Agronomic Crop Insects

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

Small Grains
We saw a significant increase in true armyworm catches over the past 10 days, especially in the Dover, Little Creek, Milford and Bridgeville areas. High numbers of moths were caught when you compare the numbers to catches at this same time last year (Table 1).

Table 1. Early Season True Armyworm Black Light Trap Comparisons

 The University of Kentucky is also reporting record numbers (http://www.uky.edu/Ag/kpn/kpn_08/pn080428.htm#armarm and http://www.uky.edu/Ag/kpn/kpn_08/pn080505.htm#armarm).

As Doug Johnson from Kentucky indicated, high moth catches do not always mean we will see a large population of caterpillars. Many other factors, particularly disease, predation and parasitism, can impact how well the eggs and caterpillars survive. However, we may be at increased risk compared to most years, so scouting will be important. Although true armyworms overwinter in our area, we may also be getting migrant moths from the South. Therefore, be sure to scout all small grains for armyworms. Although the combination sprays of fungicides and insecticides have worked in the past to control armyworms, this may be a different year so it will be important to re-check fields after application to be sure you have gotten control, especially those fields that were sprayed on the early side. In addition, barley will be susceptible to attack so both barley and wheat should be checked for armyworms and sawflies.

Alfalfa
If economic levels of alfalfa weevil were present before harvest and your fields were cut instead of sprayed, 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 regrowth can be significant. A stubble treatment will be needed if you find 2 or more weevils per stem and the population levels remain steady. The first potato leafhoppers have also migrated to our area so be sure to sample within a week of first cutting. In addition, new seedings should be watched carefully since leafhoppers can quickly damage these plantings. Once the damage is found, yield loss has already occurred. The treatment thresholds for leafhoppers are 20 per 100 sweeps on alfalfa 3 inches or less in height, 50 per 100 sweeps in 4-6 inch tall alfalfa and 100 per 100 sweeps in 7-11 inch tall alfalfa.

Field Corn
We have had a number of reports of cutworms damaging field corn, especially in no-till fields where an insecticide was not used with the burn down herbicide. In many cases worms have been larger and damage was most likely done by the clay backed cutworm. Since enough degree days have accumulated since we saw peak black cutworm moth catches, we should also start to see black cutworm larvae in fields. Be sure to scout all fields from emergence for cutworms. If soil conditions are dry, you will also start to see cutworms damaging plants below ground – either cutting below ground or causing what we term “drill worm injury” (a photo of this type of injury is available here: http://www.ent.iastate.edu/imagegal/lepidoptera/bcutworm/1658.14cutwormlarva.html). If cutworms are feeding below the soil surface, it will be important to treat as late in the day as possible, direct sprays to the base of the plants, and use at least 30 gallons of water per acre.

We have also had an increase in the number of fields with economic levels of slug damage. Although some have used liquid nitrogen applications to help plants grow ahead of the damage, the use of Deadline M-Ps should be considered if weather remains cool and wet and damage is increasing. Please refer to the May 2 Weekly Crop Update for more information on slug biology and management (http://extension.udel.edu/weeklycropupdate/?p=149).

Lastly, you will also want to watch for true armyworm damage, especially in fields that are planted into a burned-down small grain cover or if volunteer grain was present before planting. With the recent heavy moth flights, these fields may be susceptible to damage. Pyrethroids applied with the burn down herbicides have generally controlled this pest if done close to planting; however, all fields should still be sampled for damage. As a general guideline, a treatment may be needed for armyworms if 25% of the plants are infested with larvae less than one inch long.

Grass Hay Fields
True armyworm moths will also be attracted to grass hay fields so be sure to check those fields as well. Although no thresholds are available, high populations could be found in grass hay fields during the last armyworm outbreak and economic losses occurred in a number of fields. Mustang Max and Warrior are now labeled for armyworm control in grass hay fields. See labels for use rates and restrictions. http://www.cdms.net/LDat/ld8G1002.pdf and http://www.cdms.net/LDat/ld5JH041.pdf