Joanne Whalen, Extension IPM Specialist; jwhalen@udel.edu
Alfalfa
Begin sampling for alfalfa weevil larvae as soon as you see the first signs of new growth. The following is a link to a picture of an alfalfa weevil larva and damage (http://entomology.k-state.edu/extension/insect-information/crop-pests/alfalfa/alfalfa-weevil.html). With the warm fall, as well as warmer overwintering temperatures, we could start to see economic populations as early as late March. This is a direct result of more egg laying by adult beetles during the fall and early winter. Eggs are laid in the alfalfa stem any time temperatures are above 48°F. Once eggs hatch, weevils can pass through four larval stages in approximately three weeks. When fully grown, the larvae spin a cocoon on the leaves of alfalfa plants or on the ground. A second generation of adult weevils emerges around mid-June, feeds for a few weeks, but it does not produce a second larval generation. During the first visit to a field, examine 5-10 stems for damage and larvae. A full stem sample is not needed until damage or larvae are found on the plants. If leaf feeding is present, randomly collect 30 stems from throughout the field. Grasp stems at the base and place each stem upside down in a bucket. After collecting the stems, separate them into 3 or 4 bundles and beat them against the inside of the bucket to dislodge larvae from the stems. Count and record all larvae found per 30 stems. You will also need to measure 10 of the 30 stems and record the average stem height. 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
In recent weeks there have been reports of low levels of winter grain mites (WGM) in no-till small grain fields. This arthropod pest is most likely to occur in small grains planted no-till into corn stubble. Two generations occur in our area with the second generation peaking in March and April. Temperature and moisture are the two most important factors influencing WGM development and abundance. They are most active when temperatures are between 50 and 60°F. On hot, dry days when temperatures exceed 75°F, the mites stop feeding and will burrow down 4 to 5 inches in the soil profile in search of moisture and to escape the warm temperatures. Egg hatching will also cease when daily temperatures exceed 75°F. For more information on identification, sampling decision making and management, please see the following links:
http://extension.udel.edu/blog/winter-grain-mite-management-in-small-grains/.
http://entomology.k-state.edu/extension/insect-information/crop-pests/wheat/winter-grain-mite.html
As we approach/start to see spring green up, I continue to receive questions about aphids in small grains. Although populations were higher than normal in some fields last fall and into early winter, populations remain low at this point. We are in the last year of an IPM project evaluating the use of “southern thresholds” for fall aphid management as they relate to barley yellow dwarf virus (BYDV) management. We will have more information to share before the fall planting season. In the meantime, there are a couple of things to remember about aphid management as it relates to BYDV management:
(1) Although all aphids found in Delaware small grain fields can move BYDV into a field, not all aphids have the virus and different aphid species are known to vector different strains of the BYDV. Some aphids can vector more severe strains and others only mild/moderate strains. In the last 2 years of our survey, we have only detected low levels of the mild/moderate strain. We will see if that changes this spring.
(2)The most significant damage from BYDV occurs from fall infections, especially during the first 60 days after plant emergence. However, the yield impact also depends on the strain of BYDV, time of infection, aphid species, and varietal tolerance.
So what about direct damage from aphids? Although direct damage from aphids generally does not occur, there are two exceptions:
(1) Greenbug (http://texasinsects.tamu.edu/images/insects/common/images/a-txt/aimg104.html) – This aphid species can cause losses due to direct feeding on young plants. While feeding, it injects a toxic saliva that causes small grains to turn yellow, and heavy feeding damage can cause plants to die. Heavy feeding also causes typical “greenbug spots” in a field. In the centers of the spots you will see dead plants surrounded by living plants that are heavily infested and beginning to turn yellow. This generally occurs in the fall during the first 30 days after plant emergence. We saw very low levels of greenbug last fall. However, warmer overwintering conditions may have favored survival so be sure to look for them this spring.
(2) English Grain Aphid (EGA) (http://soilcropandmore.info/crops/Wheat/Insects/EnglishGrain_adults.jpg)
In the spring, this is the only species of aphid that can be found feeding in the heads of small grains. Damage from EGA feeding in grain heads can result in shrunken kernels and reduced test weight. Heavy infestations have been shown to cause up to 13% yield loss. So be sure to sample fields for EGA as soon as grain heads emerge. The treatment threshold is 15-25 aphids per head with low beneficial activity.
Timothy
Be sure to sample for 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. As a general guideline, 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. (http://www.cdms.net/ldat/ldAK3029.pdf). 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/factsheets/cereal-rust-mite-in-timothy/