Psocids are emerging pests in stored grain.
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At less than five millimeters long, psocids (often called booklice or barklice) can be very difficult to detect in stored foods — until they reach large numbers. And while a single psocid or two may have minimal impact, large populations can cause extensive contamination, tainting ingredients or even altering product taste, especially when product remains in storage for long periods. Just as these tiny insects are difficult to detect, they also are difficult to manage.

Psocid Identification. As a group, psocids are very distinctive in terms of appearance but being so small, they are often overlooked or inaccurately identified. They also are sometimes confused with mites, but look very different when viewed through a hand lens. Psocid pest species:

Psocids (a.k.a. booklice or barklice) are less than 5 mm in length.
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  • Have protruding eyes and enlarged heads with the widest point just before the start of the abdomen.  
  • Are wingless and soft bodied.
  • Can be brown to cream or white to gray in color.  
  • Measure three times longer than they are wide, giving them an elongated, flattened appearance.
  • Have long, thread-like antennae on their heads.
  • Can be seen as a few isolated individuals to discovering millions coating surfaces.

THE PSOCID CHALLENGE. Psocids are nearly microscopic, “appearing as tiny dots to the naked, untrained eye,” said IFC Director of Technical Services Sharon Dobesh. “Inspectors know where to find them and frequently do. While you might not recognize a psocid problem, your auditor surely will.”

It is often thought that psocids feed only on mold, added USDA ARS Research Leader and Entomologist James Campbell. “But that is not true since they can live on a wide range of foods of both animal and plant origin.” They do, however, require relatively high relative humidity of 70% to 80%, and they can absorb the moisture they need from the air. Additionally, he said, psocids can be quite mobile and move from areas with higher relative humidity into other areas to feed, with a tendency to be observed crawling on the outside of packaged foods.

Although psocids do feed primarily on organic matter such as molds, fungi, and algae, they also will feed on starch-based materials such as grains and glues, Dobesh said. Because of this, they often can be found in dust, since molds and other food sources can occur with these.

Liposcelis brunnea Motschulsky
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Psocid reproductive habits further add to the challenge, Dobesh said. Psocids develop from egg to adult in as little as 18 days, and can then live for up to three months. As adults, she added, “Psocids can reproduce often, quickly, and for a significant period of time, allowing their populations to multiply rapidly.” Certain psocid species have the additional advantage of parthenogenetic reproduction — that is, without the presence of a male — which further increases reproductive rates. Thus, Dobesh said, “The presence of a few psocids can rapidly increase to an infestation, unless dealt with efficiently and immediately.”

Taking the challenge of psocid management even further is that a facility often has more than one species present at the same time, Campbell said, and each species can vary in its response to management tactics.  

Because of this, once sighted, psocid treatment begins with identification of the psocid species and life stages that are present, Dobesh said, explaining, “Psocids have different life stage characteristics that can require knowledge of the life stages present.”

Psocid Management.  The efficacy of pesticides in psocid management depends on the species and the pesticide, with psocids tending to tolerate a lot of pesticides more than other stored product pests and can recover from treatments more rapidly, Campbell said. “But generally they are harder to control with pesticides than are stored product moths and beetles.”  

The egg stage is tolerant to phosphine, and the long duration egg stage makes management with fumigation challenging, he explained. However, maintaining lower relative humidity conditions can be effective in managing psocids, so reducing moisture content of grain and air relative humidity, and lowering the temperature can also help. Heat treatments, which also help lower the relative humidity, can be effective; but, he said, “Species vary in tolerance of heat.”

“Temperature control, humidity control, organophosphates, and some grain protectants are some of the more effective psocid management strategies,” Dobesh added. Although heat tolerance does vary by species, applying temperatures over 104°F can stunt the development of many species, helping put an end to population expansion.

Campbell also recommends sanitation, along with moisture and temperature management, as likely being the best tools for psocid management. Additionally, because the psocids can move in and out of high relative humidity environments, he said, “Inspection and trapping to identify where the psocids are coming from can be helpful.”

Pieces of corrugated cardboard also can be useful for trapping psocids.  

Psocid Prevention. While psocid management can be a distinct challenge, preventive measures for psocids can begin in-house once you know what to look for, Dobesh said. Effective prevention methods help reduce or even eliminate conducive conditions, that is, elements of the facility’s environment that attract pests. For psocids this includes high humidity and sanitation issues. Some best practice methods Dobesh provided for psocid prevention include:

  • Facilities located in climates with routine or occasional humid weather should inspect for psocids during the humid stretches of time.
  • Thoroughly inspect any packing supplies stored in poorly ventilated, humid areas, as these are especially vulnerable to psocid activity.
  • To help alleviate high humidity, consider adding environmental controls and preventing moisture when possible.
  • The classic recommendations to correct higher moisture levels include portable heaters, fans, dehumidifiers, and other means to move air and promote drying.
  • Prevent added humidity further by avoiding wood pallets, which tend to hold moisture.
  • Store products to avoid dust buildup, placing the most susceptible supplies or products above floor level on racks where air circulation will be best.
  • Regularly inspect bags and packages containing starchy products.
  • Thoroughly inspect areas behind storage racks since air movement is limited in these areas, allowing higher humidity, dust, and molds to accumulate.

While in-house efforts can help keep additional pests from entering, it’s best to involve your pest management professional to address conducive conditions or any existing infestations, Dobesh added. Based on the psocids species at hand and the location and extent of the infestation, your pest management provider will consider the specific needs of your facility and develop an effective strategy for control.

The author is owner of LJ Writing Services and a member of the QA Advisory Board.