Agronomic Crop Insects – June 4, 2010

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

Alfalfa
Continue to sample for potato leafhoppers on a weekly basis. Although adults are the main life stage present, we are starting to see the first nymphs. Although both life stages can damage alfalfa, the nymphs can cause damage very quickly. Once plants are yellow, yield loss has already occurred. The treatment thresholds 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.

Although we do not have any local research on potato leafhopper thresholds for fields planted with the glandular haired varieties, here is some information from Ron Hammond from Ohio regarding treatment thresholds on these varieties:

“If the alfalfa is one of the glandular-haired, leafhopper-resistant varieties of alfalfa, the economic threshold is three leafhoppers per inch of growth (24 leafhoppers for 8” tall alfalfa, for example). However, if the resistant alfalfa is a new planting this spring, growers might want to use thresholds meant for regular alfalfa during the very first growth from seeding. Because its resistance improves as the seedling stand develops, research suggests that the threshold for a resistant variety can be increased to 3 times the normal level after its first cutting.

“More information on potato leafhopper, including how alfalfa growing conditions might affect the threshold, is available at http://ohioline.osu.edu/ent-fact/pdf/0033.pdf.”

Field Corn
We continue to receive reports of larger cutworms feeding below the ground and causing plant lodging and wilting. It can be difficult to achieve control when cutworms are feeding below the soil surface and in some fields worms are large (greater than one inch long). This below ground feeding is generally done by cutworms that range in size from ½ inch to 2 inches in length (4th instars and later instars). If you can find 3% of plants damaged (cut or tunneled), larvae are one inch or less in length, and corn growth stage is between plant emergence and the 6th leaf stage, controls may still be needed. However, when damage is occurring below the soil surface ( plants appear tunneled) it is 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. For information on cutworm biology and management, please refer to the following fact sheet from Ohio
http://ohioline.osu.edu/ent-fact/pdf/0035.pdf

Small Grains
We continue to see economic levels of armyworms in both wheat and barley that did not receive an earlier insecticide treatment. As indicated in past newsletters, damage can quickly occur in barley. In many cases there is a mixture of worm sizes so the potential for head clipping is high, especially in barley. Be sure to read the label before applying any insecticide for the rate, days between last application and harvest and other restrictions. Control options are limited at this late date due to pre-harvest intervals.

Soybeans
Continue to sample for bean leaf beetles and grasshoppers. In the earliest planted and emerged fields, we have started to see an increase in activity for both insects. As barley is harvested and soybeans are planted, these fields will be especially susceptible to attack and grasshopper feeding can often cause stand loss. If stand reductions are occurring from plant emergence to the second trifoliate, a treatment should be applied. Although no precise thresholds are available, a treatment may be needed if you find one grasshopper per sweep and 30% defoliation from plant emergence through the pre-bloom stage. As a general guideline, a treatment may be needed for bean leaf beetle if you observe a 20 – 25% stand reduction and/or 2 beetles per plant from cotyledon to the second trifoliate stages.

Tags: ,

Comments are closed.