Wednesday, July 14, 2010


A morning stroll through the garden is always revealing. The low angle of the sun highlights plant features that you might miss later in the day: the intricately beautiful circular web of the orb spider; ripened fruit, ready for picking, reflected in the morning light; and, of course, that behemoth zucchini you missed last night while collecting vegetables for dinner.

California Buckeye 46 Heirloom Bush Beans
That early walk can also highlight glistening leaves on your vegetable plants. A closer look may show that it is not the morning dew. It's a "dew" of a different kind, shimmering in the sun: honeydew, the sticky secretions of that ubiquitous bad bug, the aphid. 

Turn over that leaf, and voila! There's a colony of aphids, busily sucking the life out of your plant, curling and yellowing the young foliage.

What's a gardener to do?

Predatory Aphid Fly Larva
First of all, take a closer look. There may be other critters on that leaf, the garden good guys: natural enemies such as lady beetles, lacewings, syrphid fly larvae and predatory aphid fly larvae. It's the larval stage of these beneficial insects that do the most good. 

Syrphid Fly Larva (lower left)
Syrphid fly larvae, for example, move along plant surfaces, lifting their heads to grope for prey, seizing them and sucking them dry and discarding the skins. A single syrphid larva can consume hundreds of aphids in a month.

Parasitized aphids
Look for the mummified skins of parasitized aphids, where small, harmless (to you) parasitic wasps have laid their eggs which then hatch...inside the aphid, and crawl out!  

Look for disease-killed aphids as well: they may appear off-color, bloated, or flattened. Substantial numbers of any of these natural control factors can mean that the aphid population may be reduced rapidly without the need for treatment. 

If you see these garden good guys at work on the aphids, resist the temptation to spray broad-spectrum pesticides (organophosphates, carbamates, and pyrethroids). These kill natural enemy species as well as aphids. And guess whose population builds up more rapidly after such an assault? Aphids, which are born pregnant! Natural enemy populations do not appear in significant numbers until aphids begin to be numerous. Another reason not to use broad-spectrum pesticides: aphids can develop a resistance to those products. Mother Nature bats last!

The weather is your friend, as well. Populations of many aphid species are reduced by summer heat in the Central Valley and desert areas.

Aphids on a rose bud in spring
Check the other plants, as well, especially those on the upwind side of the garden, a favorite location for aphids. One tell-tale sign of aphids: the presence of ants, crawling up and down the plants. Ants herd the aphids and defend them from natural predators, in order to harvest the honeydew secretions for the ant colony. Control the ants with ant baits, and you can control the aphid population, too.

What If You See Aphids, But None of the Garden Good Guys?
My favorite way to reduce aphid populations on plants is to knock them off with a strong spray of water from the garden hose. Most dislodged aphids will not be able to return to the plant, and their honeydew will be washed off as well. Using water sprays early in the day allows plants to dry off rapidly in the sun and be less susceptible to fungal diseases. I do this two or three times a week on plants that the aphids find attractive.

Insecticidal soaps and light, horticultural oils (such as Neem oil) will kill aphids on contact. But soaps and oils will kill any beneficial insect it contacts, as well. Still, because soaps and oils have no residual action, any migrating beneficials will not be effected. Remember, too, that spraying soaps or oils on a hot day (90 and above) may damage your plants. Test a small portion of the plant with such sprays first.

Stop Aphids Before They Start: Tips from the UC Davis Integrated Pest Management Website

Before planting vegetables, check surrounding areas for sources of aphids and remove them. Aphids often build up on weeds such as sowthistle and mustards, moving onto crop seedlings after they emerge. Check transplants for aphids and remove them before planting.

Where aphid populations are localized on a few curled leaves or new shoots, the best control may be to prune these areas out and dispose of them. In large trees, some aphids thrive in the dense inner canopy; pruning these areas out can make the habitat less suitable.

On a newly planted vegetable garden, covering the seedlings with row covers can thwart the arrival of aphids.

High levels of nitrogen fertilizer favor aphid reproduction. Never use more nitrogen than necessary.  Apply it in small portions throughout the season rather than all at once. Most organic fertilizers can be classified as low level nitrogen products as compared to synthetically manufactured fertilizers.

Another effective aphid control method: snip off heavily infected branches and leaves, and put them in the trash or compost pile (if it is a hot compost pile!).

Aphids will always be with us. But there are plenty of garden good guys out there willing to help you in the battle...if you give them a fighting chance.

1 comment:

  1. Thanks for the tip! I just noticed the leaves of one of Venus' Moon and Stars Melon glistening in the morning sun this morning -- and was surprised to discover this little pest of a problem.