Cabbage and leafy green production is huge for Colquitt County. This county is the biggest producer in Georgia for cabbage and various other brassica crops. However, it has not been easy for the growers in this county to produce these crops. Growers have been in a continuous battle with the diamondback moth (DBM), Plutella xylostella, which is one of the most destructive pest in cabbage and leafy greens if not the worse. And it seems that so far this year, the DBM’s are not letting up and have been uncontrollable in certain areas. In efforts to help our growers, we have partnered with Dr. David Riley, UGA entomologist at the UGA Tifton campus, in assessing the DBM insecticide resistance across Colquitt County.
This summer in fields throughout the county with high DBM populations, we have started collecting DBM larvae for insecticide bioassays. These bioassays allow us to monitor insecticide resistance among the DBM field populations. In the bioassay, we are testing 11 different treatments that farmers are using throughout year in attempt to control the DBM. Attached below are the results that we have so far from the bioassays. As you can see, there is variation in resistance from field to field. Therefore, it takes sampling from each field where there are high levels of this pest. It may also take sampling more than once from a field during a growing season to monitor the resistance situation. These bioassays are only representations of what insecticides are working in a field at that particular time.
We are doing the bioassays not to just survey the DBM resistance across the county, but to try to help the growers make educated management decisions in situations where the DBM populations are uncontrollable and nothing seems to be working. These bioassays are definitely not the answer to the DBM problem, but by doing this our goal is to have a better understanding on the current situation we are facing and aid in DBM resistance management across the county.
If you have any questions or have a field that may need to be sampled call the Colquitt county extension Office 229-616-7033 or email me at email@example.com.
Helpful tips on Managing Insecticide Resistance
If a DBM control failure occurs with an MOA, rotate to an alternate MOA.
Rotate insecticides with different IRAC groups each DBM generation (4 weeks in mild to 2.5 weeks in hot seasons).
Reserve the most effective insecticides to protect the harvested portion of the crop.
Never use insecticides off-label, especially in transplant production. Following labels helps to manage resistance.
Where year-round crop production occurs, use a host-free period during the warmest months of the year (2 or more DBM generations).
Consider using a regional selection-free period of one MOA to reduce the carryover of DBM resistant to a specific MOA from one season to the next.
If sequential plantings of cole crops are required, then avoid adjacent plantings and rotate IRAC groups per DBM generation time across all plantings.
Biology of the Diamondback Moth
Except for the adult stage, DBM completes its whole life cycle on the plant. DBM moths lay eggs singly on the underside of leaves. The larvae hatch in a day or so and feed on the underside.
Initial damage is small incomplete holes caused by young larvae and larger complete holes caused by mature larvae. The holes become larger as the leaf develops. The entire plant may become riddled with holes under moderate to heavy populations. Larvae also feed in the developing heads of cabbage, causing deformed heads and encouraging soft rots. The pupae of DBM are green and encased in a netlike cocoon attached to the foliage. Pupae reduce quality as a contaminant.
DBM attacks all types of leafy greens and cole crops during all parts of the growing season. DBM is a cold-hardy species, so it can survive cold temperatures in the caterpillar stage. During temperatures below approximately 50°F, larvae cease to feed. As the temperature rises above this mark, feeding resumes. The life cycle is retarded during cooler temperatures. In contrast to this, DBM populations may increase dramatically at temperatures above approximately 80°F. The life cycle may be as long as 50 days at low temperatures and as short as 15 to 20 days during high temperatures. There may be 10 or more generations during warm years.
Suppression of DBM populations with preventive treatments is the most efficient control method. Preventive treatments with biological compounds should be made on a five-day interval. A seven-day interval may be used if no worms are found, especially during cool winter weather. Cleanup sprays may be necessary periodically. Heavy rain showers may reduce populations dramatically. Monitor crops two to three times per week and make decisions on changes in control strategies.