Pserious Psocids

For years, the psocid (pronounced sô’ sĭd) was considered simply a nuisance pest. The tiny insect, it was believed, fed on grain mold but not the grain itself; was fairly easily controlled by standard fumigants or insecticides or by simply reducing moisture levels; and was not a direct economic or health hazard.

As psocid populations have grown at an disquieting rate across the globe over the last decade and a half, these common beliefs about this insect group of 5,000 species have proven to be species-specific at best and completely unfounded at worst.

In fact, these “nuisance pests” are now seen by university and USDA researchers as having “emerged worldwide as serious stored-product pests.”

“Historically, they have been considered more of a nuisance pest, primarily in tropical parts of the world,” said USDA/ARS Research Entomologist Jim Campbell. Then population growth began to be seen in Australia, where, he said, “they were getting astronomical numbers of them in grain.”

With the expansion of global trade, the insects are now becoming a problem in all parts of the world, including the U.S., and are literally being delivered into plants on and in packaging of products of all types. While a single psocid or egg is virtually undetectable by the naked eye – being pale in color and extending only 1 mm in length, with eggs measuring less than 1/3 of that – populations can grow to incredible numbers in relatively little time, damaging product and causing allergic reaction.

As a result of the increasing appearance of psocids, ever increasing amounts of research is being conducted across the U.S., overturning previously held beliefs and revealing new facts. But there are still many unknowns about psocids and much industry awareness to be built.

“We’re just shedding light on what kind of numbers are out there,” said Oklahoma State University Assistant Professor George Opit. “I am getting psocid questions from home owners and industry; the same is true for the USDA ARS.” In addition, he said, with its recognition of the increasing psocid problem in the U.S., the FDA has asked Opit to give a presentation next spring on psocid identification, biology, status and management as a means of increasing awareness about this little known insect group.

“What is worrisome,” Opit added, “is that we have very little information regarding why psocid-related concerns have suddenly increased in the U.S., and a dearth of information exists on species associated with homes and grain handling, storage, and processing facilities.”

“Psocids are a very primitive group of insects,” said USDA ARS Researcher Frank Arthur. “We don’t really know a lot about them, except that we’re hearing reports that they seem to be increasing.”

The Psocid Challenge. Campbell said he has heard psocids as a problem in the warehouse and packaging areas rather than in the production area itself. However, they can show up just about anywhere as they are proving to be a very mobile insect. “We don’t know how far they can move, but there is some suggestion of high mobility,” he said. Because of this, psocids may come in on deliveries then move to various areas of a plant to seek out the moisture essential to their survival …

Essential, that is, to the survival of some species – which symbolizes the very essence of one of the greatest challenges in control of this pest. Although a relatively small percent of the thousands of psocid species are encountered in the U.S., each of these has unique biological characteristics. This means that a method which may be useful in controlling one species may have absolutely no effect on another, explained Steritech Technical Manager Judy Black.

Many species are attracted to moisture and tend to reproduce more quickly in higher humidities; for these species, a reduction in humidity to 60 percent or less will kill existing psocids and their eggs, she said.

However, said Jim Throne, USDA ARS researcher, other species don’t need high moisture to survive. In fact, Throne said, “one species is actually better off at lower relative humidity.”

ARS research has, in fact, turned over one of the most commonly held beliefs about psocids – that they feed only on the grain mold not grain itself. “Psocids are omnivorous,” Throne said. They will feed on almost anything, including dead insects and one another.

Inspection. It can also be difficult to prevent the build up of psocids simply because of their size. Inspection is a standard method of prevention through detection; however, Black said, “When you notice psocids, you’ve got a huge problem already. Visual inspection is useful but with smaller infestations, you may not see them.”

Regular inspection to detect and contain this insect pest is also important in eliminating a population before it can become a major infestation because, while psocid species do vary greatly, they are, in general, less susceptible to fumigants and insecticides than are most other stored-grain insects.

This is because of the psocid’s biology and protective proteins but also because of its short reproductive cycle, with some species needing only three weeks to develop from egg to adult, but able to extend this if exposed to a fumigant. The key to control,” Arthur said, “is to identify the species because susceptibility to insecticides does vary depending on the species.”

The insect’s mobile nature also makes it difficult to control with fumigants or insecticides. The mobility of the adult can reduce its exposure to the pesticide, Campbell said, even temporarily slipping through building cracks and crevices to outside walls; and a psocid egg which is exposed to a fumigant can delay its hatching until the vapor is vented.

For all these reasons, inbound inspections are important for preventing infestations from being brought into the plant, Campbell said, noting, however, that the only signs of psocid presence will be the insects themselves. Unlike other pests, they leave no easily visible signs of damage, excrement or frass, and eggs are generally laid in cracks and crevices, he said. “You won’t see many signs except seeing them directly.”

“This is a challenging one,” Campbell said. Inspections are critical to keeping the insect under control, but, he added, “they are difficult to see in an inspection.”

On a positive note, you can monitor for the presence of psocids very simply and inexpensively, using a small square of corrugated cardboard. In plants, the monitoring traps should be placed in delivery and warehouse areas, and in grain bins, they can provide very effective samples when placed on the surface of the grain or near the hatch. To check for psocids, simply shake the cardboard over a sheet of paper and examine it with a magnifying glass.

Elimination. If psocids are found, they need to be eliminated as soon as possible to prevent further contamination, population growth and potential movement to other areas. While psocids have been found to be less susceptible to many pesticides, a pest management professional may recommend:
• non-residual fogging (where approved) with an insecticidal barrier treatment along the exterior against those which move outside.
• heat treatment with temperatures of 100o to 140o depending on the species. While some species are known to be to survive temperatures of 100 o, others will require that heat be of 140 o for complete eradication.
• humidity reduction to levels of 50 to 60 percent. This is, again, dependent on the species, as some psocids thrive best in low humidity environments.

With any treatment, consideration should be made for the psocid’s mobility, to ensure that they do not simply move out of the area during treatment and move back in once it is completed.

Prevention. While psocids do not generally come in from outside, a proximity to grain bins or other such attractants could bring the pest into the vicinity, then interior, of a processing plant. To help prevent this and protect your plant against other exterior pest as well, Black recommends:
• When mowing, start near the building and work your way out, which serves to push the pests away from the plant. If you mow toward the building instead, you may be literally pushing pests not only toward, but into, the plant as they seek to outrun the mowing.
• Maintain door sweeps; police entry and exit doors to ensure they are not being propped open.
• Ensure that no vegetation touches or overhangs the building, as this can provide access points for many pests.

“I think a lot of it is being aware of them and keeping an eye out for them,” Campbell said. At the same time, he said, understand that where you are seeing psocids is not necessarily where the population was building up. Rather they will move around to find the areas best suited to their survival.

Although the ever-expanding global supply chain is certainly a factor in the worldwide movement of the psocid, why psocid populations are growing at all is less of a certainty. One possibility Campbell noted as being posited in Australia is that there had been so much fumigation being conducted that insects which preyed upon psocid eggs were being eliminated, enabling these less susceptible secondary pests to build to unprecedented numbers.

“It could also be that people are looking more, and are doing more inspections,” he said. Because any form of contamination meets with less customer and consumer acceptance than in the past, there may simply be more awareness of their presence.

Either way, “it’s definitely an issue to have any insect in a plant,” Black said. “Psocids don’t breed (indoors) or spread disease, but they can cause physical contamination of the product. The focus has to be on keeping them out.