By Tom Dominguez
In the recent days I have had a flood of calls asking, “Why are the elm trees in Tucumcari all dying out?”
Another question overflowing the answering machine is, “What are these little yellow and black beetles in my yard and house?”
Well, after much observation, collection, calling and research, I have found the answer. Elm leaf beetle is one of the most common insects in our area and are known to cause sufficient damage to, you guessed it, elm trees. They really can cause damage to other trees but prefer elms.
Adult beetles are about inch long, overall yellow to brownish green in body color, and marked with black spots on the head and thorax, and broad black stripes following the outer wing cover margins. Larvae grow to about inch long and are bright orange yellow with scattered black bristles.
Adult beetles spend the winter in protected sites, occasionally in homes. Beetles fly to elm trees in the spring, feed on newly emerged foliage and deposit clusters of five to 25 lemon shaped yellow eggs in two or three rows on the underside of leaves. In about seven days, small black larvae hatch and feed for about 21 days. Mature larvae crawl down the tree trunk and pupate in the soil at the base of the tree. Adult beetles emerge from pupae in about 14 days. Three or more generations can occur per year.
Larvae and adults feed on leaves of elm trees causing unsightly damage and premature defoliation. Larvae skeletonize foliage by removing the parts of the leaves between the leaf veins, leaving the upper leaf surface intact. Damaged portions of leaves soon turn rusty, reddish brown, and dry skeletonized leaves fall to the ground. Adults eat roughly circular holes in leaves
Recognize that elm leaf beetle populations fluctuate dramatically from year to year and most trees do not require treatment every year.
When beetles are present, otherwise healthy elms can tolerate substantial defoliation. Where elm leaf beetle is a problem, use a combination of methods. Realize that no single method kills 100 percent of the pests.
Because adult beetles fly from tree to tree, management efforts directed at single trees may give less satisfactory results in comparison with control efforts aimed at all elms in an area. Because most elms in Tucumcari are very large, any pesticide spraying is best done by a professional applicator.
The best control method seems to band the bark with and insecticide before mature larvae crawl down trunks to pupate, which in New Mexico may occur from early May to late June, depending on the location and weather.
Inspect foliage weekly during may and June and band as soon as mature larvae are observed on leaves. The calendar date of peak abundance of beetles and their damage and the optimal time for banding varies greatly form year to year, depending on spring temperatures.
Monitoring temperature is highly recommended to more accurately time foliage inspection and control actions.
If an application of a soil systemic insecticide is planned, the optimal treatment time is before beetles are present and before knowing if beetles will be abundant enough to warrant control during the current or next generation of insects.
Bark banding is an inexpensive and environmentally sound technique that involves spraying a small area of the tree trunk with an insecticide. Use a hand pump sprayer or hydraulic sprayer at low pressure to spray a band of bark several feet wide around the first main branch fork. Carbaryl is most commonly used and should be applied at the rate labeled for elm bark beetles (about 2% active ingredient.)
If trunk spraying is not listed on the label of the products available for home landscape use, it will be necessary to have the trunk application done by a licensed pesticide applicator. Do not use the rate labeled for foliar application because this rate will not be effective as a trunk banding treatment.
Pyretheroids also provide control. About one half gallon of dilute material is applied on each large tree. The insecticide kills the larvae when they crawl down to pupate around the tree base after feeding in the canopy. By reducing the number of elm leaf beetles that pupate and emerge as adults, bark banding reduces damage by alter beetle generations, especially when done to all nearby elms.
Although it has not been scientifically demonstrated, relatively warm and wet winters are believed to reduce the likelihood that beetles will be a problem the following spring.
Wet winters can increase over wintering mortality of beetles form insect pathogenic fungi.
Warm winters may cause many hibernating beetles to starve to death because warmer weather increases the rate at which these insects consume their stored energy, increasing the likelihood that beetles will become weakened or starve before elm leaves appear in spring.
If elm leaf beetle damage was low the previous fall and winter is warm and wet, avoid preventive insecticide application the following spring.
It’s much too late to treat for elm beetles this year. It’s best to wait out the life cycle and be better prepared for next year. With this year’s damage being so severe be rest assured that they will survive and come out of it in full bloom next spring.
I have not seen where the damage has been detrimental enough to kill or sufficiently cause die back of branches. Wait out the storm and lets head this problem off next year.
Tom Dominguez is an agent with the Quay County Extension, NMSU Extension Service. He can be reached by emailing firstname.lastname@example.org or calling 461-0562.