Psocids (booklice) in stored grain
In the past 10 years, psocids have emerged as significant pests in grain storages in Australia. Psocids (pronounced ´so kids´, also called ´booklice´) are tiny termite-like insects. These are common grain pests, but are not seen usually because they are the size of specks of dust. When present in large numbers they appear as a moving carpet of dust on the grain, silos and sheds. Psocids infest a range of commodities including wheat, barley, sorghum, maize, rice beans pulses, oilseeds and cassava.
The most cosmopolitan species of psocid in Australia is Liposcelis bostrychophila. L. entomophila occurs mostly in warmer summer rainfall zones of Queensland and northern New South Wales. L. decolor occurs mostly in winter rainfall zones of Western Australia, South Australia and southern New South Wales. One or more of these species have been detected in almost all grain handling facilities in Australia. Central storage and export terminals are worst affected with regular population outbreaks, control failures and reinfestations.
Psocids thrive under warm, moist conditions. Under favourable conditions of 30°C and 70% relative humidity, psocids will multiply by 25 times in a month. Under these conditions the lifecycle of psocids takes 21 days.
Because of their rapid growth in warm, moist conditions, they usually reach peak numbers during late summer. They are more common in the higher humidity of coastal regions than in drier inland areas. Infestations of psocids are generally more conspicuous in commodities with high moisture content and contaminated with mould.
Damage caused by psocids
Psocids are secondary feeders (i.e. they excavate the soft endosperm of damaged or cracked grain). They feed preferentially on the germ, eating out the germ by first gaining access via damaged seed coat caused by harvesting, handling and other ´primary´ stored grain insect pests. Their feeding causes damage only when numbers are high. Huge infestations can spread to storage structures, machinery and walkways, sometimes in the form of a moving carpet of brown ´dust´. They can cause discomfort to workers handling infested grain. The presence of ´zones´ of heavy psocid populations in grain storages can cause build-up of grain temperature and high humidity. This can result in spoilage of grain.
Many markets will not accept produce infested with psocids. This reason is sufficient to justify control measures.
The three main psocid species differ markedly in their susceptibility to particular insecticide treatments. However, almost all infestations are comprised of at least two species. Therefore, treating an infestation with a single chemical insecticide where more than one species are present may result in incomplete control and subsequent reinfestation by the tolerant species. Reliance on chemical control alone will often not work.
If serious infestations of psocids persist, it may help to have the psocid populations closely examined by a specialist who can identify the species present. Send insect specimens to the Department of Employment, Economic Development and Innovation's Stored Grain team.
It is always risky to use a chemical to control a mixed infestation, specifically where the tolerant species L. entomophila is present. It is better to identify the species prior to choosing chemical controls. Check registered products.
Strict hygiene practices in and around grain or produce storage and handling areas are essential for control.
- Remove and destroy unwanted, infested produce by burning or burying.
- Fumigate infested produce that is to be kept as described below.
- Clean up and destroy grain dust and grain residues at least weekly.
- Clean grain storages and handling equipment before handling new uninfested produce.
- Spray the walls and floors of warehouses and sheds with azamethiphos (Alfacron®), 5 g/L water for steel storages, 10 g/L water for concrete storages (applied at 20 L/m²). Do not spray surfaces that come into contact with the grain or produce.
Because of their need for warm, moist conditions psocid numbers can be kept low by:
- reducing temperature below 20°C, for example by aeration, or
- reducing moisture below 60% relative humidity or 13% moisture for cereal grains, for example by drying.
Fumigation with phosphine will kill psocids only if it is done in a sealed, gas-tight storage. Dosages lower than recommended on the label often allow psocid eggs to delay their development, during the fumigation. Reinfestation due to subsequent hatchings can occur after the fumigation period is over. Apply 1.5 tablets per cubic metre and leave the enclosure sealed for seven days if the temperature of the produce is above 25°C, or for 10 days at lower temperatures. Air the treated produce before it is handled.
Contact us or the National Grain Storage Extension Team:
Philip Burrill: 07 4660 3620
Peter Botta: 03 5761 1647
Chris Newman: 08 9366 2309
- Grains Research and Development Corporation publications 'Aeration - how aeration works´ and ´Aeration - what´s possible?´