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The Pest Status of Psocids in the UK*

*This is a modified and updated version of account that was published in 1998

(Turner, B. D. 1998 Psocids as a nuisance problem in the UK. Pesticide Outlook 9, 27-30)

Introduction

Members of the insect order Psocoptera, commonly called psocids, are mostly innocuous inhabitants of trees and other vegetation. They are small fragile insects ranging in size from 1 to about 10mm long and are often cryptically coloured. Many live on bark and leaf surfaces feeding on algae, fungi or lichens. Others are found in leaf litter, or in caves. A small number have extended their range to include buildings and building materials such as thatch, stored food stocks and museum exhibits. About 16 of the 87 species that have been recorded from the UK (Turner & Ali 1996) have anthropophilic tendencies but of these, only two or three are of any significance as pests. For example,Lepinotus patruelis a round bodied, 2-3mm long, dark chocolate brown psocid is a minor storage pest of grain, used in the milling and brewing industries (eg. Finlayson 1949). It is interesting to note that this wingless species, which is normally found indoors, turned up in insects sampled from low altitude air currents (Freeman 1945) and this may be a significant means of dispersal for wingless psocids generally.

Photo 1 Lepinotus patruelis. The large dark brown individuals are females, smaller brown individuals are males and the nymphs are pale and of various sizes

Most psocid species in the UK are not pests. Where they are found in some number this is often associated with other problems such as excess moisture and fungal growth. Liposcelis bostrychophila is different. It can be found in clean well kept premises and is usually associated with kitchen food stores. When individuals are distributed throughout storage areas they are difficult to detect, but they become very obvious when they congregate on a particularly attractive food source, frequently a fresh bag of flour.

Photo 2 Liposcelis bostrychophila female

Liposcelis bostrychophila is the principal psocid pest species in the UK and in Europe. This single species, which is mainly an inhabitant of households, is responsible for virtually all of the psocid related complaints in the UK (Turner & Ali 1996). Liposcelis bostrychophila is primarily a nuisance pest and is widespread in the UK. Recent estimates are that 30% of households contain this species. It can cause considerable emotional disturbance to householders who pride themselves on cleanliness, or who are entomophobic. At high density L. bostrychophila taints foodstuffs with waste products and can physically damage grains (eg. rice). In addition they may elicit allergic reactions in sensitised persons.

Several other small (about 1mm long), flattened, wingless liposcelid species (eg. Liposcelis corrodens, L. pearmani and L. brunnea) can become quite common in industrial sites, particularly in the summer months but are uncommon in domestic premises.

 

Pest qualities

In assessing the key characteristics of an insect pest species, L. bostrychophila scores highly on most counts. It is small and very inconspicuous at low population density, able to hide in cracks and crevices. It reproduces parthenogenically, that is there are no males and the females lay unfertilised diploid eggs. It can feed on a wide range of foods and despite being wingless has a very wide distribution in the UK and the rest of the world. The map shows our present knowledge of the pest status of L. bostrychophila in the world.

The lifespan of L. bostrychophila is relatively long and varies with temperature, humidity and food sources available. The maximum recorded life span is 53 weeks feeding on skimmed milk powder (Turner 1994). Sinha's climate plasticity index (Sinha 1991), a mathematic which analyses the climatic tolerance of the population dynamics of a pest thereby indicating a species' relative pest potential, produces highest values for the flour beetleTribolium castaneum and the flour moth Anagasta kuehniella . The value for L. bostrychophila lies between these top two (Turner 1994), which explains its ubiquity.

 

Life history

Liposcelids lay eggs singly or in small batches. The eggs are large in relation to the female (about 1/3rd body size) and usually only develop one at a time. Egg production rises to a peak when the females are about three weeks old and then steadily declines until death. Maximally three to four eggs per day can be laid, a remarkable rate of production considering the relative sizes of the eggs and female. Liposcelids have 4 nymphal stages (one less for males). The nymphs are almost colourless and difficult to see on light coloured backgrounds, and populations can increase rapidly without notice until the nymphs mature into brown adults.

The generation time (from egg to egg laying adult female) depends on temperature. For L. bostrychophila it takes approximately 56 days at 20°C and shortens to 22 days at 30°C (Turner 1994).

