Viral Bioinsecticide for Control of Corn Earworm in Soybeans
According to insect loss surveys for soybeans in the Midsouth, economic losses attributed to insects (lost yield + insect management costs) during the 2011-2017 period in the combined four states of Arkansas, Louisiana, Mississippi, and Tennessee ranged from 407.7 to 695.2 million dollars/year. Also according to these surveys, corn earworm (CEW–Helicoverpa zea) is the pest that has caused the most or nearly the most yield loss in Arkansas and Mississippi every year since 2011.
CEW must always be accounted for in late-season scouting to ensure that control measures are meted out when the pest reaches economic threshold. According to the 2018 Insect Control Guide for Agronomic Crops (p. 36) published by Miss. State Univ. Extension, there are numerous insecticides and insecticide classes that can be used to manage/control this pest. One such product is the viral bioinsecticide Helicoverpa NPV (HaNPV–Helicoverpa armigera nucleopolyhedrovirus–trade names Heligen and Helicovex) that is relatively inexpensive at $3 to $6 per acre according to Univ. of Ark. Guide AG1306 that was published in 2018.
The Univ. of Ark. guide and product labels outline what you need to know to successfully use the HaNPV product to control CEW. Major points from those resources follow.
• HaNPV only kills CEW and tobacco budworm (TBW– Heliothis virescens). Thus, proper identification of larvae that are present is vital. When sampling with a sweep net, count the number of CEW and TBW larvae separate from other larvae that may be present. See the section on “How to Separate from Armyworms” in the above AG1306 publication.
• HaNPV only kills larvae that are less than 0.5 in. (~12 mm) long (3rd instar). These small larvae will die within 4 to 6 days after infection. If large larvae are present, do not use this product since these larger larvae will cause significant damage even when infected with the product. The Heligen label states that the application will be most effective when the majority of CEW larvae are less than 0.3 in. (7.5 mm) long (2nd instar).
• Use a reduced economic/action threshold of 2 to 5 small larvae per 25 sweeps. This is lower than the threshold of 9 worms per 25 sweeps for the other insecticides listed in the table on p. 36 of the MSU-ES guide cited above. Do not use when there are more than 5 large larvae per 25 sweeps.
• Soon after infection by the applied virus, larvae stop feeding even though they are still alive. Before death they will migrate to the top of the canopy and begin secreting viral droplets. Complete liquefaction and death occurs followed by the release of viral particles that will be available to infect healthy larvae–i.e., capable of causing secondary infection.
• According to the Heligen label, daytime temperatures of 75°F to 95°F are ideal for optimum activity of HaNPV. Greater control under these ideal conditions will be expected on smaller larvae that are less than 0.3 in. long.
• Residue of HaNPV can remain viable on leaves in the crop canopy for several weeks without larval hosts being present, but this biocontrol agent is most effective when fresh CEW or TBW larvae are continually present. Thus, one application of HaNPV is capable of infecting multiple generations of these pests when their presence is sustained.
• According to the Helicovex label, the product should be stored at temperatures below 41°F to ensure viral activity at the time of application, and bioactivity may be impaired when stored at temperatures above 86°F. Storing the product under cold or freezing temperatures will extend its shelf life. AG1306 states that storing at temperatures below 59°F is necessary if the product is kept for longer than 6 months, and that it can be stored at room temperature (≤77°F) for up to 6 months.
• Do not leave the product in direct sunlight for more than 2 hours and do not leave the product mix in the application tank for extended periods of time before application.
• Because the product acts through ingestion of treated plant material by the targeted organism, thorough spray coverage is essential for optimum control. The Helicovex product label states a minimum of 5 gal. of spray solution per acre by aircraft and AG1306 states a minimum of 10 gal. of spray solution per acre by a ground applicator.
• If applied in water, the product can be tank-mixed with herbicides, fungicides, foliar fertilizers, or other insecticides as long as the mixture does not exceed pH 8.
• There is no preharvest interval (PHI) and the re-entry interval (REI) is 4 hours after application.
This viral bioinsecticide should be considered for control of CEW in soybean if the above conditions/criteria are met. This will help delay or mitigate the development of insect resistance to the chemical insecticides that have been used/are still being used for control of this insect pest.
REMEMBER, THIS PRODUCT ONLY KILLS CORN EARWORM AND TOBACCO BUDWORM.
Composed by Larry G. Heatherly, July 2018, larryheatherly@bellsouth.net. Thanks to Jan de Regt for providing impetus for this article.