Issue: March 26, 2005

Pests on Mexican Elder

I was reading the NMSU news release article on "cowpea aphids" that attack the alfalfa fields and wondered if these are the same aphids that attack the Mexican Elder Trees every spring? If they are not the same aphids, what species are they and how do I treat them?

D. Hall
Las Cruces, NM

According to Dr. Carol Sutherland, NMSU Extension Service entomologist, the aphid on the Mexican elder in Las Cruces is probably a cowpea aphid but not the very virulent strain that attacks alfalfa in central New Mexico. The toxin injected by aphids in Las Cruces doesn't cause as dramatic an affect on the growth of the plant. She explained that as higher temperatures develop, the problems with this aphid rapidly diminish.

Dr. Sutherland indicated that washing the aphids from the tree with water would probably not be effective since too many aphids will remain in the tree and, because of their rapid rate of reproduction, they would continue to damage the tree. She suggested that insecticidal soap may be used if sprayed frequently because such treatment will also miss some of these rapidly reproducing pests. However, frequent use of insecticidal soap or application at too great a concentration can cause leaf scorch and some growth malformation.

Dr. Sutherland suggests that imadacloprid insecticide applied according to label directions as a soil drench may be the most effective treatment. Be sure the product you purchase is labeled for lawn and ornamental pest management, then follow label directions for soil application. When applied in this manner, the insecticide will be absorbed by the roots and translocated to the leaves where the aphids are feeding.

She also mentioned that the larvae of a small caterpillar, the Zootheca tranquila caterpillar, may also be present on the Mexican Elder (green, yellow, black colors). The young caterpillars roll foliage and live inside the rolled leaves while they feed on the foliage so that you do not see them until they are mature and it is too late to control them.

Dr. Sutherland said that the application of Bacillus thuringensis insecticide when the caterpillars are less than 3/4-inch in size may give some control. She said that Malathion® or Sevin® insecticides may possibly work. With any treatment, it is important to treat early in the leaf roll stage when the caterpillars are small and before the damage is done. Later treatments may kill the insects, but the damage is already done.

She explained that the caterpillar has only one generation on Mexican Elder. Subsequent generations feed on other native elder species that grow at higher elevations. Because of their habit of rolling the leaves to form a sanctuary for themselves, they are not easily visible in those locations.

If the caterpillar persists until it pupates and emerges as a moth, it may be recognized as a sage green miller moth. The miller moths are the Noctuidae (night flying moths that often collect around our porch lights or enter our homes at night and fly around lights inside the house).

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Send your gardening questions to:
Yard and Garden, ATTN: Dr. Curtis Smith
NMSU Cooperative Extension Service
9301 Indian School Road, NE, Suite 112
Albuquerque, NM 87112

Curtis W. Smith, Ph.D., is an Extension Horticulture Specialist with New Mexico State University's Cooperative Extension Service. New Mexico State University is an equal opportunity/affirmative action employer and educator.