The food moth (Plodia interpunctella), alternatively spelled Indianmeal moth, is a pyraloid moth of the family Pyralidae. P. interpunctella larvae (caterpillars) are commonly known as waxworms. It is important to note that they are not the same species as the waxworms often bred as animal feed. Rather, they are a common grain-feeding pest found around the world, consuming cereals and similar products. Substantial efforts have been taken in the United States to control the moth’s damage to grain crops.
The larvae of this species have the ability to bite through plastic and cardboard; thus, even sealed containers may be infested. Once found, the moths are difficult to eradicate. The last instar larvae is also able to travel long distances before pupating; thus, a given infestation site may be located a great distance from the nearest pupation site.
Plodia interpunctella has been found on every continent in tropical habitats, excluding Antarctica. Within the United States, the moth is most commonly found in Florida, where it thrives in the tropical habitat. The moth lives in a wide range of conditions, making it a persistent pest. It is often found at food storage facilities worldwide, specifically in grain bins or grain storage buildings.
Adults are 8–10 mm in length with 16– 20-mm wingspans (c. 3⁄4 of an inch). The distal two thirds of their forewings are generally reddish brown in color with a copper luster. They can also be bronze or dark gray. The more proximal parts of the wings are yellow-gray or white-gray, with a dark band at the intersection between the proximal and distal regions. The hindwings, in general, are uniformly gray.
The eggs of the food moth are white, ovate, and very small. It is difficult to see them with the naked eye. Newly-hatched larvae are equally difficult to see. They are mostly off-white in color, have brown heads,and develop through five to seven larval instars. When these larvae mature, they measure about 12-14 mm long. Larvae also have three sets of legs near the head and five sets of prolegs protruding from the abdomen. The legs help the larvae move over long distances in order to find pupation sites.
Diet and eating habits
Food moths feed on a large range of plants, grains, and other human food products.
- Plant-based foods
Moths feed on many plant-based foods. These foods include dry pet food (plant based), birdseed, cereal, soup mixes, bread, pasta, rice, flour, spices, dried fruits and nuts. There is strong evidence that the northern Manitoba wheat supports the development of the moth. Other optimal diets include sultanas, American yellow corn and almonds. Groundnuts and maize meal, on the other hand, result in a longer development time for the moths.
- Non-plant foods
Food moths are also known to cannibalize larvae. However, this often leads to viral granulosis infections spreading through a food moth population, though healthy larvae are picked more often than unhealthy larvae for cannibalism. Typically, cannibals do not eat closely related individuals, which might share genes with them, but when given the choice between siblings and unrelated individuals, food moths tend to cannibalize their siblings.
- Foraging flights
Though food moths generally do not migrate over long distances, they do engage in long distance foraging flights. These flights take place during the twilight hours during which blue light (400 to 475 nm) rather than UV light (10 to 400 nm) is dominant and attracts the moths. To add to this, blue light’s role in the food moth foraging behavior has recently been developed as a form pest control since it is attractive to the moth.
Usually, the life cycle of a food moth colony starts in a location where grain is present. The temperature within a grain bin must exceed fifty degrees Fahrenheit (10 °C). The eggs of the moth are grayish white and have a length between 0.3 and 0.5 mm. Eggs can be laid directly on the food source either singly or in groups of between 12 and 30. A mature female may lay between 100 and 300 eggs at a time. Larvae begin to hatch in approximately two to fourteen days. The larvae have between five and seven instars. Newly-hatched larvae feed on grain while more mature larvae feed on grain germ. The larvae are an off-white color, but can be pink, brown, or greenish. They are about 12 mm long and have prolegs for movement purposes. Fully-grown larvae are able to spin webs and leave silk threads in their path of travel. Mature larvae make silk also make threaded cocoons. The pupae are often seen on grain surfaces and on the walls of grain bins. The adults emerge in four to ten days. They then mate and the cycle begins again. The entire life cycle of this species may take 30 to 300 days. A typical life cycle takes 50 days. Under optimal conditions a life cycle can be as short as 28 days but cooler winter months prohibit this from happening. There is a potential for seven to nine generations of moths to live in a year.
Diapause is defined as a delay in animal development due to certain external factors. It can end once the adverse environmental conditions wear off. Thus, the duration of diapause varies. This kind of behavior is witnessed in food moths. It is especially prevalent late in the breeding season. During the egg stage, if the temperature of the moth’s environment exceeds 25°C (77°F), it can cause a delay in hatching. Additionally, in the moth’s early larval stage, temperatures of below 20°C (68°F) can cause a similar diapause. Different strains of P. interpunctella have differing tendencies to enter diapause.
Food moths as pests
Discovering a food moth infestation in your kitchen can be a bit of a shock. Finding out that these bugs have been laying eggs in your food, and then realizing you may have eaten some of them while enjoying a snack of cheese and crackers. Food moths lay eggs directly on their food source, which can include:
- Dried fruit
- Dried herbs
- Powdered milk
- Also: Bird seed and dry pet food
That means that until you discovered the infestation, you could have eaten a live moth, its eggs or its larvae. Equally concerning is that it is also possible you consumed their refuse, including their waste material or skin cast-off during molting.
Aside from the insects themselves, food infested by pantry moths may also have silk webbing present on the surface. Food moths or their larvae, eggs and webbing cannot make you sick if you accidentally eat them as are not known to spread any known diseases, parasites or pathogens.
Discovering an infestation should prompt you to consider how these insects got into your food in the first place. First, the food may have been left exposed to these pests at some time during the production or packaging process. This may also make you wonder what else got into the food. Further, an infestation means your food is probably far from fresh — after all, the pantry moths had enough time to go through their reproductive life cycle while inside the package. Both of these thoughts are understandably unsettling.