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In This Issue:
A) Corn Rootworm Activity on Corn
B) Bean Leaf Beetle Activity on Soybeans
C) Two Spotted Spider Mites on Soybeans
D) Salvaging Drought Damaged Corn
During the past two weeks, we have been evaluating a number of trials for rootworm management on continuous corn. Trials at the Western Branch on continuous corn following a late planted trap crop exhibited minimal rootworm injury. Such results were anticipated since adult rootworm activity in the late planted corn trap crops last year was very low. In contrast, a trial of standard rootworm treatments on continuous corn at the Farm Focus site in Van Wert County generated interesting results with untreated check plots having an average root rating of 3.8, differences were observed among treatments, and adult western corn rootworms were swarming above the tassels.
At the Farm Focus site, a rootworm trial was also conducted on first year corn, but root ratings of untreated checks averaged only 2.2 despite the fact that this area has been documented as an area of first year corn rootworm activity. The site of the continuous corn trial was used for a first year corn trial in 1998. Rootworm injury in the first year corn trial in 1998 was very low, but rootworm injury at the same site the following year in continuous corn was heavy. Our experience to date in conducting trials at the Farm Focus area in Van Wert demonstrates that rootworm activity in first year corn may not be economic despite the presence of first year corn rootworm activity. Rather, rootworm injury in 2nd year corn may be more significant than that traditionally observed under Ohio conditions.
The summer survey of adult corn rootworm activity in soybeans and first year corn was initiated during the past week. Initial visual counts of first year corn survey sites in Marion, Delaware, Union, Madision, Clarke, Darke, Shelby, Mercer, Auglaize, Champaign, Prebble and Miami counties detected relatively low levels of adult corn rootworm activity. The highest visual count of western corn rootworm adults reported in the counties listed was only 0.3 WCR beetles per plant at a Mercer County site and a Darke County site. Sweep net sampling of soybeans detected no western corn rootworm activity in the soybeans, but occasionally a single adult northern corn rootworm was collected. Results of the summer survey for rootworm will be periodically reported as reports of field observations are reported during the forthcoming weeks.
First generation adult bean leaf beetle (BLB) activity in central Ohio soybeans is higher than normal based on sweep net collections which ranged from 0.5 to 3.0 adult BLB per sweep on soybeans that are predominantly in the early bloom stage of development. Defoliation of fields sampled in central Ohio ranged from about 3 to 10%, but the average level of defoliation is less than 5%. The level of BLB activity observed to date is unlikely to cause significant defoliation to warrant rescue treatment, but does indicate that a significant potential exists for 2nd generation BLB pod injury problems in the late summer if BLB population activity continues to multiply this summer. This year, the emergence of significant over-wintering generation in the spring combined with early soybean plantings and a hot summer has generated a significant level of first generation BLB activity. If warm weather leads to heavy 2nd generation BLB activity, then pod feeding problems are predicted, especially if moist conditions develop in the late summer favoring development of seed mold in insect damaged pods.
Southern and eastern regions of Ohio experiencing severe draught conditions continue to have problems with two spotted spider mites (TSSM). A report was recently received that some no-till fields planted into clover in Defiance County have been recently treated. The geographic distribution of the spider mite problem is almost the opposite of regions impacted during the 1988 TSSM outbreak which primarily impacted northwest Ohio. Problem fields may likely be found anywhere in the State if one finds soybean fields under draught stress that have a habitat that would have enhanced TSSM establishment.
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Spider mite damage to soybean leaves. |
In early June, I reported a case of TSSM we had in our experimental area at the Western Branch station. Although a set of treatments was applied to the infested soybean blocks, TSSM activity declined in both the treated and untreated plots until the past week when we observed a resurgence of TSSM activity. However, other blocks on the station do not appear to have a mite problem. This case demonstrates that once TSSM is established in a field, it may decline and then build up again as draught conditions favor spider mite development. In such situations, it is important that soybean foliage injury be monitored closely and inspected for presence of spider mites. Where TSSM is a problem, discoloration of foliage will be evident, TSSM feeding sites will exhibit speckling, and all stages of mites (adults, nymphs, and eggs) will be readily detected by inspecting the underside of impacted foliage with a 10X hand lens.
In a coffee shop conversation this past week with some ag-service dealers, the question was raised about a threshold for TSSM. The question was primarily targeted at whether there was a magic number of mites that may appear on a sheet of paper used to collect mites shaken from soybean foliage. My response was basically that a decision to treat should be based on a combination of factors, namely the following:
In the 1988 TSSM outbreak, the problem peaked in late July and early August. With the onset of cooler weather in early August, the TSSM infestations rapidly declined. Thus, it is very important to note the presence of mites, especially the immature stages which indicate the trend of population development.
Field Crops Pest Management Circular # 24 Twospotted Spider Mite
For those of you confronted with questions on salvaging drought damaged corn, I'd suggest you consider using a publication from the National Corn Handbook entitled "Utilizing Drought-Damaged Corn" (NCH-58). Some of you may be familiar with this publication from 1988. It's available via the web at the following address-
http://www.agcom.purdue.edu/AgCom/Pubs/NCH/NCH-58.html
This publication provides an overview of drought stress effects on corn and describes alternative uses for drought damaged corn, i.e. using it for green chop and silage. Hard copies of NCH-58 will be available on request.
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C.O.R.N. is a summary of crop observations, related information, and appropriate recommendations for Ohio Crop Producers and Industry. C.O.R.N. is produced by the Ohio State University Extension Agronomy Team, State Specialists at The Ohio State University and Ohio Agricultural Research and Development Center. C.O.R.N. Questions are directed to State Specialists, Extension Associates, and Agents associated with Ohio State University Extension and the Ohio Agricultural Research and Development Center at The Ohio State University.
Contributors to C.O.R.N. this week include:
Contributors to C.O.R.N. this week include: State Specialists: Peter Thomison(Corn Production), Anne Dorrance (Plant Pathology), Hal Willson (Entomology), Ron Hammond (Entomology); District Specialists: Jim Jasinski (Southwest District, Entomology); Barry Ward (Champaign), Steve Prochaska (Crawford), Dennis Baker (Darke), Gary Wilson (Hancock), Greg LaBarge (Fulton), Clark Hutson (Seneca), Roger Bender (Shelby), and Andy Kleinschmidt (Van Wert)
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