Agronomic Crop Insect Scouting

David Owens, Extension Entomologist, owensd@udel.edu

Early Season Moth Activity
Cutworm numbers have come up a bit in the Seaford trap. Armyworm flight continues to be low, and should not improve this week. Trap counts for the week are as follows, with thanks to Joanne Whalen, Emily Zobel, and Maegan Perdue.

Small Grain
Wheat fungicides are going out soon, and many folks think it is a convenient time to add a pyrethroid. The thinking is that it would save a future trip across the field. At this point, the only pest that we would be concerned about between now and harvest is true armyworm. This year, activity has been fairly low, and I have not heard of any reports of more than the rare worm showing up in fields. Generally, a pyrethroid spray is only going to have a week or so residual activity. So, would a cheap pyrethroid pay for itself? I wish I had a way to tease out 1/5^th of a bushel in research plots, but it washes out with plot variability, let alone the insects. One critter we will hit is brown stink bug, which some folks believe is harder to kill in other crops with pyrethroids because they often get exposed to tank-mixed pyrethroids. Unless a scouting report indicates that worms or cereal leaf beetle are present, it may add up over enough acres to hold off on the tank-mix.

Corn and Soybean
With the cool weather pattern we are in, it may be a good idea to scout fields for slug activity. There is a good video of Bill Cissel explaining slug sampling with roofing shingles (https://youtu.be/-5YD2BArGOg). There is nothing magical about shingles; corrugated cardboard that is weighed down a little bit would also work. The key is to look at your refuge trap early in the morning before the sun gets hot and heats up the covering.

We do not have any good thresholds for slugs, in part because their activity is so weather dependent. I prefer to think of it as ‘population of concern’, and is generally thought to be around 3 per square foot. In Delaware we have 2 common species of slugs – marsh slugs and gray gardens. There may be damage potential differences between the two, but I have seen fields require replanting from both. I have also seen fields with very high slug populations, much above the 3/ft and not have any significant damage because of warm weather and warm soils promoting rapid seedling growth. If soil is warm, weather is forecast to be warm, sunny, low humidity, or windy, that will limit slug activity and promote soybean growth.

If Deadline is going to be used as a rescue treatment, you need to keep a sharp eye out at emergence, scouting fields probably twice a week. I have seen situations where a deadline application was made, but perhaps a bit too late and the field was replanted anyway. By the time the field was replanted, the temperatures were 15 degrees warmer. So what worked? The deadline or just delaying things until better planting conditions? It’s hard to know, especially when the 3-10 day forecasts don’t seem to be that reliable this time of year.

Soybeans also have really good compensatory ability to recover from stand loss. As long as their unifoliates are out and the trifoliates are coming on, its difficult (but not impossible) for slugs to kill them. I have also seen corn take a lot of feeding and grow out of it, but have heard many horror stories of corn replanting too.

Our best management for slugs includes reducing insecticide useage as much as possible, soil disturbance, and promoting good growth conditions. Ground beetles are the most important slug predators and are harmed if a pyrethroid is included with cover crop burndown sprays or post herbicides. Right now, pest moth flight seems to be fairly low. Neonic seed treatments can also be detrimental to ground beetles, although it is much harder to find corn without it. Using row cleaners, getting good seed slot closure, and strip tillage will also help promote seedling growth. Joanne Whalen did a study a few years back that suggested that vertical tillage could also help reduce slugs. Tillage is by far the most effective slug management tool. John Tooker at Penn State has been doing a lot of work with planting into a green cover crop for slug management. There are also cover crops that slugs REALLY like, including legumes and brassicas. Brassicas may actually promote a slug population and it may be well to avoid putting brassica cover crops in sluggy fields.