Joanne Whalen, Extension IPM Specialist; jwhalen@udel.edu
Alfalfa
Continue to sample for potato leafhoppers on a weekly basis. We continue to find adults and nymphs in fields. 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.
Field Corn
We are continuing our stink bug survey of corn fields next to wheat to evaluate movement of stink bugs from wheat into corn. Just this past week, we have found a few stink bugs in corn along field edges adjacent to wheat. The following information developed in the south (North Carolina and Georgia) should provide guidance for management in our area:
(a) Until the V6 stage, the economic threshold is four stink bugs per plant.
(b) Corn is most susceptible to stink bug injury during ear formation before the tassel stage (VT). Bugs feed through the sheath, causing a dead spot on the ear. As the ear expands it becomes distorted and curves usually outward.
Feeding during silking and pollen shed (R1) will also kill kernels on the ear. Once the ear has elongated, stink bug feeding during the blister and milk stages can blast individual kernels usually causing them to abort.
(c) When the ear is forming, during ear elongation, and during pollen shed, the treatment threshold used in the south is one stink bug per four plants (25% infested plants).
(d) From the end of pollen shed to blister/milk stage, the threshold is one stink bug for every two plants (50% infested plants).
Soybeans
Be sure to sample seedling stage beans for bean leaf beetles, grasshoppers, and thrips.
(I) Grasshoppers: Population levels are starting to increase in a few of the earliest emerged no-till full season fields. As barley is harvested and soybeans are planted, these fields will be especially susceptible to attack by grasshoppers which can 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 maybe needed if you find one grasshopper per sweep and 30% defoliation from plant emergence through the pre-bloom stage.
(II) Bean Leaf Beetle: 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. These treatment thresholds should be reduced if bean pod mottle virus is present in your area or you suspected virus the previous season.
(III) Thrips: At this time, soybean thrips and other thrips species are present in seedling stage fields. Thrips can feed and reproduce on the leaves and buds of soybean seedlings. Their feeding creates bleached-out lesions along the leaf veins and gives a silvery/bronzed appearance to the leaf surface when damage is severe. These insects are very small (less than 1/10 inch) and are torpedo shaped. While thrips always occur on seedling stage soybeans, it is only during outbreak years that they cause concern. In particular, during dry weather and on earlier planted full-season soybeans, thrips populations can explode when plants are growing slowly. Under these circumstances thrips injury will occasionally kill seedlings. Other stressors, such as nutrient deficiencies and herbicide injury, can add to thrips damage and cause plant loss. Yellowing can occur from thrips but there are also a number of other factors that can cause yellowing so it is important to scout fields to identify what is causing the yellowing. Although no precise thresholds are available, as a general guideline, treatment may be needed if you find 4-8 thrips per leaflet and plant damage is observed.
We are also seeing slugs attacking seedling stage soybeans, especially in areas with heavy cover. Slugs are more difficult to manage in soybeans because the damage can occur below the ground before plants emerge. Damage to soybean can be more severe than damage to corn because the plant’s growing point is within the emerging cotyledons. If soybean plants are able to emerge, the plant may be able to send out the unifoliate leaves where slug feeding will be noticeable. In 2012, there were a number of fields were Deadline MPs were applied and provided effective control when fields were carefully scouted , plants were just emerging/ or emerged and before there was significant feeding on the growing point.