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How common are psocids in households?

introduction to the survey, what the traps caught, psocid/mite interactions, countrywide patterns, importance of building age, other insect pest problems, allergies, damp conditions, heating, methods of food storage, responses to infestation, summary

Surveys carried out in 1986/7 of 863 households showed that the psocid or booklouse Liposcelis bostrychophila was present in 15 percent of the households. But this study could not identify any specific characteristics of the household that were linked with psocid presence.

.Liposcelis bostrychophila adult female

Psocids are still the major cause of consumer complaints to the flour industry and the survey was repeated ten years later to determine whether there has been any changes in the levels of prevalence in households with time.

The 1997 survey closely followed the methods used in 1986/7 and participating households were solicited in similar ways. Each household was asked to place a number of small non toxic, yeast-baited traps in their kitchen cupboards where dry foods were stored. The traps were exposed for the last two weeks in September 1997, then recovered and sealed in a small plastic bag. The traps were accompanied by a questionnaire.

727 of the 3654 sets of traps and questionnaires sent out, were returned (20% response). As in the 1986/7 surveys there was a strong bias to households situated in the SE. The south west and Wales were poorly represented in the survey and the low numbers of households from those areas may bias the regional analyses.

The findings

What the traps caught Thirty two species of invertebrate (listing) were trapped by the yeast traps in kitchen cupboards. This surprisingly long list contains a number of quite large species for which the traps were not designed. Consequently, it is likely that species other than psocids or mites are under represented.

• Forty percent of the returned traps contained some arthropods.
• Just under 30% of households contained psocids
• Approximately 14% contained mites.

15% of households contained psocids in the 1986 survey, 18% in the 1987 survey. By 1997 these figures for psocids had doubled. In contrast the incidence of mites was slightly lower in 1997 than in 1986/7 (17-19%). Levels of occurence of other species such as dermestid beetles and the biscuit beetle, Stegobium, in the traps were unchanged between the surveys.

Psocid/mite interactions There is a strongly significant negative association between psocids and mites. Kitchens have one or the other but not both. In the laboratory mite infestations of psocid cultures invariably lead to the death of the psocids. The negative association between these two groups may a feature of natural kitchen based populations also.

Countrywide patterns Although the average number of households with psocids for the country as a whole is 29% this statistic conceals some marked regional differences (Figure 1).

There is a clear decrease in the prevalence of psocids in households in the more northern areas, with psocids in only 12.4% of Scottish kitchens. To the west the prevalence is much higher. More than 50% of kitchens in Wales and the west country have psocids.

The picture with mites also varies over the UK (figure 1). The country wide average is 9.9% with lowest levels in the south east (<5%) and generally increasing towards the north and west.

These high figures of incidence in the west country and Wales must be viewed in the light of the small samples obtained from those areas

Figure 1 The prevalence of psocids (lefthand map) and mites (righthand map) in the UK, broken down by area

Is the age of a house important? The two surveys were similar in respect of the range of age structure of the households but the 1997 survey was more biased to housing built in the mid-war period with relatively little new housing available. In 1986/7 a non-significant correlation between age of property and presence of psocids was noted and this situation remains in the 1997 survey.

Are psocids found in houses with a history of insect pest problems? An analysis of the levels of infestation in domestic kitchens based on the responses of householders, indicated that 36.5% had had trouble with insects in the past. One third of the respondents (32.3%) knew what psocids were.

For the most part, the response of householders to previous pest infestations bears no relationship to the current presence or absence of psocids in their kitchens. Interestingly there was a significantly greater prevalence of psocids in households that carried out their own pest control treatments using insecticides. This may be interpreted either as an inefficiency of the control methods or insecticides used, or that past use has led to the development of some resistance in the psocids to the insecticides.

What about allergies? People with allergies or asthma were found in almost 40 % of the households in the survey. Whilst for some the condition was present all year round, most reported only seasonal allergies. There was no significant relationship with psocids, but there was a significant correlation between the presence of mites (all types) in kitchens and of allergy in the household.

