Agronomic Crop Insects – April 30, 2010

Joanne Whalen, Extension IPM Specialist; jwhalen@udel.edu

Alfalfa
If economic levels of alfalfa weevil were present before harvest and you decide to cut instead of spray, be sure to check fields within one week of cutting for damage to the regrowth. If temperatures remain cool after cutting, there is often not enough “stubble heat” to control populations with early cutting. In some cases, damage to re-growth can be significant. A stubble treatment will be needed if you find 2 or more weevils per stem and the population levels remain steady.

Small Grains
During the past week, there has been a significant increase in populations of cereal leaf beetle larvae, especially in areas where adult egg laying has been abundant over the last few weeks. In many cases, larvae were very small (about the size of a pin-head) early in the week; however, they will quickly increase in size with the predicted warm temperatures. In addition, they can be found throughout the plant canopy so you need to look at the entire plant when sampling. Damage can occur quickly under these conditions so be sure to scout carefully for cereal leaf beetle larvae. The treatment threshold is 25 eggs and/or small larvae total per 100 tillers.

With the predicted warmer temperatures, we will also see an increase in true armyworm catches. Although true armyworms overwinter in our area, we can also get migrant moths from the South. Therefore, be sure to scout all small grains for armyworms at this time. Remember, on barley, head clipping can occur in a relatively short time. As a general guideline, the threshold for armyworms in barley is one per foot of row and for wheat one-two per foot of row.

In addition to armyworms, do not forget to watch for sawflies since larvae can be found in fields throughout the state. As a review, adult sawflies generally emerge in early April and begin to lay eggs in the leaf margins of small grains. Most egg laying is complete by early May but can be delayed by cooler temperatures. The first small larvae generally feed on the lower leaf blades and larval development takes approximately 21-30 days. Barley and wheat are both damaged by sawflies; however, during years of high population pressure, barley may experience more damage. Sawfly larvae prefer to feed on the stems and can be more damaging than armyworms. Stem clipping often occurs before leaf feeding is complete and/or the grain reaches physiological maturity. Since sawflies can clip heads quickly, be sure to scout carefully for larvae and watch closely for clipped heads. As a guideline, a treatment should be applied for sawflies when you find 2 larvae per 5 foot of row innerspace or 0.4 larvae per foot of row. However, remember if the number of clipped heads is twice the worm count for sawflies then it may be too late to treat for them.

Since aphids feeding in the heads of small grains can result in a loss in test weight, be sure to watch for movement of aphids into the grain heads. As a general guideline, a treatment should be considered if you find 20 aphids per head and beneficial insect activity is low. You need at least one beneficial insect per every 50-100 aphids to help crash populations.

Lastly, before treating for any insect be sure to check the days between last application and harvest when selecting a spray material.

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