Joanne Whalen, Extension IPM Specialist; jwhalen@udel.edu
Alfalfa
Continue to scout fields for both alfalfa weevil and pea aphids. Under dry weather conditions, you may need to reduce the following thresholds, especially when both insects are present in a field. 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. Beneficial insects can help to crash aphid populations and 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; 17 – 18 inches tall – 2.5 per stem.
Small Grains
It appears that we could see head emergence in some barley fields by next week. Once grain heads have emerged, you should begin sampling for grass sawfly and armyworm larvae. Although we can see economic damage from local overwintering armyworm populations, we often see significant outbreaks in years when moths coming from the south migrate to our area. You can look at the following link from Kentucky that compares their moth flights this year compared to 2006 & 2008 – which they consider outbreak years. (http://www.uky.edu/Ag/IPMPrinceton/counts/taw/tawgraph.htm ). Currently moth populations are low in their traps but this week of warm weather could cause a significant increase in moth activity. Armyworm larvae are nocturnal so look for larvae at the base of the plants during the day. As a general guideline, a treatment should be considered if you find one armyworm per foot of row for barley and 1-2 per foot of row for wheat. Although armyworms initially feed in lower canopy on the leaves, under drought conditions we could see them quickly moving to the heads. In addition, barley is more susceptible to damage and early head clipping so be sure to scout carefully for armyworm. Since sawflies feed on the plants during the day, small sawfly larvae can often be detected early using a sweep net. However, there is no threshold for sweep net samples. Once sawfly larvae are detected, sample for larvae in 5 foot of row inner space in 5-10 locations in a field to make a treatment decision. You will need to shake the plants to dislodge sawfly larvae that feed on the plants during the day. As a guideline, a treatment should be applied when you find 2 larvae per 5 foot of row inner space or 0.4 larvae per foot of row. If armyworms and sawflies are present in the same field, the threshold for each should be reduced by one-half. The higher rates of insecticides are needed for grass sawfly control.
Continue to watch for winter grain mite activity in no-till wheat fields, especially in fields planted into corn stubble. Although the current summer-like conditions are not favorable for this mite, the predicted cooler temperatures this weekend are favorable for increases in populations. Remember that 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. Heavy spring infestation can result in reduced yields, so be sure to check for mites if fields appear off color.
Timothy
Cereal rust mites remain active in fields so if you have not checked for this pest, be sure to sample all fields. 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.