The latest Arizona Vegetable Integrated Pest Management Update from the University of Arizona (UA) Cooperative Extension in Yuma, Ariz. released Dec. 14, 2011.

Green peach aphids in desert lettuce

By John Palumbo, UA Research Scientist and Extension Specialist

In a recent update, we anticipated that pest control advisers would soon find aphids infesting local lettuce and cole crop fields. Last week (week of Dec. 5) we readily found winged (alate) aphids plus small colonies of apterous (wingless) green peach aphids, Myzus persicae, on untreated lettuce plants at the Yuma Agricultural Center.

The timing of aphid colonization in lettuce and other leafy vegetables varies by species, and depends largely on temperature, rainfall, and planting dates. Previous studies suggest that green peach aphids are historically most severe in late October and early-mid November plantings of lettuce. By late February, green peach aphid populations generally begin to rapidly decline as temperatures increase.

However, this is not always the case on other crops including cabbage and spinach where heavy green peach aphid infestations can occur in late March and early April. Other aphids including foxglove and lettuce 'red' aphid typically cause problems on later lettuce plantings (i.e., late November and December) because these species are better suited to warmer winter temperatures.

There are exceptions to the rule and aphids can occur when least expected.

For more information on aphid population dynamics on desert lettuce see the aphid dynamics report.

It's important to also note that in our recent inspection of lettuce plants, field plots treated with a soil at-planting application of imidacloprid (Admire Pro, 7 oz) were aphid free. This is not surprising since imidacloprid at the 0.25 pounds active ingredient per acre rate still provides long residual control of most aphids (less consistent on foxglove and lettuce aphid).

Research trials from last spring showed imidacloprid soil applications in head lettuce provided protection against aphid contamination nearly season long. In many cases, an application of a foliar alternative is generally required near harvest to prevent unacceptable aphid contamination.

The trick is to determine when the imidacloprid residual begins to decline and aphids begin to colonize. Of course, this will require close examination of heads well in advance to harvest. Remember - "When in doubt – scout."

Contact Palumbo: (928) 782-3836 or jpalumbo@ag.arizona.edu.