Joanne Whalen, Extension IPM Specialist; jwhalen@udel.edu
Alfalfa
Continue to scout fields for both alfalfa weevil and pea aphids. Economic levels of alfalfa weevil larvae and pea aphids can both be found in alfalfa fields at this time. 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 cooler temperatures have slowed their activity. As a general rule, you need one beneficial insect per every 50-100 aphids to help crash populations. For alfalfa weevil, 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.
Small Grains
With the cool rainy weather this past week and the anticipated warmer weather, be sure to watch for increases in aphid populations. Based on research done in VA in past years, they found that small grains can tolerate a lot of feeding, especially lower in the canopy. As a general guideline, the treatment threshold for aphids in wheat and barley over a foot tall is 300 aphids per foot of row. Since we are past the time of barley yellow dwarf transmission (fall transmission is the most important), the next important time to consider aphid management in small grains is at grain head emergence. Since aphids feeding in the heads of small grains can result in a loss in test weight, it is important to look for aphids as soon as the grain heads emerge. As a general guideline, a treatment should be considered if you find 20 aphids per head and beneficial insect activity is low. Although beneficial insects can help to crash aphid populations, the cooler temperatures have slowed their activity. As a general rule, you need one beneficial insect per every 50-100 aphids to help crash populations.
We have also seen an increase in winter grain mite activity in no-till wheat fields. As a reminder, this is a cool weather mite so be sure to watch fields for this pest. These mites do not cause the yellowing characteristic of spider mite feeding. Heavily infested fields appear grayish or silvery, a result of the removal of plant chlorophyll by mite feeding. When high infestations feed on the plants for several days, the tips of the leaves exhibit a scorched appearance and then turn brown. Many of the infested plants do not die, but become stunted and produce little forage or grain; damage on young plants, however, is more severe than on large, healthy ones. Damage may also be greater in plants stressed by nutrient deficiencies or drought conditions. There are two types of damage to the small grains, namely, reduced amount of forage throughout the winter and reduced yields of grain in the spring. For pictures of this mite and more information, please refer to our webpage (http://ag.udel.edu/extension/IPM/index.html).
Timothy
Another cool weather mite species that feeds on timothy is the cereal rust mite. If you have not checked fields for this pest, be sure to sample fields since they are easily found in timothy fields at this time. 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. As indicated in past newsletters, these mites are microscopic, so the use of a 20x-magnifying lens is often needed to detect the mites. The only effective and labeled material on timothy is Sevin XLR Plus under a 24(c) label (special local needs label).The following is a link to the 24(c) label for Delaware (http://www.cdms.net/ldat/ld332028.pdf). You must have this label in your possession at the time of application.