 

The situation in the UK

There has been a gradual increase in the prevalence of psocid related complaints in domestic kitchens in the UK. It is impossible to judge whether this has been the result of an actual increase in numbers and distribution of L. bostrychophila or as the result of a decrease in the tolerance of householders towards insects in the home. There are no central databases collecting pest statistics so that long term trends cannot be easily determined. The food industry has data on psocid related complaints which plots the early, almost exponential increase through the 1970’s (fig.1).

Levels peaked in the early 1980’s but since that time the data have become less reliable. In the 1990’s there has been an increasing tendency for supermarket chains to respond to psocid complaints in foods themselves, rather than pass the evidence to the manufacturers. Such data are often vague and imprecise.

Within this long term pattern there is some evidence that climate influences annual complaint levels. Consideration of the numbers of complaints for the period from when complaint levels peaked until the data became less reliable (1982-1992) shows considerable year to year differences (fig 1). The patterns of complaints show a strong seasonality, peaking in the autumn and being lowest in the spring (fig. 2).

 

Numbers of complaints rise during the summer period as the psocids multipy in the warm months of the year. The UK summer is notoriously variable and there is a good agreement between the numbers of psocid related complaints and whether the summers were above or below average (fig. 3). In Figure 3, 0 on the monthly deviations axis represents an average summer (equal to the long term average) whilst negative values denote colder than average summers and positive numbers warmer than average.

This suggests that the size of the window (time v. temperature) of favourable summer conditions affects the growth of liposcelid populations in the home and therefore the probability that an infestation will be noticed.

Intriguingly these observations are not limited to the UK. In Denmark, data are collected centrally on pest infestations. Denmark has experienced an identical increase in the numbers of psocid related complaints over time to that described for the UK (Turner 1987; Turner & Ali 1996).

This pattern of increase in the prevalence of psocid related complaints in domestic environments has not been recorded in other parts of the world and appears to be isolated in the UK and NW Europe. Elsewhere in the world, psocid and particularly liposcelid, infestations are linked primarily with grain store environments.

 

Problems of controlling Liposcelis bostrychophila

Liposcelis bostrychophila is an extremely difficult insect to control. Its small size enables it hide in crevices and other inaccessible places, avoiding contact by many standard types of treatment. They are able to survive without food for considerable periods of time (Turner & Maude-Roxby 1988) and appear able to spread readily to new areas, possibly on clothing or in baggage. Thus apparently complete control is sometimes short-lived and problems frequently recur. A single individual or even one egg is sufficient to start a new population and, as all adults are female, L. bostrychophila can quickly establish itself.

Control can be very effective by permanently providing physical conditions (cold and dry) that are inimitable for population growth. However such conditions are not viable options currently in most domestic or commercial situations.

Being parthenogenetic, each population of L. bostrychophila is genetically isolated from all others. Studies by Ali (1994) have demonstrated that there is a surprising degree of variability between populations from different parts of the UK and even between populations drawn from the same geographic area.

Variability has been demonstrated in a number of characteristics (size, colour, egg production, insecticide tolerance), some of which appear to be linked with geography whilst others seem to be random (Turner & Ali 1996).

We now know that the parthenogeneticity in this species is because all individuals are infected by a bacterium called Wolbachia (Yusuf et all 2000) and that variability in egg production is at least in part related to the amount of bacteria in the liposcelid's ovaries. Wolbachia loads vary during the adult stage, getting bigger as liposcelids get older and also vary between individuals and populations.

It has been known for some time that L. bostrychophila is tolerant of pyrethroid (and carbamate) insecticides (Turner 1988; Turner et al. 1991) but this observation was limited to a laboratory culture of the insects of many years standing. Ali's (1994) findings have shown that although the majority of populations studied were still suceptible to the pyrethroids (permethrin, deltamethrin and cypermethrin) some were tolerant, in one case far more so than the previously mentioned laboratory culture. Tolerance is more common in L. bostrychophila populations from southern England.

This tolerance to pyrethroids can be countered by using synergists. A mix of piperonyl butoxide and permethrin (5:1 or 10:1) is highly effective at killing even the tolerant populations (Turner et al. 1991) however such a formulation has not been popular with pest control services since: a) it is more costly, b) it is oil based and therefore leaves an oily film on treated surfaces and c) the residual activity of the pyrethroids is compromised leading to contract difficulties.

