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Identifying and Controlling Armyworms - a destructive insect pest in vegetables, field crops and grasses

picture of flying bug

General information plus a brief review of the five most destructive armyworms

Once again we are bedeviled by lingo, one more label that won't go away... 'armyworms'. It is misleading because the insect also becomes (adult) moths. After all, hardly a moth looks like a worm. The establishment's solution? Call the caterpillar stages 'larvae' (not worms).

It should first be understood these particular insects of the family Noctuidae go through stages of development known as a 'complete metamorphosis' and only the larvae stage is damaging to crops whereas the adult stage (moth) is not. There are five species of 'armyworms' identified as major plant pests; the Fall armyworm, True Armyworm, Beet armyworm, Southern armyworm, and the Yellowstriped armyworm (all addressed below). Of these, the Beet armyworm is the most devastating... attacking a wide range of crops. Actually, the word 'armyworm' is a generic term because no insect is correctly known by that name... it is slang and often causes confusion as to which 'armyworm' is being referred to. Its usage, however, is either in reference to an unidentified armyworm, or the family Noctuidae as a whole.

To describe Noctuidae as a whole, the larvae feeds on many vegetables, field crops and grasses. They generally feed on just the foliage but may attack the stems, fruit and tubers of certain plants. They can (and do) cause extensive damage and, except for the winter months, are always active. Their color, size and markings are somewhat similar making them often hard to tell apart. Most will go through 5 to 6 larval stages within 14-21 days. Each larval stage is called an 'instar' by the appropriate number (i.e. 3rd instar, 5th instar) and their ability to cause damage increases with each instar stage (they become larger). They soon develop the ability to chew large holes in the leaves and eventually can strip the entire plant of foliage. After the final instar, the armyworm larvae will move down to the soil, bury themselves about 1" deep, and then pupate. Within 7-14 days, they will emerge as adult moths. Counting all stages, their lifecycle is 24-36 days. They over-winter (hibernate) most often as pupae in the soil and in the spring the adults will disperse themselves in all directions which includes migrating long distances north to lay their eggs. The climate of their final destination determines how many generations there will be that year.

For gardeners preferring to use insect predators (beneficial insects) alone as control measures, identifying the species of armyworm isn't really necessary because the same predators will work for all five. However identification is often necessary when considering pesticides and other biological controls. No recommendations of pesticides are made here however due to the unknown variables and legal ramifications… consult with your local pesticide supplier, university extension agent or nursery.

A.O. Kime - former Arizona and California agricultural Pest Control Advisor (1970-1992) and family farmer (1973-1998)

fall armyworm - this is a 'fair use' image, clicking it will take you to the photograph 

		source (another website)

Fall armyworm

Spodoptera frugiperda

Fall armyworms have three thin yellow lines running down their back, plus a dark strip and a wavy yellow stripe along each side. Their most distinguishing feature is an inverted white "Y" atop its dark head (like a brand on cattle). Fully mature larvae (6th instar) are about 1.5 to 2 inches long. The adults (moths) will feed on nectar and other liquids and deposit their egg masses usually under leaves. Larvae will emerge from eggs in 3-4 days, then completely develop within 2-3 weeks, then pupate, then emerge from their pupal cases as adults (moths) in 7-10 days... all of which completes a generation in 24-36 days. The Fall armyworm is primarily a sweet corn and field corn pest. Before feeding on the leaves and developing ears, it prefers to feed on the tenderness of the whorl (the spiral) first. The Fall armyworm isn't exclusively a corn pest however, it may also feed on lodged cereal crops.

Note: While the Fall armyworm is recognized as most responsible (of the family Noctuidae) for the damage done in both sweet corn and field corn, sometimes the True armyworm and/or Beet armyworm are also found in corn. Infestations would depend largely on the geographic latitude since Noctuidae (armyworms in general) can seldom survive hibernation in the cold northern winters. In order to be in the northern states and Canada therefore, they must migrate there annually each spring (or summer) ... and it appears the Fall armyworm is inherently predisposed to fly north both sooner and farther than other species of Noctuidae. The time it takes to migrate north is largely the reason it is usually only found there in late-planted corn. The warmer southern states would have more incidences of having more than one species attacking corn than the northern states and Canada.

True armyworm - this is a 'fair use' image, clicking it will take you to the photograph 

source (another website)

True armyworm

Pseudaletia unipuncta

True armyworm larvae are brownish in color and about 1.5 to 2 inches long. Being similar in size to a Fall armyworm, it is frequently mistaken for one. This is curious because the True armyworm would not have the "y" brand atop its head. At any rate, like most all 'armyworms', they will feed on crops at night and during overcast days. On sunny days, the larvae will hide under crop debris... making daytime field-checking more difficult. Aside from their occasional forays into corn, true armyworms feed on wheat, oats, rye, barley, pasture grasses and 'sometimes' (it is said) broadleaf plants such as alfalfa, cabbage and turnips. If true, they likely migrated there from grain fields harvested earlier. It could be, perhaps, a case of mistaken identity and more likely Beet armyworms (at least more likely in the southern states). But when they're in corn, True armyworms will feed on both the leaves and ears.

