Where do psocids come from?
(summary of Turner, B.D. (2006) Psocid infestations and domestic kitchens. Journal of Environmental Health Research 5:(2) in press)
Over the last couple of years pairs of bags of flour were purchased from retail outlets in a number of locations in England and Northern Ireland. One of the two bags was immediately sealed inside a plastic bag whilst the other was placed in a typical domestic kitchen in the location. After one week this bag too was sealed in plastic and both bags were sent to King's College where they were "incubated" for 3 months.
Twenty two of the 133 bags that spent a week in kitchen cupboards were infested with psocids. None of the 135 control bags contained psocids. The commonly held idea that bags of flour purchased from shops are sometimes contaminated with psocids is not supported by this study.
These findings are in accord with other, largely unpublished, work carried out by the author in which there is a partitioning of psocid species found in homes compared to warehouses (Turner and Ali 1996 -see references) . Liposcelis bostrychophila , although ubiquitous needs warm conditions, in excess of 20°C, to reproduce. It cannot live outdoors throughout the year in the UK and is the normal species found in domestic property.
Most if not all situations of consumer complaints of psocids on new flour and other farinaceous products are caused by the attraction and concentration of undetected scattered psocids in kitchens onto the product. This occurs rapidly within one or two days. If a reasonable number of psocids colonise the new bag by the time it is used it will be seen to be infested. The natural explanation is that the product arrived in the house already carrying the psocids. If that scenario was an option then at least some of the control bags in this small study would have also been infested.
This study does not help to answer the wider question as to how the psocids spread from one household to another -how did the primary infestation of the property occur if not via contaminated foods? It is highly likely that almost anything that is bought into a home may carry psocids. In the case of L. bostrychophila it is important to note that it is parthenogenetic so a single egg or a juvenile or adult can found a new population.
This idea that they may be on anything derives from an unpublished study for a company that makes plastic packaging who were finding their product contaminated with psocids. The psocids in this case were moving on to the pallets of product during transportation in containers. Every one of five containers sampled contained psocids, living in the crevices of the floor of the container and feeding on spillages of powdered food which was swept into the cracks between the floor boards between use. In this case the psocids had made their way into the cardboard boxes containing the packing. This product is totally inedible and its only attraction to wandering psocids may have been slightly warmer or cooler temperatures found in the boxes compared to the container. Commercial shipping containers carry all sorts of products including foods all over the world. The containers used by the packaging company are now pre-treated with an insecticide fog to prevent contamination of their product but that is not a standard practice and therefore the possibilities for cross contamination are very high.