What about damp conditions? High humidity has been suggested as being important for psocid infestations. To address this, the questionnaire asked contributors to rate their condensation problems on a 1-6 scale. No statistically significant correspondence was found between levels of condensation and the presence of psocids or mites, although there was a non-significant trend of increasing mite prevalence and condensation levels. This accords with the country wide increase in mite prevalence in the more northerly and westerly areas which are more humid than the south east.

Or heating? A large proportion (71.4%) of the kitchens were heated during the winter but this had no relationship with psocid or mite prevalence. Similarly there were no connections to be made between kitchens, which were classified as either warm (50.2% of total) or cool (45.5%) in the summer, and psocid or mite presence.

Are food storage conditions important? With the demise of the larder in most modern homes, food is stored in a variety of places. Psocids were not correlated with where food is stored but mites appeared to be strongly influenced by storage location. Households where food is stored in wall mounted cupboards had a significantly lower than expected mite presence whilst those where food is stored in floor mounted cupboards were significantly above expectations.

Respondents stored different foods in different ways. Of the main dry foods (flour, cereals, semolina, rice, and sugar) a significant link between food storage methods and prevalence of psocids was evident where flour kept in the original packaging was stored in a storage container.

Surprisingly there is no significant relationship between the frequency with which flour (and rice -the semolina sample too small to analyse) is purchased or the length of time it is stored and the presence of psocids. However it is of interest to note that there is a slightly higher than expected number of households with psocids among those where flour is bought more frequently and the converse it true of households that buy flour less frequently. The crossover point is 5 months. This fits with the observation that as flour ages it dries out somewhat and becomes much less attractive to psocids.

Mites are also most attracted to kitchens where there is a regular supply of fresh moist flour but again the relationship is not statistically significant.

Since flour is the primary product that is attractive to psocids, respondents were asked to name their usual brand(s) of flour There is no correlation between brand of flour and the prevalence of psocids

The majority of respondents (88%) shop in supermarkets. Only 1.5% use local shops exclusively whilst the rest used both. There was no statistically significant correlation between the type of shop used and the presence of psocids, but for mites there is a significant correlation between whether local shops are used or not.

Respondents were asked whether they had ever thrown food away and if so why. Almost three quarters (70%) stated that they had thrown food away in the past. Of these, most were thown out for being past the sell by date (75%) with insect infestation being the second most common reason (23%).

Psocid infestation of farinaceous products is the most important cause of consumer complaint for these types of foods. Survey respondents were asked what they would do if they found a psocid infestation in their kitchens. The responses to this question are presented in the following table. The vast majority said they would deal with the problem themselves by throwing the infested food away and cleaning. The numbers that said they would complain was small (less than 5%).

Response of householders to an infestation of psocids in their kitchens

These results indicate that householders do not rate psocid infestations as something with which they would involve the local Environmental Health Office. This was born out by the responses to the brief questionnaire sent to EHO’s of England and Wales. Just over half (55%) of these EHO’s dealt with 5 or less psocid complaints per year and only 12% dealt with more than 11 complaints per year. The majority (60%) of EHO’s felt that psocid complaints are decreasing and only 20% felt they are on the increase.

Summary

There has been a marked increase in the prevalence of psocids in households in the past 10 years, these increases being primarily concentrated in the south and west parts of the UK. As in the 1986/7 survey there are no strong indicators that particular characteristics of the house or kitchen; cleaning, buying or storage practices; or food preferences predispose households to psocid infestations. There appears to be a 50:50 chance of having a psocids at all and about a 30% chance of gaining them in a 10 year period. Once they are resident the chance of losing them is lower (about 15% in 10 years) hence the net increase in the numbers of households with psocids during the inter-survey period.

Acknowledgements

This study was made possible by support from the Pre-packed Flour Association to BDT.

[this is an abridged version of the paper by Turner, B. D. & Bishop, J. 1998 An analysis of the incidence of psocids in domestic kitchens: the PPFA 1997 household survey (What's bugging your kitchen). Environmental Health Journal 106, 310-314.]