Battling Armyworms

For many farmers armyworms can be a very costly pest. By correctly identifying the armyworms and choosing the most effective insecticide for their crops, farmers can save time and money.

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The three most common armyworms found in Florida are the southern armyworm (Spodoptera eridania), fall armyworm (Spodoptera frugiperda) and the beet armyworm (Spodoptera exigua) though several other species also exist. Armyworm species can be difficult to tell apart. All armyworm species are ravenous feeders that can cause serious crop damage. Armyworms tend to feed on foliage but may also attack other parts of the crop including the stems and fruit.

The southern armyworm is one of the larger armyworm species. It is sometimes referred to as climbing cutworm. When larvae are fully grown they can be up to 1.5 inches in length. Their head usually has a yellow to red tint, while their bodies have a tan to pink color. Southern armyworms can be serious pests of beet, cabbage, carrot, collard, cowpea, eggplant, okra, pepper, potato, sweet potato, tomato, and watermelon. They can also damage avocado, citrus, peanut, sunflower, velvet bean, tobacco, and ornamental flowers.

The fall armyworm often causes the most damage to vegetable and hay producers. Unlike the southern armyworm the fall armyworm has a black or brown head. Its body ranges in color from tan to gray to green. The fall armyworm can easily be identified by the inverted “V” found on its head. Mature larvae can be up to 1.5 inches in length. Fall armyworms can feed on numerous plant species. However their preferred food sources are grasses. They often cause the most damage to field corn, sweet corn, sorghum, and bermudagrass. Fall army worms can also attack alfalfa, barley, buckwheat, cotton, clover, oat, millet, peanut, rice, ryegrass, sugarbeet, sudangrass, soybean, sugarcane, timothy, tobacco, and wheat. Sweet corn is the only vegetable crop that fall armyworm causes significant damage to.

Beet armyworms are slightly smaller than the fall armyworm and the southern armyworm. The larvae are typically 1.25 inches when fully grown. Their bodies are typically green with dark stripes along their entire length. The distinguishing characteristic for beet armyworms is the large black spot behind their head. Beet army worms can be a serious pest for a wide variety of vegetable and agronomic crops. They can attack asparagus, bean, beet, broccoli, cabbage, cauliflower, celery, chickpea, corn, cowpea, eggplant, lettuce, onion, pea, pepper, potato, radish, spinach, sweet potato, tomato, and turnip, alfalfa, corn, cotton, peanut, safflower, sorghum, soybean, and tobacco.

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The striped grass looper (Mocis repanda) is sometimes mistaken for armyworms. It is often found in the same crops as armyworms. Striped grass loopers are thinner than armyworms and move in a distinctive crawling motion similar to inchworms. Striped grass loopers also have 4 prolegs compared to the 2 found in armyworms. In addition, striped grass loopers can be distinguished from armyworms by the numerous vertical stripes that cover their head. Striped grass loopers are can be serious pests of forage crops particularly bermudagrass and stargrass.

It is important to use Integrated Pest Management (IPM) principles when trying to control armyworms or any other lepidopteran larvae. Scouting for armyworm population is critical to determine the timing for insecticide applications. If most of the larvae on the crop are greater than 0.5 inches in length, treating them with insecticides is usually not worth the cost since they will soon stop eating and pupate. However, if more than three to four larvae that are less than 0.5 inches are found per square foot, they need to be sprayed quickly. If the crop has been sprayed before the treatment threshold drops down to two larvae per square foot. Armyworms and other lepidopteran larvae have numerous natural enemies. When choosing a product to treat worms look for insecticides that have low toxicity to beneficial insects. Using broad spectrum insecticides that kill larvae’s natural enemies can lead to greater worm populations later in the season.

Insecticides for worm control have two routes of entry: stomach and contact. For stomach insecticides to be effective, the insect must first ingest the pesticide where it then moves to the gut and is absorbed by the insect. In contrast to stomach insecticides, contact insecticides do not need to be ingested by the insect. The pesticide is absorbed through the insect’s cuticle. Systemic insecticides move throughout the plant in the xylem tissue. In contrast, locally systemic insecticides move only within the leaf that was treated. Most locally systemic insecticides also have translaminar activity. Translaminar refers to the pesticide moving from the top of the leaf, through the leaf tissue, to the bottom side of the leaf.

The following insecticides are commonly used to control lepidopteran larvae. The Insecticide Restience Action Committee (IRAC) groups insecticides together according to their mode of action. It is critically important to rotate insecticide applications with products from different IRAC groups. Repeated use of insecticides with the same mode of action has and will lead to insecticide resistance. To simplify information available in this CEU series, it is sometimes necessary to use trade names of insecticides. No endorsement of these insecticides is intended, nor is criticism implied of similar insecticides not mentioned.

