Indianmeal Moth, Plodia interpunctella (Hübner) (Insecta: Lepidoptera: Pyralidae)
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The Indianmeal moth, Plodia interpunctella (Hübner), is a very common household pest, feeding principally on stored food products. In fact, it has been called the most important pest of stored products commonly found in American homes or grocery stores. The larvae are general feeders, which may feed on grain products, seeds, dried fruit, dog food, and spices. The Indianmeal moth received its common name from the United States where it was found to be a pest of meal made of "Indian corn" or maize.
This insect is found in a wide range of climates in stored products and food storage facilities around the world. It is very common in Florida, where it also lives successfully outdoors.
Eggs of the Indianmeal moth appear grayish-white and range in length from 0.3 to 0.5 mm. Eggs are oviposited singly or in clusters and are generally laid directly on the larval food source.
There are five to seven larval instars. Their color is usually off-white, but has been observed to be pink, brown, or almost greenish, depending on the food source. The mature larvae are about 1.27 cm in length. They have five pairs of well-developed prolegs that help them move considerable distances to pupate.
The larvae pupate either in a silken cocoon or unprotected. The pupae measure 6 to 11 mm and are pale brown in color. Pupation takes place away from the infested material. In fact, late instar larvae can travel such distances that they are often mistaken for clothing pests. Within pantries small larvae often climb to other shelves before pupating. This misleads people trying to find the source of the infestation.
Adults are a common sign of an infestation. Flying adults often appear to be fluttering instead of maintaining a direct line of flight. They are attracted to light and may move to distant rooms in the house away from the infestation. As a result, they are also commonly mistaken for clothing pests. Adults do not feed. However, even though unnecessary for egg production, adults have been reported to be interested in fruit juice and sugar baits (Kullberg, personal communication).
Adults are about 12.7 mm long with a wing span of about 16 to 20 mm. The forewings of this moth are reddish brown with a copper sheen on the outer two thirds and gray on the inner third. At rest the wings are held roof-like over the body. The head and thorax of the moth appears gray and the posterior brown, with a coppery sheen.
A life cycle can be completed in 27 to 305 days. A single female can lay up to 400 eggs after mating. Mating and egg laying occurs about three days after adult emergence. The eggs can be laid singly or in clusters and are generally oviposited directly on the larval food source. The eggs hatch in seven to eight days at 20°C and three to four days at 30°C. Upon hatching, the larvae begin to disperse, and within a few hours can establish themselves in a food source. The larvae can complete their development in six to eight weeks at temperatures ranging from 18 to 35°C. The number of larval instars varies from five to seven (depending on the food source and the temperature). The pupal stage can last from 15 to 20 days at 20°C and seven to eight days at 30°C.
The larvae are surface feeders. Most of the "damage" to stored products occurs when the larvae spin massive amounts of silk that accumulate fecal pellets, cast skins, and egg shells in food products. The damage to stored products due to this contamination exceeds the amount of food eaten by the insects. Homeowners and managers of food processing plants, warehouses, groceries, and granaries should be on alert for signs of infestation.
Monitoring and sampling can be accomplished with pheromone sticky traps. In some cases of low-level infestations, traps can be used for control as well. Traps are also useful in timing and evaluation of control procedures. In a study of pheromone-trap effectiveness in attracting Indianmeal moths, traps containing (Z,E)-9, 12-tetradecadien-l-yl-acetate (ZETA) proved useful in monitoring populations. One drawback of the pheromone sticky trap seems to be that population size and number trapped hinder their effectiveness. For example, if large number of moths become trapped, the ability to capture more moths decreases. The continued use of sticky traps (with pheromones) could lead to improved control programs and less incidence of insects in stored products on grocery shelves.
Elimination and exclusion are key elements in controlling pest populations of this moth. If a population is discovered, all infested food must either be discarded or treated. Any susceptible food source should be placed in sealed containers. Dog food and bird seed usually are overlooked as infestation sites, and these items should also be kept in sealed containers. All stored food products brought home from the grocery store should be examined for the tell-tale "white worms" and webbing.
Freezing the product for several days has proven to be an effective control measure. Heating in an oven or microwave also kills larvae and eggs.
Numerous insecticides have been used to control Indianmeal moth populations, but effectiveness is limited. In a study evaluating the effectiveness of an insect growth regulator, it was found that even after treatment with these chemicals, Indianmeal moths were in corn storage bins. In an additional study using the same insect growth regulator as aforementioned, it was found that at rates of 5 and 10 ppm, no control of fifth instar larvae was accomplished. Only at 20 to 30 ppm was control of fifth instar larvae observed. Studies show that resistance increased with increasing selective pressure. Resistance was also observed in studies performed with the microbial insecticide Bacillus thuringiensis. Resistance was just as likely with exposure to single strains of B. thuringiensis as with mixtures of sequences of the insecticide. Resistance, once achieved, appeared to be inherited as a recessive trait, and with time was observed as genetically stable.
See Pantry and Stored Food Pests (https://edis.ifas.ufl.edu/publication/ig095).
Population suppression has been observed in the laboratory using egg and larval parasites. Both the larval parasite, Bracon hebetor (Hymenoptera: Braconidae) and the egg parasite, Trichogramma pretiosum (Hymenoptera: Trichogrammatidae) have demonstrated moth population suppression. When the parasites were used in combination, an 84.3 percent suppression was observed. Trichogramma pretiosum acting alone offered a 37.3 percent suppression rate while Bracon hebetor provided a 66.1 percent suppression rate.
Arthur FH, Highland HA, Mullen MA. 1991. "Efficiency and longevity of two commercial sex pheromone lures for Indianmeal moth and almond moth (Lepidoptera: Pyralidae)." Journal of Economic Entomology 26: 64–68.
Arthur FH, Simonaitis RA, Throne JE, Zehner JM. 1990. "Evaluation of chlorpyrifos-methyl and chlorpyrifos-methyl plus methoprene as protectants of stored corn: small bin tests." Journal of Economic Entomology 83: 1114–1121.
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Brower JH, Press JW. 1990. "Interaction of Bracon hebetor (Hymenoptera: Braconidae) and Trichogramma pretosium (Hymenoptera: Trichogrammatidae) in suppressing stored-product moth populations in small in-shell peanut storages." Journal of Economic Entomology 83: 1096–1101.
Brower JH. 1988. "Population suppression of the almond moth and the Indianmeal moth (Lepidoptera: Pyralidae) by release of Trichogramma pretiosum (Hymenoptera: Trichogrammatidae) into simulated peanut storages." Journal of Economic Entomology 81: 944–948.
Dowdy AK, Hagstrum DW, Lippert GE. 1994. "Early detection of insects in stored wheat using sticky traps in bin headspace and prediction of infestation level." Environmental Entomology 23: 1241–1244.
Mallis A. 1997. Handbook of Pest Control. 7th Edition. Franzak & Foster Co. Cleveland. 1990. 1152 p.
McGaughey WH, Tabashnik BE. 1994. "Resistance risk assessment for single and multiple insecticides: responses of Indianmeal moth (Lepidoptera: Pyralidae) to Bacillus thuringiensis." Journal of Economic Entomology 87: 834–841.
Smith EH, Whitman RC. 1992. Field Guide to Structural Pests. Dunn Loring, VA: National Pest Management Association.