Stored Product Pests, Others that Infest Various Products
Silverfish and Firebrats
These insects can be readily identified by the carrot-shaped bodies that terminate in 3 elongated appendages. In addition their body is covered by scale-like appendages which may or may not be silvery in color.
Silverfish normally live outdoors under rocks, bark and leaf mold, in the nests of birds and mammals, and in ant and termite nests. However, many are found in houses and are considered a pest, or at least a nuisance, by homeowners. They lay relatively few eggs (for insects at least) and require several years to reach several sexual maturity. As a result large populations are rarely encountered in the home. If so it is quite likely that the infestation has been there for years.
Occasionally, they are found in bathtubs, sinks and wash basins. They crawl in seeking food or moisture and can’t climb out. These insects prefer vegetable matter with a high carbohydrate and protein content. However, indoors they will feed on almost anything. A partial list includes dried beef, flour, starch, paper, gum, glue, cotton, linen, rayon, silk, sugar, molds and breakfast cereals.
Feeding damage of silverfish. Note the chewed areas of paper (chewing mouthparts) and the feces staining (black). Image courtesy of Clemson University.
The young nymphs are very much like the adults except for size. This is one of the few insects that have an undetermined number of molts. With almost all insects there are a set number of molts (typically 3 to 6) between the egg and adult stage and molting does not occur in the adult. Silverfish may molt up to 30 times as nymphs and continue to molt as adults.
One group of silverfish is the firebrats. Their name comes from the fact that they prefer very warm environments. As a result they are frequently found in bakeries, around water heaters, furnaces or any other similar locations of high heat.
These tiny insects are either winged or wingless with chewing type mouthparts and a swollen clypeus which basically mean they look like they have a fat lip.
Outdoors species of booklice. Image courtesy of Troy Bartlett.
The most common booklouse (Liposcelis spp.) is a small, grayish and has a very flat shape superficially resembling the shape of head lice. This common house-dwelling booklouse is wingless with the size of an adult approximately 1/25 to 1/12 inch. Booklice cause little direct damage to plants and wood because they feed chiefly on mold. They are found commonly in confined areas like the bindings of books, where they eat the starch sizing in the bindings and along the edges of pages. They prefer warm, moist conditions that are conducive to the growth of mold and mildew and require humidity of at least 60%. They are sometimes found on houseplants where they may be feeding on honeydew (a protein-rich substance excreted by plant-eating insects such as aphids), or more likely, on the sooty mold that grows on the honeydew. Outdoor species feed on organic matter including algae, lichen, fungi, pollen, decaying plant particles (detritus), and occasionally dead animal matter. The habit of psocids includes living or dead foliage, ground litter, bark of trees, and inside human habitations.
On occasion huge numbers of these insects emerge from the walls of newer housing developments. In these cases developers have used green lumber to frame the homes. Green lumber typically has not been kiln dried and has high moisture content. Due to the excessive moisture fungi eventually develops on the wood resulting huge populations of these tiny insects. When the wood eventually looses its excess moisture the fungi die and as a result the booklice emerge from the walls looking for a more favorable environment. Of course this is a temporary situation but try to tell that to a home owner.
Stored Product Pests
There are a few dozen species of beetles, moths and mites that commonly infest dried and or processed plant and animal products. The list of the type of materials that these insects breed in is quite extensive but some of the more common products include grain of any type, pasta products, flour, nuts, spices, drugs (including marijuana), dried meat, dried dog food, tobacco, decorative corn, dried flowers, breakfast cereals, candy bars and energy bars to name a few.
These insects are always present in the environment and will attack any of these products if given time. The important point here is that if any of these products remain unprotected on the shelf, in a super market, home, warehouse or any other situation for that matter for an extended period of time they will become infested with one or more of these insects. The length of time before an infestation occurs will vary depending on the situation but a year or more is way more than a product should remain unprotected, sold, consumed or discarded.
As to the where the source of the infestation initiates there are many possibilities. Take for example a hypothetical package of infested noodles that is stored in a kitchen cabinet. If they have been in the cabinet for a long period of time (years) there is a pretty good chance that the infestation initiated in the cabinet. But those noodles have probably had quite a route prior to reaching the homeowner and anywhere along where they could have been infested. They could have been infested at the manufacturing plant or in trucks or ships while they were transported to a warehouse or even at the grocery story prior to purchase.
As far as protecting these products from infestation this is a difficult matter. The larval and/or adult forms of these pests are capable of penetrating most types of packaging including cardboard, tinfoil and soft plastics. They cannot eat their way through glass, canned goods or harder plastics.
Generally speaking most of these pests have a few characteristics in common. Namely most are small averaging 1/8th to 1/4th inch or smaller (mites) in length in the adult stage. More importantly the early immature stages (eggs and early instar larvae) are nearly microscopic in size. Of course this makes early detection of an infestation difficult. In addition most of these insects lay a large number of eggs (several hundred per female) and have a relatively short life cycle (several weeks from egg to adult). As a result a rather large infestation can develop quite quickly.
Successful control starts with identification of the pest and determining the extent of the infestation. Any small moths or beetles found in the vicinity of stored products are a pretty good indication that an infestation does exist. At that point an inspection of the products is necessary. Most of these pests leave the host when approaching the pupal stage and frequently eat their way out of the packaging. As a result small emergence holes (typically about the diameter of pencil lead) will appear on the outside of the packaging. An inspection of the suspect product may reveal webbing (produced by some of the moth larvae), the feeding larval stages, larval feces or damaged product.