Group 1A: Organophosphates

Organophosphates are toxins that target the nerve synapse. The synapse is the gap where one nerve cell ends and the other begins. For nerve cells to communicate they release chemicals called neurotransmitters that travel across the synapse and stimulate the adjacent nerve. The neurotransmitters are then broken down and the process repeats itself. The organophosphates bind to the enzyme acetylcholinesterase (AchE). AchE is responsible for breaking down the neurotransmitter acetylcholine so the nerve can return to its resting state. Without AchE the neuron is stuck in the “on position” leading to uncontrolled nerve firing and insect death. Organophosphates were first developed in Germany during World War II as part of its chemical weapons program. During the war, nicotine for pest control became in short supply and Germans began testing derivates of their chemical weapons as substitutes for nicotine. Later on organophosphates use became widespread after the war as a substitute for the organochlorine insectides such as DDT. Organophosphates are chemically unstable and have a short half-life. Malathion (generic, many brand names) was introduced in 1950. Unlike many organophosphates, malathion has low mammalian toxicity. It is a broad spectrum contact pesticide with a short residual period. It does however have high toxicity to many beneficial insects.

Group 1B: Carbamates

The carbamates were first developed in the 1950s in the United States. They are synthetic derivates of physistigmine, a natural toxin from the calabar bean (Physostigma venenosu). Carbamates are nonselective, broad spectrum insecticides. Like the organophosphates, carbamates inhibit the enzyme AchE. Two of the most commonly used carbamate insecticides labeled for armyworm control are carbaryl (Sevin, many others) and methomyl (Lannate). Carbaryl is an old insecticide, discovered in 1956. It is a contact insecticide that is sold in a variety of formulations including wettable powder, suspension and dust. Carbaryl has good knock down ability, however it has a short residual period. Methomyl is systemic insecticide. It is a restricted use pesticide due to its high human toxicity. It too has good knock down ability but short residual period. Both of these insecticides are highly toxic to beneficial insects.

Group 3: Pyrethroids

Pyrethroids are synthetic insecticides based upon the on the natural insecticide pyrethrum. Pyrethrum is naturally derived from chrysanthemum flowers. The pyrethroids insecticides were developed to be more stable in UV light and have a longer residual period compared to pyrethrum. Pyrethroids are nerve toxins that disrupt the sodium channels in the axon of the nerve cell. The axon is the part of the nerve cell that carries electrical impulses away from the cell body. Interfering with the sodium channels leads to uncontrolled nerve firing that quickly results in the insect’s death. Pyrethroids are potent, fast acting insecticides that have low use rates. They are however, very broad spectrum and are highly toxic to many beneficial insects. Pyrethroids are contact insecticides with most having a residual period of 5 to 10 days depending on their chemistry and environmental conditions. There are numerous pyrethroid insecticides. Some of the more common ones include but are not limited to permethrin (many brand names), bifenthrin (many brand names), zeta-cypermethrin (Mustang Max) and esfenvalerate (Asana). Most pyrethroids are Restricted Use Pesticides due to their toxicity to fish and other aquatic organisms.

Group 5: Spinosyns

The spinosyns insecticides are toxins produced by soil bacteria (Saccharopolyspora spinosa). To produce the insecticides, cultures of S. spinosa are fermented and then purified to extracted the toxin . Spinosyns bind to the acetylcholine receptor in the nerve cell leading to uncontrolled nerve firing. Spinosyns also bind to GABA (gamma-aminobutyric acid) receptors. GABA is a neurotransmitter that regulates chloride channels in the nerve cell. Spinosyns have low mammalian toxicity. The insecticide Spinosad (Tracer, Conserve, Entrust, Spintor) is a mixture of spinosyns A and D (hence the name spinosAD). Entrust is OMRI label for use in organic production. Spinosad is a contact insecticide that has a lower impact on beneficial insects. However, it is toxic to foraging bees up to three hours after spraying. Spinetoram (Radiant and Delegate) are synthetic spinosyns that have a longer residual period. Spinetoram is an EPA Reduced Risk insecticide with low mammalian toxicity. Like spinosad, spinetoram has low impact on most beneficial insects. Spinetoram is safe to bees three hours after spraying.

Group 11: Microbial Disrupter of Insect Midgut

Only one insecticide falls into Group 11 Bacillus thuringiensis more commonly known as Bt. Bt is a soil dwelling bacteria whose spores produce toxic proteins that effect a number of insect species. Different strains of Bt control different insect species. The crystalline toxins found in the Bt spores rupture and disintegrate the cell membranes of the gut lining, leading to gut paralysis and insect death. The Bt varieties kurstaki and aizawai are only effective on lepidopteran larvae and will not damage beneficial insects. The insecticidal properties of Bt were first discovered around the turn of the 20th century. It was not until the 1960s that the first commercial formulation of Bt became available for lepidopteran larvae control. Bt is a stomach insecticide and must be consumed by the insect in order to be effective. Bt has very low mammalian toxicity. Bt is a generic product available in a number of brand names including DiPel, Xentari, and Javelin and Deliver.