 

Where next?

This account has briefly described the current situation regarding psocids, particularly L. bostrychophila, in the UK. In 1986/7 a survey was carried out to find out the degree to which households were infected with psocids. In a sample of over 800 domestic kitchens 15% contained L. bostrychophila. In many cases the presence of the psocids were unknown to the owner. In 1997 the survey was repeated in virtually the same way and the incidence of liposcelids in domestic kitchens had more than doubled in 10 years (Turner and Bishop 1998).

Increasing prevalence will naturally increase the amount of insecticide applied to control these populations. The obvious outcome of this scenario is an increase in the numbers of insecticide tolerant populations. The new FFAST technology used in the production of AquaPy® (Lucas et al. 1996) may prove useful in preventing a build up of tolerant populations by delivering oil dispersed insecticide-synergist mixes in a way that is easy, acceptable and effective.

Psocids are known to stimulate allergic responses in sensitised individuals (Rijckaert et al. 1981; Turner et al. 1996) and current research is focussed on the importance of psocid allergy in the UK. In addition we are investigating the role and potential of microbial agents, like Wolbachia, but whilst this is of some considerable academic interest it is unlikely to offer an alternative to chemical control in the forseeable future.

 

Bibliography

Ali, N. (1994) Biological and biochemical variability in the stored food pest Liposcelis bostrychophila: King's College, University of London.

Finlayson, L. H. (1949) The life history and anatomy of Lepinotus patruelis Pearman. Proceedings of the zoological Society of London 119, 301-323.

Freeman, J. A. (1945) Studies in the distribution of insects by aerial currents. The insect population of the air from ground level to 300 feet. Journal of Animal Ecology 14, 128-154.

Lucas, J. R., Mooney, M., Bowron, M. J. & Powell, T. C. (1996) The application of advanced water based technology in the control of public health and hygiene insect pests. In Proceedings of the 2nd International Conference on Insect Pests in the Urban Environment (ed. K. B. Wildey), pp. 309- 317. Edinburgh: ICIUPE.

Rijckaert, G., Theil, C. & Fuchs, E. (1981) Siberfischchen und Stabläuse als Allergene. Allergologie (Germany, West) 4, 80-86.

Sinha, R. N. (1991) Storage ecosystems. In Ecology and management of food-industry pests (ed. R. J. Gorham), pp. 17-30. Arlington, Virginia: Association of Official Analytical Chemists.

Turner, B. D. (1987) Forming a clearer view of L.bostrychophilus. Environmental Health 95, 9-13.

Turner, B. D. (1988) Psocids: a problem to control. In Pest Control without pesticides: proceedings of a symposium of the Society of Food Hygiene Technology, pp. F1-F10. Huddersfield, UK: SOFTH.

Turner, B. D. (1994) Liposcelis bostrychophila (Psocoptera, Liposcelididae), a stored food pest in the UK. International Journal of Pest Management 40, 179-190.

Turner, B. D. & Ali, N. (1996) The pest status of psocids in the UK. In Proceedings of the 2nd International Conference on Insect Pests in the Urban Environment (ed. K. B. Wildey), pp. 515-523. Edinburgh: ICIUPE.

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.

Turner, B. D. & Maude-Roxby, H. (1988) Starvation survival of the stored product pest Liposcelis bostrychophilus Badonnel (Psocoptera, Liposcelidae). Journal of Stored Products Research 24, 23-28.

Turner, B. D., Maude-Roxby, H. & Pike, V. (1991) Control of the domestic insect pest Liposcelis bostrychophila (Badonnel) (Psocoptera); an experimental evaluation of the efficiency of some insecticides. International Pest Control 33, 153-157.

Turner, B. D., Staines, N., Brostoff, J., Colin & Cooper, K. (1996) Allergy to psocids. In International Conference on Insect Pests in the Urban Environment (ed. K. B. Wildey). Edinburgh.

Yusuf, M., Turner, B. D., Miles, R. J., Whitfield, P. J. & Pacey, J. 2000 Electron microscopical evidence of a vertically transmitted Wolbachia-like parasite in the parthenogenetic, stored-product pest Liposcelis bostrychophila (Psocoptera: Insecta). Journal of Stored Products Research 36, 169-175.

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Last modified: July 18, 2002 3:00 PM by; Bryan Turner