Agronomic Crop Insect Scouting

David Owens, Extension Entomologist, owensd@udel.edu

This is the last week that we will be maintaining the true armyworm and black cutworm traps. Many thanks to the folks that let us put traps up in their fields and to Maegan Perdue, Emily Zobel, and Joanne Whalen for checking traps.

Counts are as follows:

Slugs
There are reports of corn fields that have been treated for slugs or should be treated for slugs. Carefully watch the plants the next few days. If plants start going ‘backwards,’ a treatment may be necessary. While slugs generally do not like dry windy conditions, they retreat underground and in open seed slots and can cause unobserved damage. This also makes scouting for their presence a bit trickier. Any rain we might get will be favorable to both crop and slug, and if cool conditions persist, probably more favorable to the slug. There are two chemical options that have shown promise: UAN and Deadline.

Galen Dively (UMD) is putting together a slug control pamphlet and writes that Deadline “is effective both as a nerve poison if ingested at high concentrations and as a dermal irritant that causes slugs to secrete large amounts of mucus, resulting in desiccation. The efficacy of metaldehyde baits is highly dependent on temperature and humidity. Effective control is more likely if the bait is applied following a rain or irrigation that increases slug activity; however, heavy rains following application can lead to control failures because of reduced residual action and increased slug recovery from the water loss effects.”

Deadline M-Ps ‘has longer residual action and contains a slug attractant which presumably increases control efficacy. This product is a pellet-type bait applied on corn as a broadcast or banded over-the-row application at 10 to 40 lbs/acre, respectively. Studies have shown that Deadline Bullet treatments can significantly reduce the number of slugs active at night and the amount of feeding injury. Both rates of 10 and 40 lbs/acre applied at planting provided greater than 90% control of slugs and reduced feeding intensity by 80%. The broadcast application at 40 lbs/acre gave slightly better control but not significantly different from the 10 lbs/acre banded treatment. Because of the high price, the application of 10 lbs. of bait banded over-the-row is the most cost-effective. An early treatment applied at planting or as plants emerge is necessary to prevent feeding injury to the most vulnerable plant stages. However, baits may be ineffective if slugs are feeding below the surface in the seed slot.

Regarding UAN, Joanne Whalen wrote in 2012 that “In years past, 30% UAN applied at night when the plants are dry and there is no wind has resulted in variable levels of success (the rate used in past years was 20 gallons per acre of 30% UAN on corn in the spike to one-leaf stage and the mix was cut 50/50 with water to reduce – but not eliminate — plant injury).”

Efficacy is heavily dependent on slug activity – this is contact only. In an experiment that Galen Dively performed where slug density ranged from 1 to 3 per plant, after 24 hours “spray volumes of 20 gallons/acre consisting of 5, 10, and 20 gallons of the nitrogen formulation reduced slug densities by 48%, 74%, and 81%, respectively. The 10 gallon rate mixed 1:1 with water may be the best choice because of the reduced cost and less risk from phytotoxicity to the corn foliage. Although the cost of a nitrogen spray could be partly offset by the added fertility, it is doubtful whether much of this nitrogen becomes available for plant growth.”

Soybeans
Slugs are not the only potential stand reducer in soybean. The other potentially significant early season feeder is bean leaf beetle. I have not heard of any reports of BLB activity yet, but they can be expected soon. Defoliation thresholds are 40% with 2 beetles per plant.