Agronomic Crop Insects – April 15, 2011

Joanne Whalen, Extension IPM Specialist;

Continue to scout fields for both alfalfa weevil and pea aphids. 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
Aphids and cereal leaf beetles can still be found in fields throughout the state. With the cool rainy weather this past week, be sure to watch for increases in aphid populations, especially if we get a brief warm up. In general, small grains can tolerate a fair amount of feeding in the spring, especially lower in the canopy. As a general guideline, the treatment threshold for aphids in wheat and barley over a foot tall is 150 aphids per foot of row with low beneficial insect activity. 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. Cereal leaf beetle adults and eggs can be found in fields throughout the state; however, cooler temperatures have slowed development. There are reports of larvae being found in fields in Virginia and we can expect to see larvae as soon as temperatures increase.

Continue to watch for winter grain mite activity in no-till wheat fields, especially in fields planted into corn stubble. The recent cool weather has been favorable for increases in mite 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.

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. If you are familiar with this microscopic mite species, 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.


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