Insect Strip

Insect Strip

Wednesday, November 7, 2018

Insecticidal Virus Products for Pest Control: What are the Latest Facts

The arsenal of insecticides and acaricides that we have to select from for controlling arthropod pests is extremely important. Our arsenal may be limited due to cost of the product, which pesticide class will be effective, pests developing resistance, and the impact of a pesticide on beneficial populations or possible flaring of other pests. Therefore, we are always looking for new pesticides to add to our arsenal. 

Toward the end of our growing season this year there was talk of a new insecticide for control of corn earworm/headworm/cotton bollworm. The active ingredient in this product is the nucleopolyhedrovirus (NPV) that is being used in Australia. What is interesting is that this “new insecticide” was studied as early as 1960’s and extensively during the 70’s and 80’s as the nuclear polyhedrosis virus.  And, it was first registered in 1975 by EPA and marketed as Elcar©, Biotrol VHZ©and Viron/H© for Heliothis zea control in beans, corn, lettuce, peanuts, sorghum, soybeans, strawberry, tomato, and cotton. (Yes, I am dating myself). Registration for all of these old products has been cancelled. 

Now new products of the same NPV are being marketed as Heligen©, Helicovex©, and Gemstar©. Are these new products any different from the older products? The virus itself is the same, but how the product is formulated or processed may or may not be different. So, what facts are available about NPV and these products.

Numerous NPVs have been identified for different lepidopteran insects, but each NPV is specific to a particular insect host or narrow range of hosts.These above NPV products are specific to larval stages of Helicoverpa (aka corn earworm, cotton bollworm, tomato fruitworm, soybean podworm, and sorghum headworm) and Heliothis virescens (tobacco budworm). There is an NPV marketed as Fawligen©that is specific to Spodopteralarvae (fall armyworm and beet armyworm). Another NPV product (Loopex©) is specific to the cabbage looper, Trichoplusia ni. These few examples of NPVs will not infect other insects or arthropods, including pests and beneficial predators and parasites. 

Figure 1. Old world cotton bollworm, Helicoverpa armigera, cadaver from NPV infection. Photo: O.P. Sharma,
As a simple illustration of how the virus works, a larva feeds on NPV treated plants where the virus infects the cells of the gut lining and begins to replicate throughout other cells in the larva. These infected cells rupture causing the larva to die. Viral particles are released from the larval cadaver onto the host leaves where healthy larvae can become infected when eating on the plant. 

Figure 2. Top, Dead Beet armyworm from NPV infection. Photo: David Nance, USDA-ARS,
Larval infection and mortality are dependent on the amount of NPV consumed and the larval size at infection. The infection rate of the target pest to the NPV in larvae is age-dependent (Engelhard et al. 1995, Virology). As with any biological insecticide, it may take a few days to a week for larvae to die. Often times dying larvae will climb to the top of the plant.  One great virtue of NPV insecticides is that they target a very narrow pest range, have no direct effect on beneficial insects, and offer little disruption to the arthropod complex in the field. These biological insecticides could be an alternative control method when the target pest has developed resistance to other available insecticides. However, with any product for controlling a pest there is a potential with increased exposure for the pest to develop resistance to the product over time, even NPV. For example, velvetbean caterpillars developed resistance following multiple generation exposure to NPV (Abot et al. 1996, Biological Control).

We do not have any current data on the effectiveness of NPV product applications for control of the above listed pest species with field crops in the Texas High Plains. There was a trial this past summer conducted by Dr. Katelyn Kesheimer, former Extension agent–IPM (Lubbock, Crosby counties), with Heligen©for control of sorghum headworm. At the time of application, the headworm population was very low, below economic threshold, which prevented any measurable results of the effectiveness for control of headworms in sorghum. However, she occasionally found diseased dead larvae when scouting the plots. Since we do not have any definitive results, we (Texas A&M AgriLife Extension) are hesitant to suggest the use of these products, but that does not mean that the products could not provide effective control under the right conditions. If you are interested in trying any of the NPV products the following are facts about the products and general guidelines to help obtain optimum results:

Application volume
Ground rig – 10 gal/ac
Aerial – use only water with > than 3 gpa 

Applied through ground sprayers, chemigation or by airplane. Use sufficient water to obtain thorough, uniform coverage.

