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United States Department of Agriculture

Agricultural Research Service

Research Project: BT RISK ASSESSMENT FOR LEPIDOPTEROUS PESTS OF COTTON

Location: Southern Insect Management Research Unit

2013 Annual Report


1a.Objectives (from AD-416):
The objective of this cooperative research project is to conduct risk assessment research for Lepidopterous pests of Bt-crops. This research should enhance Bt-resistance management strategies which are designed to delay the onset of resistance development in target insects.


1b.Approach (from AD-416):
This agreement will determine the effect of Bt crop production on the population genetics of bollworm, tobacco budworm, and fall armyworm. Specific issues will be:.
1)a better overall understanding of gene flow and population structure for the pests,.
2)Bt-resistance allelic frequency estimates over time,.
3)the impact of changing refuge strategies and dynamic agroecosystems on managing resistance to Bt, and,.
4)the impact of Bt-suppressed population densities on insecticide resistance, e.g. the recent pyrethroid resistance in bollworm. The cooperator will be actively involved in all phases of this research including the collection of test insects from across the U. S. Cotton Belt. Pyrethroid resistance assays will be conducted in cooperators laboratory. Insect tissue will then be sent to the USDA-ARS for use in genetic marker analysis and carbon isotope analysis. Other technologies (e.g. secondary plant chemical detection in insect tissue and oxygen and nitrogen isotope analysis) will be used as they become available to further understand the population ecology of the pests in relation to Bt resistance management.


3.Progress Report:

Bollworm larvae were collected from Non-Bacillus thurigiensis (Bt) and VT3P field corn in 2011 and 2012. Pupal duration and pupal weights were determined for the parental generation. Backcrosses and reciprocal crosses were made and the offspring neonates were subjected to dose mortality bioassays on lyophilized Bollgard II cotton tissue. Male bollworm larvae collected from VT3P field corn had a longer pupal duration compared to males collected from Non-Bt field corn. Female pupal duration was not significantly different for individuals collected from non-Bt field corn and VT3P field corn. Populations collected from VT3P field corn had higher pupal weights than larvae collected from Non-Bt field corn. In 2011, progeny from females reared on VT3P field corn had a higher LC50 compared to progeny resulting from females reared on Non-Bt field corn regardless of paternal host. In 2012, progeny from all reciprocal crosses and the backcross larvae of non-Bt had similar LC50 values. The progeny from the VT3P backcross had a higher LC50 value than all other crosses. Based on these results, bollworms collected from VT3P field corn are healthier and more robust than those collected on non-Bt field corn. These results suggest that there are multiple minor genes that influence bollworm survival on Bollgard II cotton. Those genes may be associated with resistance traits, but are most likely influencing overall fitness of larvae that develop on VT3P field corn. Results of these experiments will be important for developing resistance management plans in areas where dual-Bt toxin corn hybrids and dual-Bt toxin cottons are grown in close proximity. An experiment was conducted in 2012 to evaluate the impact of insecticide applications on dual toxin Bt cottons. Non-Bt, Widestrike, and Bollgard II cotton varieties were planted in a split-plot design with 4 replications.Each variety had a sprayed and unsprayed treatment.The sprayed plots were treated, as needed, with Prevathon, Belt, or Tracer to control lepidopteran insect pests.Overall, lepidopteran densities were relatively high in this trial. Overall, insecticide applications on significantly reduced the numbers of damaged squares and bolls on non-Bt cotton, but not on Widestrike or Bollgard II cotton. Insecticide applications targeting lepidopteran pests significantly increased yields of non-Bt and Widestrike cottons. In contrast, no significant differences were observed on Bollgard II cottons. These results demonstrate that dual-gene Bt cottons are not immune to injury from bollworm. As a result, management of bollworms in dual-gene Bt cottons with foliar insecticides may be an important component for resistance management.

An experiment was conducted in 2012 to determine the impact of sequential low rate applications of Prevathon on bollworm injury and yield in Bollgard II cotton. The rates of Prevathon used included 1.5, 3.0, and 6.0 fl oz/A. These treatments were co-applied with a tarnished plant bug application when tarnished plant bugs reached the current economic threshold of 3 per 5 row ft. Each rate was sprayed with either every tarnished plant bug application or every other application and compared to Prevathon and Belt sprayed at their normal use rates (27 fl oz/A for Prevathon and 3 fl oz/A for Belt). A total of six applications were made for tarnished plant bugs in this trial. Therefore, treatments 1-3 were sprayed six times and treatments 4-6 were sprayed three times. Treatments 7 and 8 were sprayed one time when bollworm densities reached threshold. All of the insecticide treatments reduced square and boll injury from bollworm in Bollgard II cotton compared to the unsprayed plots. Applications of 3.0 fl oz/A of Prevathon sprayed six times and 6.0 fl oz/A Prevathon sprayed three and six times provided similar levels of control to that observed with Prevathon sprayed at threshold with 27 fl oz/A. All insecticide treatments resulted in higher yields than untreated Bollgard II. These results suggest that low rates of Prevathon can be sprayed sequentially beginning prior to flowering and reduce the impact of bollworms in Bollgard II cotton. This approach can reduce input costs for growers and may serve as an effective resistance management tool in dual-gene cottons. Low rates of 3-6 fl. oz/A applied throughout the season may result in a third toxin being present in dual-gene cottons throughout the season and further reduce the selection pressure on the Bt genes. These preliminary results will need to be repeated and refined before specific recommendations can be made. A colony of bollworms was collected from Bollgard II cotton and given to Southern Insect Management Research Unit for further testing.

Insecticide resistance in all insect pests of cotton threatens the economic viability of dual gene Bt cottons. Increased costs of foliar insecticides associated with resistant insects compounded with technology fees of dual-gene cottons results in a significant economic cost for growers. Surveys of cotton aphid susceptibility to neonicotinoid insecticides were conducted from 2008 to 2012. Cotton aphids were collected from commercial cotton fields across Mississippi, Louisiana, Arkansas, and Tennessee. Additional populations were tested from Texas and North Carolina. Aphids were collected from fields where control failures with foliar neonicotinoids were reported. Additional collections were made from cotton fields in these states where no foliar applications of neonicotinoid insecticides had been made. Leaf dip bioassays were used to measure their susceptibility to neonicotinoids. Cotton aphids collected from fields where control failures had occurred showed significant levels of resistance each year. Mean LC50 values averaged across all collections increased from 12.75 ppm in 2010 to 60.06 in 2011 and 27.0 in 2012. All of the populations tested in 2012 showed moderate to high levels of resistance to thiamethoxam regardless collection site (sprayed or unsprayed field).

In 2013 experiments evaluating the impact of bollworms on yields of Bollgard II and Widestrike cotton will be continued. In addition collections of bollworms from Bt crops will be continue.


Last Modified: 9/10/2014
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