Agronomic Crop Insects – June 15, 2012

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

Alfalfa
Continue to sample for potato leafhoppers on a weekly basis. We can find both adults and nymphs in fields at this time. 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
In recent years, we have seen an increase in native brown stinkbug damage to developing corn ears, especially when fields are adjacent to wheat fields. Remember, corn is most susceptible to this type of 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. In the last 2 years, we have also seen kernel damage, not distorted ears, on the edges of corn fields resulting from Brown Marmorated Stink bug (BMSB) feeding. We are continuing to survey fields to evaluate the extent of the damage from all species this season. This past week we have observed low levels of native brown stink bugs present in whorl stage corn. We also found the first BMSB eggs on corn in New Castle County.

We also observed and received a few reports of heavy aggregations of Japanese beetle adults in the whorls of field corn in New Castle County. Other areas of the country are also reporting earlier than normal emergence of beetles. Currently, they do not appear to be feeding on the plants. As a general rule, whorl stage corn is very tolerant to defoliation. The following link provides information on the potential for yield loss in bushels per acre from whorl stage defoliation.

http://www.caes.uga.edu/commodities/fieldcrops/gagrains/documents/Buntin_InsectControlinFieldCorn.pdf

As we indicated earlier this season, we were able to confirm the presence of Western Bean Cutworm moths in pheromone traps for the first time in 2011. The counts were extremely low and no damage was observed to corn ears, but this is a pest we will need to watch for in both field corn and sweet corn. Adult moths generally fly in mid-summer (we could see earlier flights in 2012) and females lay eggs on the upper surfaces of corn leaves. Unlike black cutworm that feeds on seedling stage corn, this is a later season corn pest, which feeds on tassels, silks, and developing kernels and can cause severe damage. Factors that contribute to the risk of potential problems include: (a) sandy soils, (b) a high percentage of acres in reduced and no-till production, (c) high humidity, and (d) presence of multiple host crops. Since these conditions fit Delaware, we will need to watch and see if this insect becomes a serious pest over the next couple of years. We have expanded our survey this year and will keep you updated if we see an increase in populations.

Soybeans
Be sure to sample fields for bean leaf beetles, potato leafhoppers, thrips, grasshoppers, green cloverworm and spider mites. Grasshopper populations have increased, especially in no-till fields. As barley and wheat are harvested and soybeans are planted, these fields will be 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. Numerous products are labeled for grasshopper control including a number of pyrethroids, dimethoate, Lorsban (chlorpyrifos), Orthene 97 (acephate) and Sevin XLR (carbaryl). Be sure to check all labels carefully before combining insecticides and herbicides since there are a number of restrictions on the labels.

Continue to watch carefully for spider mites. Be sure to scout the entire field for mites since windy conditions can result in mites being found throughout a field. Labeled materials include dimethoate, Lorsban, Hero (zeta-cypermethrin + bifenthrin) as well as a number of stand-alone bifenthrin products. All of these products need to be applied before mites explode. Be sure to read the labels for use rates and restrictions – including but not limited to combinations with herbicides, number of applications as well as the time between applications.

For those who read Weekly Crop Update, you know that the presence of soybean vein necrosis virus was confirmed for the first time in 2011 in Delaware soybean fields. The following link provides pictures and more information on this virus. http://agdev.anr.udel.edu/weeklycropupdate/?p=3727 . Although we do know that thrips transmit this virus, we do not know which species transmit the disease or when transmission occurs. Work is being done in the Midwest to identify the thrips vectors and possible other hosts of the virus that may harbor it and allow thrips feeding to move it to soybeans. In 2011, it did not appear that yield loss occurred; however, more information is needed to determine if it will cause yield reductions or lower seed quality in our area. In 2012, we will include thrips monitoring in our soybean surveys as well as look for virus symptoms.

By now, most growers should also be aware of another potential new soybean insect pest, the Kudzu bug. It has not been found in Delaware as of this date. In 2011, this bug was found as far north as one southern county in Virginia near the North Carolina border. It was first found in Georgia in 2009 and within two years made its way through North and South Carolina. We have received funding from the Delaware Soybean Board to survey for this pest so we will be able to alert you if it makes it to Delaware in 2012.

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