Joanne Whalen, Extension IPM Specialist; jwhalen@udel.edu
Alfalfa
Be sure to watch for an increase in pea aphid and alfalfa weevil populations. When sampling for aphids and weevils, collect a minimum of 30 random stems throughout a field and place them top first in a white bucket. For aphids, you want to count the number present per plant as well as any that have dislodged from the stem into the bucket. As a general guideline, you should consider a treatment in alfalfa less than 10 inches tall if you find 40-50 aphids per stem. The treatment threshold for alfalfa 10 inches or taller in height is 75-100 per stem. Although beneficial insects can help to crash aphid populations, the recent cool temperatures has slowed their activity. As a general rule, you need one beneficial insect per every 50-100 aphids to help crash populations. For alfalfa weevil, you will want to record the number of weevil larvae per stem. The following thresholds, based on the height of the alfalfa, should be used as a guideline when making a treatment decision: up to 11 inches tall – 0.7 per stem; 12 inches tall – 1.0 per stem; 13–15 inches tall – 1.5 per stem; 16 inches tall – 2.0 per stem and 17–18 inches tall – 2.5 per stem.
Field Corn
As a result of the cool spring temperatures, we have not seen the early slug egg hatch that we saw in 2012. However, with the predicted warm weather next week we will start to see an increase in egg hatch. Two new videos on slug management in field corn and soybean were posted in Plant Management Network News. These videos provide a good review of slug biology and management and can be accessed at
http://www.plantmanagementnetwork.org/edcenter/seminars/soybean/SlugManagementSoybean/
and http://www.plantmanagementnetwork.org/edcenter/seminars/corn/slugmanagementcorn/
Small Grains
We have had a few reports of winter grain mites being found in no-till small grains fields. The second generation develops from eggs laid by the first generation, and populations peak in March and April. Adult and immature mites feed on the plant’s chlorophyll giving plants silver or frosted appearance. Severe feeding may result in dead or stunted plants, and it has the potential to significantly reduce yield. Young plants are more susceptible to the feeding damage than older, more robust plants. Plants that are either drought stressed or nutrient deficient also exhibit more severe feeding damage than unstressed plants. More information on identification, sampling and management can be found at the following link: http://extension.udel.edu/factsheet/winter-grain-mite-management-in-small-grains/
Timothy
Be sure to watch for an increase in cereal rust mites which are favored by cool temperatures. Symptoms can appear as retarded growth, leaf curling, stunting, and plant discoloration. Injured plants appear to be drought stressed even when adequate moisture is available for plant growth. There are no established economic thresholds for the pest; however, treatment is recommended in fields with a previous history of cereal rust mites and/or when 25% of the plant tillers exhibit curled tips of the new leaf blades within several weeks following green-up. The use of a 20x-magnifying lens is often necessary to find mites on leaves. The only effective and labeled material on timothy is Sevin XLR Plus. Be sure to read the label for information on the number of applications per season as well as the days to harvest. For effective rust mite control, the use of the higher labeled rate and at least 25 gal/acre of carrier to get good coverage of leaf surfaces generally results in better control.
http://extension.udel.edu/factsheet/cereal-rust-mite-in-timothy/