Beet armyworm - this is a 'fair use' image, clicking it will take you to the photograph 

source (another website)

Beet armyworm

Spodoptera exigua

The colors of the Beet armyworm larvae changes somewhat as it grows. In the 1st and 2nd instar, they are pale green (or yellow) and in the 3rd have pale stripes. In the 4th instar they are a little darker with a dark lateral stripe. In the 5th (and/or final) instar, the larvae are about 1-1/4" long in varying shades of green with dark (but sometimes white) bands running lengthwise. Occasionally there is a black spot on its side above the second pair of legs. Without hairs or spines, they are smooth to the touch. Since it is important to stop this insect before it reaches the final instar, detailed familiarization is necessary and one should not rely on a single source for reference purposes.

Left unchecked, the larvae can completely devastate a crop. They feed primarily on foliage but also on fruit... tomatoes being especially susceptible to damage and they'll quickly ruin a head of lettuce. Beet armyworms attack an incredibly wide range of crops. The host vegetables include, but are not limited to, asparagus, beans, beets, broccoli, cabbage, cauliflower, celery, chickpeas, corn (sweet), cowpeas, eggplant, lettuce, onions, peas, peppers, potatoes, radishes, spinach, sweet potatoes, tomatoes and turnips. The field crops are alfalfa, field corn, cotton, peanuts, safflower, sorghum, soybeans, sugar beets and tobacco. Evidence of their presence is the skeletonized foliage they leave behind (they generally will not eat the veins of a leaf).

It is important to note that certain weeds nearby can serve as breeding grounds for the Beet armyworms. The weeds most likely to harbor them are pigweed, lambsquarters, mullein, purslane and Russian thistle (tumbleweeds).

Southern armyworm - this is a 'fair use' image, clicking it will take you to the photograph 

		source (another website)

Southern armyworm

Spodoptera eridania

Presenting a problem only in the Southeast, and therefore alien to us Arizonans, the mature larvae of the Southern armyworm are about 1.5 inches in length and described as being "either gray or pinkish" (contrary to what the picture indicates however) and that they strongly resemble the Yellowstriped armyworm (as pictured below). It is also said that the head of the Southern armyworm is "usually yellow to light-orange". Wrong picture? This was checked and, according to the University of Florida, it is indeed a picture of the Southern armyworm. And lastly "The lateral stripe on the side of the body is interrupted by a large dark patch at the beginning of the abdomen". That is one description.

Another description is: " Larvae are green or blackish green with a uniform light brown or reddish brown head throughout the period of development. Larger larvae bear a narrow white line dorsally, and additional stripes laterally. Each side normally bears a broad yellowish or whitish stripe that is interrupted by a dark spot on the first abdominal segment, although in some cases this spot is weak. A series of dark triangles is usually present dorsolaterally along the length of the body."

Yes, the same armyworm is being described. This is a common problem... there seems to be no 'agreed upon' description for any particular insect.

This species has a broad range of host plants which includes vegetables, fruits, field crops and ornamentals. The vegetable crops are beets, cabbage, carrots, collard, cowpeas, eggplant, okra, peppers, potatoes, sweet potatoes, tomatoes and watermelons. Other crops include avocadoes, citrus, peanuts, sunflower, velvet beans, tobacco and various flowers.

Typically, they are most active at night and the larval stage lasts 14 to 20 days.

Yellowstriped armyworm - this is a 'fair use' image, clicking it will take you to the photograph 

		source (another website)

Yellowstriped armyworm

Spodoptera ornithogalli

Effectively confined to the southeastern U.S. (like the Southern Armyworm), and likewise strangers to us Arizonans, the larvae of the Yellowstriped armyworm are said to have a "brownish head with a pale-yellow inverted V on the upper front" and also has "distinct bright-yellow lines on the top of the sides of the body". And, "the armyworm occurs with both overall pale and dark-colored bodies". And finally "It has two rows of black triangle-shaped markings running the length of the body. Each row is offset from the center of the back. A thin white line runs lengthwise through each series of dark triangles."

The Yellowstriped armyworm host plants include alfalfa, beans, beets, cabbage, clover, corn, cotton, cucumbers, grapes, grass, onions, peas, peaches, peanuts, soybeans, sweet potatoes, tobacco, tomatoes, turnips, wheat and watermelons. Not so typical however, it is said most of the damage consists of defoliating young plants.

Fairly typical, the larvae is active over a 3-week period and the 6th instar will burrow into the soil to pupate. Two weeks later the adult moths will emerge. 3-4 generations occur each year in the southern and central Atlantic states.

-------------------

Note: Invariably descriptions of the same armyworm will differ between sources... leaving one to wonder which description most often applies. Since we do not have specimens to work with, we therefore had no choice but to rely on past experiences (memory) to judge the credibility of the information we gathered. For some data, we had to 'trust' our sources but we crosschecked (as much as possible) to have the confidence these overviews are accurate.

Credits: Clicking on any thumbnail image above will take you to the photograph source (another website). The 9th U.S. Circuit Court of Appeals has ruled that a thumbnail image is ‘fair use’ provided it contains a hyperlink to the webpage where the full-size photograph was obtained, Nonetheless, if any owner of the copyright objects to our usage, upon notification we will immediately withdraw the thumbnail image. (Note: In checking 5-18-13, the webpage source for these armyworm images no longer exists)

Last modified: 03/13/16