Group 15 & 18: Insect Growth Regulators

Insect Growth Regulators (IGRs) that are effective against lepidopteran larvae include the Group 15 Chitin Synthesis Inhibitors (CSIs) and the Group 18 Ecdysone Receptor Agonists. Unlike the most insecticides, which are nerve toxins, IGRs target physiological functions that effect insect growth. CSIs block the biochemical pathway that produces chitin. Chitin is the main component of the insect’s exoskeleton. Insects produce new chitin during the molting process. If the insect is unable to produce chitin for its new exoskeleton, it results in the insect death. CSIs are most effective on larvae that are in their early growth stages. Two CSIs that are commonly used include diflubenzuron (Dimilin) and novaluron (Rimon). Both of these products primarily have stomach activity but have some contact activity as well. CSIs have low toxicity to most beneficial insects. Diflubenzuron is a Restricted Use Pesticide due to its toxicity to aquatic invertebrates. However, it has very low mammalian toxicity. In pastures diflubenzuron provided control of armyworms for over 40 days.
 
The Group 18 Ecdysone Receptor Agonists are insecticides that disrupt the insect hormone ecdysone. Ecdysone is responsible for starting the molting process in insects. Lepidopteran larvae treated with these insecticides undergo a lethal premature molt. The treated larvae take several days to die but during that time, they are unable to eat and are referred to as “walking dead worms.” The only Ecdysone Receptor Agonists registered in Florida is methoxyfenozide (Interpid). Methoxyfenozide is a systemic insecticide that has both contact and stomach activity. It is an EPA reduced risk insecticide and has a residual of 14 days or more. It has low toxicity to most beneficial insects as well as mammals.

Group 28: Ryanodine Receptor Modulators

Ryanodine Receptor Modulators are a relatively new class of insecticides that provide long lasting control of lepidopteran larvae. These insecticides are muscle toxins that activate the insect’s ryanodine receptors. The ryanodine receptors regulate amount of calcium ions that leave the muscle cell during muscle contraction. Ryanodine Receptor Modulators adhere to the receptors causing an uncontrolled release of calcium out of the muscle cell. This prevents further muscle contraction. Insects that have been treated with these insecticides appear sluggish or paralyzed and unable to feed. Ryanodine Receptor Modulators have very low mammalian toxicity due to structural difference between insect and mammalian ryanodine receptors. Insects’ ryanodine receptors are 400-3,000 time more susceptible to these insecticides compared to ryanodine receptors found in mammals. Group 28 insecticides have two active ingredients available for use in Florida: Chlorantraniliprole (Coragen, and a component of Durivo and Voliam) and flubendiamide (Synapse and a component of Vetica). Both of these active ingredients have low toxicity to beneficial insects. Chlorantraniliprole is a systemic and translaminar insecticide. It is primarily a stomach insecticide though it has some contact activity. Flubendiamide is a locally systemic and translaminar insecticide with stomach activity.
 
Credits
Suiter, D.R. and M.E. Scharf. 2008. Insecticide Basics for the Pest management Professional. Bulletin 1352. 
Ware, G.W. and D.M. Whitacre. 2004. The Pesticide Book. W.H. Freeman and Company. 
Johnson, F.A. and J.L. Castner. 2003. Armyworms. http://edis.ifas.ufl.edu/in016.
Capinera, J. L. 2005. Southern Armyworm, Spodoptera eridania (Cramer) (Insecta: Lepidoptera: Noctuidae). http://edis.ifas.ufl.edu/in263.
Capinera, J. L. 2005. Fall Armyworm, Spodoptera frugiperda (J.E. Smith) (Insecta: Lepidoptera: Noctuidae). http://edis.ifas.ufl.edu/in255.
Capinera, J. L. 2005. Beet Armyworm, Spodoptera exigua (Hübner) (Insecta: Lepidoptera: Noctuidae). http://edis.ifas.ufl.edu/in262
R.K. Sprenkel and E.A. Buss. 2011. Insect Management in Pasture.http://edis.ifas.ufl.edu/ig061
Mislevy, P. Dimilin® – An Economical Insecticide for Grass Worm Control.http://rcrec-ona.ifas.ufl.edu/in-focus/IF7-1-08.shtml. 
Coragen Technical Bulletin. 2008. Reorder No: K-14833.http://www2.dupont.com/Production_Agriculture/en_US/assets/downloads/pdfs/K-14833_Coragen_Tech_Bulletin.pdf
Spinetoram Technical Bulletin. 2006.http://www.dowagro.com/PublishedLiterature/dh_0072/0901b8038007298a.pdf?filepath=usag/pdfs/noreg/010-80088.pdf&fromPage=GetDoc.
 

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