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Lettuce Variety Comparison, Insect Resistance Evaluations, and Organic Treatment Trials for Red Lettuce Aphid

Project Leader: Lewis Grant, Grant Family Farms, Wellington, CO

Technical Advisor: Whitney Cranshaw, Professor/Extension Specialist, CSU, Fort Collins, CO

Project Year: 2005

pictures from: www.images.google.com

Project Summary

Leaf lettuce is an increasingly important high value crop in northern Colorado, but because of the variability of growing conditions there are many problems in the production of lettuce. A significant problem in lettuce production is the Red Lettuce Aphid ( Nasonovia ribes-nigri ). The aphid colonizes the growing point of the plant where it can be protected from predators and it is not easily removed by washing. Although regular applications of some systemic insecticides can control this insect, controls for organic production have not been identified. The Red Lettuce Aphid was identified in Colorado 5 years ago and has caused significant damage to the northern Colorado organic lettuce industry.

The proposed research will involve field trials to determine: 1.) optimal varieties for different parts of the growing season, 2.) relative susceptibility of various lettuce cultivars to red lettuce aphid, and 3.) tests of potential biological and cultural controls for red lettuce aphid management. The results of these field trials will help organic growers determine lettuce varieties best adapted to various growing conditions; provide information on the susceptibility of various varieties to the Red Lettuce Aphid; and will identify practices that can best manage this insect in a manner that is compatible with organic production practices.

Project Methods

•  Lettuce Cultivar Evaluations/ Host Plant Resistance Monitoring

The objective of this study is to evaluate the yield and quality of leaf lettuce cultivars grown during different periods and to determine the susceptibility of lettuce cultivars to Red Lettuce Aphids.

Approximately 3-4 dozen cultivars will be included in these studies. Plantings will be made at three different times during the growing season: early summer, midsummer, and late summer maturity.

•  BioControl Studies

The objective is to identify the potential of applied biological controls for the Red Lettuce Aphid that is compatible with organic production. The emphasis will be on green lacewings and entomopathogenic fungi.

This study will involve two planting dates: midsummer and late summer maturity to increase the likelihood of aphid infestation. Green lacewing standard application will be one egg/plant, dropped by hand onto the plant. Application will be made at three intervals three weeks prior to harvest. The entomopathogenic fungi application will be made at three different times of plant growth applied concurrently with green lacewings.

•  Mulching Studies

The objective of this study is to evaluate the use of straw mulch on lettuce aphid infestation- reducing landing rates and retarding colonization of the crop.

The study will be conducted at two different planting dates late in the season to ensure aphid infestation. It will consist of applying straw mulch at these two times of plant growth.

Final Report Summary

Thirty-four lettuce cultivars were tested for resistance to the red lettuce aphid and it was shown that all four of the Rijk-Zwaan cultivars had the most resistance to this insect. Among other cultivars, Galisse and Fireball supported the least number of aphids.

In the biological control portion of this study, it was shown that the green lacewings were ineffective at suppressing the aphid population and the only significant insect predator present in the fields was the flower fly larvae. Some organic insecticides were tested and none showed any effectiveness.

There was a trend towards lower numbers of aphids on lettuce receiving the straw mulch application. Significant differences were detected on one date, which suggests that the use of mulch, or other methods of diversifying crop background, may assist in red lettuce aphid management.

 

Colorado State University College of Agricultural Sciences Department of Horticulture and Landscape Architecture