Aerial – minimum 5 gpa spray solution.
Applied using standard ground, aerial application, and overhead sprinkler chemigation.
Apply in sufficient water to obtain thorough coverage without excessive runoff. 
Sorghum only – can be aerially applied at 1 gpa with 3 fl. oz of anti-evaporation additive (emulsifieable oil/mineral oil)
Use of non-ionic or oil-based spreader/sticker and ultraviolet screening agent may enhance performance.

Silicone-based spreaders should not be used
Performance may be enhanced with Pinolene or related sticker, latex, methylated seed oils, humic acid, and powdered skim milk.

Avoid silicone-based adjuvants intended only as “spreaders”.

In organic crops, performance may be improved by addition of a feeding stimulant such as sugar or molasses at 5 pounds per acre.
Nozzle selection and boom setup
Avoid flat fan nozzles, unless in row crops with multiple nozzles per row

Prefer use of TwinJet or hollow cone nozzles

Spray solution pH
Spray water pH should be neutral (7). pH above 8 may damage the virus and performance will be reduced.
pH of 5 - 8.5
If pH is 9 or higher, buffer to pH of 7
Environmental conditions
Best to apply in humid (above 50% RH) and warm (77-95oF) for good coverage and for larvae to actively feed

Ideal conditions often before 8 am.
Temperatures above 100oF will degrade the virus and result in a loss of quality
Short term exposure at temperatures above 90oF (a few days) will not immediately deactivate the virus. Avoid spraying in intense sunlight. 

Effectiveness may be increased by spraying in the late afternoon or early evening.
Original container out of direct sunlight at or below 40oF or in a freezer at 0oF
Store at temperatures below 41oF. 
Store at room temperature for up to two months. Refrigeration or freezing will extend shelf life to one year or longer. Store in original container in cool, dry place, out of direct sunlight
60% - 90% control depending on conditions

When to Apply
Sorghum – In fields with even flowering apply when 50% of heads have reached 100% flowering. For uneven flowering fields apply earlier than 50% flowering.

0.7 fl. oz/ac for larvae smaller than 0.3 inches.

1.4 fl. oz/ac for larvae between 0.3 inches to 0.5 inches).

Do not treat if larvae are larger than 0.5 inches.

Cotton – only be applied when larvae are less than 0.3 inches and actively feeding.

Soybeans – 1.0 fl. oz/ac used pre-podding of pre-threshold larval populations.

1.6 fl. oz/ac when populations reach economic threshold – the addition of a molasses-based additive is for threshold sprays of soybeans.
Target smaller larvae and eggs that are about to hatch. If applied on large larvae, damage might not be avoided.

Lower rates may be used during vegetative stage/before fruit set or when tank mixed with other insecticides. Preferable to apply several smaller doses than a single high dose.

For most crops apply 1 – 2.5 fl. oz/ac with reapplication after 6-8 days.  

If the plant tissue is growing fast, such as sweet corn silks, apply 0.5 – 1.25 fl. oz/ac in an interval of only 3 days.
Application should be targeted against first or second instar larvae (smaller than 0.3 inches).

Application rate – 4 to 10 fl. oz/ac

Lower rates may be used during vegetative stages or when tank mixed with other insecticides also effective against Heliothis or Helicoverpa.

When flowers, fruit, or other harvested structures are present use higher rates and/or increased frequency of sprays, or tank mix with other insecticides having contact knockdown activity.

Sweet corn: Application should be made from early vegetative growth stage to tasseling and before emergence of silks. Retreatment may be required at 2 - 3 day intervals depending on egg counts and crop growth rate due to short residual activity of the virus.
REI – 4 hours

PHI – not restrictions on applying up to the time of harvest.
REI – 4 hours

PHI – can be applied up to and including the day of harvest.
REI – 4 hours

PHI - Can be applied up to and including the day of harvest and storage.
The information in this table is for informational purposes only and does not include all information that is available on the product label. Read and follow the label before use. 

The following links were used to obtain information from company brochures, technical bulletin, fact sheet, and product labels about each of the above NPV products for control of larval stages of Helicoverpa zea) and Heliothis virescens (tobacco budworm).