If the infestation occurs in a kitchen cabinet or any other area where multiple products occur everything must be inspected except those products that are packaged with those types of materials that can not be penetrated by the pests. Keep in mind that since many of the stages of these small insects cannot be readily detected a product that appears to be pest free may be infested.
Any products that are obviously infested should be discarded or destroyed. The question then remains as to what should be done with those that may or may not be infested keeping in mind that the eggs and early instars of these pests are almost impossible to see with the unaided eye. Discarding these may not be an option depending on their value. If this is the case there are really only two options to kill the existing pests without contaminating or harming the products. Fumigation is a possibility but is generally limited to large scale operations or very valuable products. On a small scale heat is viable option. As with all insects stored product pests cannot survive when held at temperatures much above 100 F for any extended period of time. A few minutes in a microwave are also very effective. Insects are much more tolerant of very low temperatures than high temperatures. As a consequence freezing or refrigeration may not be an effective means of treatment.
Finally it should be kept in mind that many of these pests will leave the infested product and pupate in nearby areas. As a consequence all shelving, racks and other storage areas where the products occur should be thoroughly cleaned and possibly sprayed with an insecticide that carries a food label.
The following are some of the more common stored product pests:
Confused Flour Beetle
The confused flour beetle is very similar in appearance to its close relative the red flour beetle. Both are amongst the most important stored product pests of grocery stores and homes. Adults of both are about 1/8th inch in length. They attach a wide variety of stored products but are unable to feed on whole grain.
The confused flour beetle adults
Infestation or red flour beetle. Image courtesy of Jim Kalish, University of Nebraska Entomology.
This beetle and it close relative the bean weevil belong to a small family of insects that is characterized by their unique wedge-shaped body. Both species are pests of stored legumes laying their eggs on whole seeds and completing their life cycle within. The bean weevil has a broader range of hosts than the cowpea weevil developing within peas, beans, lentils, lima beans and other seed while the latter only attacks cowpeas.
Adult cowpea weevil. Image courtesy of Jim Kalish, University of Nebraska Entomology.
The is a pest of dried and processed meats and other materials such as such as jerky, dried fish, bacon, ham, cheese, feathers, horns and stuffed animals. Historically it has become less of a pest with the advent of refrigeration.
The adult larder beetle. Image courtesy of Jim Kalish, University of Nebraska Entomology.
The larvae of a larder beetle. Image courtesy of Jim Kalish, University of Nebraska Entomology.
The rice weevil and its close relative the grainary weevil are two of the oldest and most important pests of stored grain. They are primarily a pest of whole grain with the larvae developing inside grains such as wheat, barley, rice, corn, milo, oats, millet and sorghum. Since their larvae develop inside these grains they are not attracted to milled products such as flour, breakfast cereals, corn meal, etc. and therefore are not a major pest to the homeowner.
Adult rice weevil. Image courtesy of Jim Kalish, University of Nebraska Entomology.
Lesser Grain Weevil
As with the rice and granary weevil the lesser grain borer is primarily a pest of whole grain. They are one of the most important pests of whole grain. As with all stored product pests it is worldwide in distribution. This of course is due to shipping of grain and these pests all over the world.
Lesser Grain Beetle. Image courtesy of Jim Kalish, University of Nebraska Entomology.
Saw Tooth Grain Beetle
This is probably the most common stored product pest found in kitchen pantries and grocery stores. It gets its name for the saw tooth like spikes found on the margins of the prothorax. Unlike the lesser grain beetle and rice and grainary weevils it cannot survive or feed on whole grain but is very common in processed grain products like pasta, flour, breakfast cereals and many more.
The saw tooth grain beetle. Image courtesy of Jim Kalish, University of Nebraska Entomology.
These tiny insects are frequently associated with damp conditions feeding chiefly on mold. The presence of mold is not needed for their survival but damp conditions are. These insects have a tremendous reproductive capacity and a population of a few can reach millions in a few short months. As a result their main damage is due to contamination of various food products.
Psocids or Psocoptera. Image courtesy of Jim Kalish, University of Nebraska Entomology.
Indian Meal Moth
This small moth (app. 1/2 inch) prefers coarser grades of flour, and is therefore the most common pest in whole-wheat or graham flour and cornmeal but will attach a wide variety of other stored products. Heavy webbing produced by the larvae typically extends throughout the infestation. If present the adult moth can be found flying around the structure. Their coloration is quite diagnostic of the species.
Adult Indian meal moth. Image courtesy of Jim Kalish, University of Nebraska Entomology.
There are a number of species of mites that feed any kind of organic mater but often infest cheese, dried meat, four, seeds and similar food products. Due to a very short life cycle these can breed to tremendous numbers in a few weeks not only damaging and contaminating these products but often get on humans and cause an itching of the skin resulting in a condition called grocer’s or miller’s itch.
A heavy but typical infestation of mites. Image courtesy of Jim Kalish, University of Nebraska Entomology.
Yellow Meal Worm
This is one of the larger species of beetles that infests stored grain and grain products approaching 3/4 inch in length. The larvae are commonly raised and sold in pet stores as fish and reptile food.
Life stages of yellow meal worm. Imaged courtesy of Jim Kalish, University of Nebraska Entomology.