The fall armyworm (Spodoptera frugiperda) is a destructive agricultural pest native to the Americas, posing a significant threat to various crops and plant life. These ravenous caterpillars are known for their collective eating habits, swarming together in large numbers to consume a wide array of plant species. This article delves into the dietary preferences and feeding habits of the fall armyworm, providing insights into their impact on agriculture and landscapes.
Introduction to the Fall Armyworm
The fall armyworm (Spodoptera frugiperda) is a caterpillar of a tropical moth in the Noctuidae family. This insect is native to the Americas, unlike many other pests. The fall armyworm is sensitive to temperature, so it can only overwinter in South Florida or South Texas in the United States. Traditionally, fall armyworms migrate northward with each generation. Storms can carry these hitchhikers hundreds of miles. The bedraggled moths lay eggs, and within a week or so, caterpillars feast on plants.
Dietary Preferences of Fall Armyworms
Fall armyworms are polyphagous pests, meaning they have a broad host range and can feed on a variety of plants. These caterpillars are known to consume over seventy different plant species, including:
- Grasses: Bermudagrass, bahiagrass, dallisgrass, and other forage grasses are prime targets for fall armyworms. Lush, well-fertilized grass is particularly attractive to egg-laying moths. Even early-planted winter grazing crops like wheat and ryegrass are at risk.
- Crops: Corn, alfalfa, soybeans, sorghum, rice, and millet are all susceptible to fall armyworm infestations. Sweet corn, in particular, may require multiple insecticide applications per week during the silking and ear stages to protect against damage.
- Landscape Plants: Fall armyworms can also infest turfgrass and various landscape plants, causing cosmetic destruction.
- Weedy Grasses: Some wide-leafed weedy grasses, such as barnyardgrass and broadleaf signalgrass, are more preferred than bermudagrass, and cattlemen and hay producers learn to watch stands of these grasses as an early indicator of fall armyworm problems.
Fall armyworms are not particularly picky eaters and will consume any part of the plant they find suitable. This includes leaves, stems, and even reproductive structures.
Feeding Habits and Life Cycle Stages
The feeding habits of fall armyworms vary depending on their life stage:
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- Eggs: The bark-colored moths lay white, fuzzy egg masses on crops and vertical surfaces. Eggs hatch in as few as three days and develop into caterpillars.
- Early Instars: Tiny caterpillars nibble only young, tender growing tips of plants at night, hiding in debris during the day. Newly hatched caterpillars are too small to feed all the way through a leaf blade. They begin by feeding on the underside of the leaf but leave the clear upper epidermis intact. This creates tiny “windowpanes” in the grass blades that appear silver or white.
- Later Instars: As the caterpillars grow in size, so do their mandibles and appetites. Soon, they will feed both day and night, marching army-style, into adjacent areas and consuming nearly all plants in their path. As with most caterpillars, about 80 percent of total leaf consumption occurs in the last 2-3 days of the caterpillar stage.
- Pupation: The fall armyworm feeds for two to four weeks and reaches a length of 1½ inches. Then the mature caterpillar burrows into the ground to pupate for one to four weeks.
- Adults: When the moth emerges, it flies to new areas. Depending on the temperature, each generation can take as little as a month or as many as three months. With each generation, the insect moves northward.
Impact on Agriculture and Landscapes
The feeding habits of fall armyworms can have significant economic and ecological consequences:
- Crop Damage: Fall armyworms can cause extensive damage to crops, leading to yield losses and economic hardship for farmers. In some cases, crops like sweet corn may require frequent insecticide applications to prevent severe damage.
- Pasture and Hayfield Damage: Fall armyworms can quickly decimate pastures and hayfields, destroying valuable forage for livestock. In Mississippi, for example, these caterpillars can destroy a cutting of bermudagrass in just a few days, resulting in significant financial losses.
- Landscape Damage: Fall armyworms can also damage lawns and ornamental plants, detracting from the aesthetic value of landscapes.
- Economic Thresholds: Treatments using insecticides should be made when insect populations and/or damage levels reach economic thresholds. The threshold for management is not well established, but generally, nine or more fall armyworms per square yard warrants treatment.
Factors Influencing Fall Armyworm Populations
Several factors influence the population dynamics of fall armyworms:
- Temperature: Fall armyworms are sensitive to temperature and cannot survive freezing conditions. In the United States, they can only overwinter in South Florida or South Texas. Development rate is faster at higher temperatures.
- Rainfall: The more rainfall in the summer, the more of these pests you can expect to see.
- Natural Predators: Fall armyworms have many natural predators, including wasps, ground beetles, birds, skunks, and rodents.
- Host Plant Availability: The availability of suitable host plants can influence the abundance and distribution of fall armyworms. Lush, well-fertilized grass is particularly attractive to egg-laying moths.
- Migration: Fall armyworms are migratory pests, and their populations can be influenced by the arrival of moths from other regions.
Management Strategies
Effective management strategies are crucial for minimizing the impact of fall armyworms:
- Scouting: Regularly scout fields and landscapes for signs of fall armyworm activity. Look for windowpanes in grass blades, frosted appearance of grass, and the presence of caterpillars.
- Sudsy Flush Test: Perform a sudsy flush test, also called a drench test, to scout for fall armyworms that like to hide during the day. Combine 3 to 4 tablespoons of dish detergent (preferably lemon-scented Joy or Ajax) with two gallons of water. Pour evenly over a square yard of turf. Watch the area for 10 to 15 minutes. Count the caterpillars as they rise to the surface.
- Insecticides: Insecticides can be used to control fall armyworms, particularly when they are less than ½ inch long. Tiny caterpillars respond well to short-lived biological products like Bacillus thuringiensis kurstaki. Larger caterpillars require synthetic pyrethroids.
- Cultural Practices: Raise the mower blade height to allow plenty of grass leaves for photosynthesis to create nutrients for the plant's health. It is essential to irrigate the lawn to keep the turfgrass crowns well hydrated.
- Biological Control: Numerous species of parasitoids and predators affect FAW (Abbas et al. 2022). The most frequent wasp parasitoids reared from larvae in the United States are Cotesia marginiventris (Cresson) and Chelonus texanus (Cresson) (both Hymenoptera: Braconidae) species that are also associated with other noctuid species (Meagher et al. 2016). Among fly parasitoids, usually the most common species is Archytas marmoratus (Townsend) (Diptera: Tachinidae).
- Crop Rotation and Intercropping: Crop rotation and intercropping practices are also critical for disrupting the insect pest life cycle and maintaining the soil health [21].
- Natural Enemies: Conserve and promote the natural enemies of fall armyworms, such as parasitoids and predators.
Resistance to Insecticides
Some resistance to insecticides has been noted, with resistance levels varying regionally (Yu 1991, Yu et al. 2003, Yainna et al. 2023). Treatments using insecticides should be made when insect populations and/or damage levels reach economic thresholds (Overton et al. 2021).
Fall Armyworm Interaction with Other Pests
Fall armyworm larvae not only prey on BPH eggs but also prolong the developmental duration of BPH eggs by triggering a systemic increase in levels of JA-Ile, ABA and defensive compounds in the rice leaf sheaths on which BPH eggs are laid. In addition, compared to those that do not prey on BPH eggs, FAW larvae preying on BPH eggs grow faster.
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