Managing Diamondback Moths and Cabbage Loopers in Cabbage

he diamondback moth (DBM) became a severe problem for Valley growers in the fall of 1986 with control failures due to in secticide resistance occurring through most of the late 1980’s. During the 1990’s, DBM T populations experienced a few control failures in the spring with levels reaching 38 larvae per plant while in the fall these levels only reached two larvae per plant. The implementation of a cabbage IPM program in the early 1990’s was successful at reducing the number of sprays as well as controlling the worm complex, including DBM, and producing an acceptable yield of high-quality cabbage. Populations of DBM remained at manageable levels throughout the following decades.

However, last year was a tough one for controlling this pest. There was significant pressure and some reports of insecticides not providing adequate control. The diamondback moth is renowned for its ability to develop resistance to insecticides, sometimes within just a few years. Prophylactic or “calendar spraying” using conventional/synthetic insecticides has led to frequent, repeated exposure to many effective insecticides, resulting in decreased efficacy.

In South Texas we can have multiple generations of this pest throughout the growing season, which favors the development of resistance. Insecticides select individual DMB that can tolerate exposure to a toxin and survive to pass this trait on to future generations. Generally, tolerance to one insecticide results in tolerance to all insecticides with the same mode of action (MOA), categorized by IRAC group numbers. Each MOA has a designated IRAC number and all insecticides with the same MOA will have the same IRAC number. To restore the efficacy of current insecticides and extend the usefulness of new, novel insecticides, it is incredibly important to use them judiciously, rotate between IRAC groups, and include non-chemical methods whenever practical. In other words, employ an integrated pest management plan.

Biological/natural control for DBM already present in the Valley include two larval parasitoids. Diadegma insulare and Microplitis plutellae, which have been found to reach between 20 and

BY JUAN ANCISO AND HOLLY DAVIS

60% parasitization of DBM larvae if populations are not disrupted by spraying non-selective insecticides. Other regions in the U.S. have reported as high as 80% parasitization of DBM by native parasitoids. Many investigators have reported that populations in the U.S. and Canada are held in check by a multiplicity of environmental factors, chiefly biotic such as parasitoids, but serious outbreaks do occur. While the cabbage looper is typically the major pest to Valley growers, it has been kept under control with synthetic chemical insecticides. Biological/natural control for the cabbage looper present in the Valley include Voria ruralis and Cotesia marginiventris although they are generally responsible for less than 5% parasitization in unsprayed fields.

The success of the cabbage IPM program of the early 1990’s was accomplished by monitoring fields regularly, using an action threshold rather than a scheduled treatment, and by selecting the most appropriate insecticide for the pests present and the growth stage of the crop (Fig. 1). The use of commercially available pheromone lures can be a relatively easy way to monitor for DBM activity, alerting consultants and growers when adult activity begins. While these traps will not provide significant control, they indicate that scouting for larval activity should begin in 7-10 days. Note that there can be a lot of variation in activity from one field to another so each field will need to be monitored individually.

Fig. 1 Action Threshold for diamondback moth larvae throughout the season.

A successful cabbage IPM program will involve selecting Bacillus thuriengiensis (Bt) products for DBM early in the season as it is least disruptive to parasitoid populations because it is only effective on caterpillars where it must be consumed as a stomach poison. Later in the season, synthetic insecticides that are more effective for cabbage looper, or beet armyworm can be used. Bt’s have been found to be most effective for control of DBM when applied before head formation as good coverage may become a problem after head formation.

Past results indicate that good control (70-85% control) of DBM larvae was achieved with Bt’s at the higher rates. These Bt’s did provide better control than the synthetic insecticide chemical standard (permethrin) under the tested population and infestation level. It’s worth noting that, even though limited synthetic pyrethroids had been used in this field, larval samples exposed to pyrethroids using the vial technique demonstrated that the population was quite resistant to this group of insecticides.

The fact that Bt’s do provide moderate control of cabbage looper (60-70% control) and good control of DBM should further strengthen the use of Bt’s early season (until cupping or small head formation). This will preserve the biological/natural control that exists for these two pests.

Early season, Bt products should be used alone for DBM and cabbage looper, provided good coverage is achieved and infestation levels are not heavy. It is recommended to alternate between the kurstaki and aizawa strains to prevent DBM resistance to Bt. Growers can then change to synthetic insecticides mid-to late season for DBM and cabbage looper while keeping the number of these applications down to a minimum. To help delay resistance development, rotate between insecticides with different modes of action for each DBM generation.

If control fails using one mode of action, rather than treating again with the same insecticide, switch to a product with a different IRAC number. This resistance management strategy will insure the judicious use of Bt’s and synthetic chemical insecticides and proper control of these two serious pests in cabbage for the Lower Rio Grande Valley, Texas.