How Humidity Levels Impact the Most Common Pest No One Knows They Have

Psocids are a group of insects containing about 5,500 species that are commonly referred to as barklice and booklice. Despite the fact that psocids do not bite, research performed in the last 20 years has shown that parasitic lice, like head and body lice, actually share a common ancestor with booklice and therefore should be lumped into the same order called Psocodea.

Psocids are important pests in stored grains, where they can result in contamination, damage, and possible rejection of commodities. They can also be a concern in warehouses, museums and even homes. Book lice are actually one of the most ubiquitous pests that no one knows is present in their homes.  A few years ago, a research team at North Carolina State University investigated which arthropods were most commonly found in homes in North Carolina.  Out of the nearly 200 homes inspected, nearly every single one had booklice present.

Since these pests are so small, they are often overlooked. But in recent years, commercial control in grain storage has become more difficult since some populations have developed resistance to certain pesticides, including some fumigants like methyl bromide and phosphine. It is known that psocid infestations often occur in locations where moisture is an issue. New research at Oklahoma State University sought to determine what humidity levels are required to kill psocids. 

This research was performed by Abena Ocran and her colleagues and was published in the Journal of Economic Entomology in April 2021. The researchers exposed all life stages (eggs, nymphs and adults) of four different psocid species to 43, 50, or 75% relative humidity at a constant temperature of 30 degrees Celsius (86 degree F) for different time periods. 

Here’s what they learned:

  • All of the psocids kept at 75% RH increased in population size after 14 days.
  • Lower humidity levels were lethal to psocids
  • Eggs were a little bit more tolerant than nymphs and adults, but all life stages of all four species were killed after 16 days at 50% RH.

One of the most common recommendations that PMPs make to clients with psocid infestations is to lower humidity levels, so this research lends credence to that common recommendation and provides more data to help set customer expectations. That is, your customer should expect to see control 16 days after lowering the humidity below 50%. Additionally, this information can also be used in grain storage facilities to help supplement pest control efforts through IPM approaches. By understanding and communicating information such as this, PMPs can deliver data driven recommendations for clients that will help manage expectations and control one of the most common pests in homes.

Jim Fredericks, PhD, BCE


Bertone MA, Leong M, Bayless KM, Malow TLF, Dunn RR, Trautwein MD. 2016. Arthropods of the great indoors: characterizing diversity inside urban and suburban homes. PeerJ 4:e1582

Abena F Ocran, George P Opit, Bruce H Noden, Frank H Arthur, Bradford M Kard, Effects of Dehumidification on the Survivorship of Four Psocid Species, Journal of Economic Entomology, 2021;, toab066,

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