leaves.