Corn earworm and pyrethroid susceptibility, soybean aphid, and Mexican bean beetle update
CORN EARWORM: Moths are continuing to exhibit high levels of survivorship in our adult pyrethroid vial tests (see the attached line graph). We are striving to test several hundred moths each week as we feel this is important information for growers. As of this week, we are hearing of some fields where growers are getting less than satisfactory results with their pyrethroid sprays.
Earworms are reported to be in peanut, cotton and soybean fields in our area. It is time for folks to begin scouting throughout the state. In areas that are mostly dry, the problem is worse. In wetter areas, numbers are slower to build. The moth flight is increasing gradually, but has still not peaked. Rains are keeping the corn green, which tends to hold them in the corn, and in general, moth activity is less during rainy periods. Moth activity is higher in dry areas. If things dry out over the next two weeks, we can expect a large flight. But if it stays wet, the moth flight and subsequent earworm numbers will be suppressed.
SOYBEAN APHID: Weather conditions (generally cool in the low to mid 80s/wet) are nearly perfect for soybean aphid survival in some parts of the state and consequently, we are getting reports of high numbers in some areas. We will provide more detail next week, but as of today, many fields in the Gloucester/Mathews area, the Goochland area and the Shenandoah Valley have populations. I recently spoke with one of the entomologists in the north central states who was part of the large effort to develop the soybean aphid thresholds and here are his main ‘talking points’. Thresholds only apply up to the R5 growth stage. After that, beans can withstand 1000s per plant with no detectable problem. From R1-up to R5, the threshold is 250/plant IF POPULATIONS ARE INCREASING. Even maintaining at 250/plant does not result in a yield loss. An insecticide application at 250, when populations are increasing, prevents that increase. This is an important concept to be aware of. It takes consecutive field visits and good record keeping to determine if the population is holding, increasing, or decreasing due to predation by predators (lady beetles for example). He also warned that tank mixing an aphid insecticide (a pyrethroid) with a herbicide will only provide aphid control if the spray system is set up for high volume (10 + gpa) and small droplet size. The large droplet systems used for herbicides will not work well for aphids. Hollow cone spray patterns with small droplet size are needed to get product to circulate throughout the soybean canopy, to the undersides of leaves where aphids are feeding.
MEXICAN BEAN BEETLE: Prior to when I came to Virginia (in Aug 1988) Mexican bean beetle had been a major pest of soybeans. For a number of debatable reasons, in the years just before I arrived populations crashed. Since then I have seen them a total of two times in isolated areas, only in years and areas where it stayed wet and cool (relatively) all summer. Galen Dively in Maryland reported today that they are seeing a few outbreaks on the Maryland Eastern Shore. As cool and wet as it is in some parts of Virginia, it would not surprise me to hear of a few outbreaks. Keep a watch out for them. They do the worst damage when they go undetected in the first couple of generations. They gradually build to higher and higher numbers until the last generation, and in what appears to be ‘overnight’ they can totally defoliate a soybean field&not a pretty sight.
Additional information: cew-cyper-aug-13-09-herbert-ppt