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International Biodeterioration 24 (1988) 175-187 Insects and Packaging - - A Review* J. N e w t o n Research and Development Division, Rentokil Ltd, Felcourt, East Grinstead, West Sussex, RHI9 2JY, UK ABSTRACT This paper reviews the interactions of insects with food packaging, and measures which may be taken to limit damage. INTRODUCTION Insects may deleteriously affect packaging in two main ways. First, and by far the more important in practice, they may contaminate packaging by their mere presence, and thus contaminate the materials to be packaged. Second, they may physically damage packaging by boring through it, generally when attempting to gain access to, or when emigrating from, the packaged product. Most complaints of insect contamination or damage associated with packaged goods arise from consumers, who find insects or parts of insects in or on the packaged goods, and complain to the local health inspectorate or to the retailer. At the complaint stage it is generally difficult to determine whether the infestation commenced in the consumer's kitchen, at the retailer, the product manufacturer or at the packaging manufacturer. The only stages at which it is possible to determine whether the packaging itself is the source of an infestation are *Paper presented at the Seventh International Biodeterioration Symposium, 7-11 September 1987, Cambridge, UK. 175 International Biodeterioration 0265-3036/88/$03.50 © 1988 Elsevier Science Publishers Ltd, England. Printed in Great Britain 176 J. Newton manufacture and use, i.e. during fabrication and storage of packaging at the manufacturer, or during transport to and storage at the packer. Detection of any infestation further downstream may be attributable to the packaging, the product being packed, or the conditions under which the product is packed, stored or transported. C O N T A M I N A T I O N D U R I N G T H E M A N U F A C T U R E OF P A C K A G I N G MATERIALS Packaging materials are generally rather inedible, so most of the insects found in packaging manufacturing premises are either casual intruders from outdoors, or general scavengers living indoors. The exceptions to this are woodboring insects living in wooden packaging, and, in the tropics or subtropics, termites (Isoptera) which will eat cellulose-based packaging. At the manufacturing stage the presence of insects is primarily a problem of contamination rather than damage. Insects may be accidentally incorporated into multilayer plastic films. This can happen if insects alight on the single film, or are attracted by the electrostatic charge which builds up on such film during manufacture. Trapped insects may appear simply as non-descript blemishes on the film, but in a transparent film insects show up very well, especially when wrapped around a pale-coloured foodstuff! W h e n asked for advice in just such a case, the author suggested that antistatic bars on film lines might help to reduce the problem. W h e n rigid plastic, glass or metal containers are stacked mouth upwards they can function like arrays of'pitfall traps'. In a recent case, for example, insects drawn in through a ventilation system were falling into the upturned open ends of alloy toothpaste tubes. Not all insects were able to crawl out, and there was an obvious risk of contamination of toothpaste when the tubes were filled. Davis (1986) reports a similar problem in the USA. Psocids living on pallets in an unheated warehouse became trapped in empty sauce bottles, and subsequently contaminated the sauce, despite caustic washing of the bottles. If containers cannot be stacked mouth downwards, a turnover loop with an air blast should be installed at the start of the filling line, so that insects and other foreign bodies are expelled. This is particularly important if there is no pre-fill washing. The 'casual intruders' may include anything from cereal thrips to stag beetles, and are basically outdoor-living insects which are attracted into the premises, or just wander in. To some extent their numbers can be kept down by keeping vegetation cut well back, grass short and the site clean Insects and packaging 177 and well maintained. Blocked gutters, for example, may be breeding grounds for midges and other small flies. The building should be kept in good repair, so that insects find entry difficult. Gaps under doors can be proofed with bristle strip; windows can be fitted with insect screens; regularly used entrances can be fitted with strip curtain doors. Ideally there should be positive-pressure ventilation with well-filtered air. The episode referred to earlier of insects becoming trapped in plastic film was due in part to the fact that the outer fabric of the building contained many holes, and ventilation was by exhaust only. Added to this, production continued into the night, so that night-flying insects, which abounded in the rural location, were attracted to holes by light. Several species of beetle and moth may be classed as general scavengers as they feed on a wide variety of dry plant and animal materials. Before pupation, mature larvae of the Australian spider beetle, Ptinus tectus, tend to bore into and damage inedible materials such as cardboard boxes, sacks and even plastic and wood. Larvae of the brown house moth, Hofmannophila pseudospretella, and the white-shouldered house moth, Endrosis sarcitrella, when mature may bite through soft materials such as cardboard in search of pupation sites. Spider beetles and house moths are often associated with birds' nests. Yet more closely associated with birds' nests are the following insects (which prefer a diet high in animal protein, which may include keratin): clothes moths (Family Tineidae); carpet beetles (Anthrenus spp.); fur beetles (Attagenus pellio); larder and hide beetles (Dermestes spp.). Yellow mealworm beetles (Tenebrio molitor) usually feed on cereals but will eat animal protein, and are associated particularly with pigeons' nests. These general scavengers are best controlled by thorough and regular cleaning of premises. Removal of birds' nests and exclusion of birds is essential. Disused areas and roof voids should be kept clean. Regular spraying with residual insecticides will help to eliminate any insects surviving the cleaning. Packaging manufacture and storage areas should be considered to be food processing premises rather than engineering works. BOOKLICE u THEIR PRESENCE ON PACKAGING MATERIALS The one group of insects which frequently lives on packaging itself is the booklice or Psocids, Order Psocoptera. They are not infrequently found 178 J. Newton on paper and cardboard packaging; in books, they probably feed on animal glues in bindings or on microfungi growing on the surface of books. Booklice infest a wide range of materials, ranging from chocolate to salami, bat guano to bagged nuts, birds' nests to milk powder (Dodd, 1981). It is assumed that on packaging they feed on microfungi or on some other organic edible surface contaminant. They can breed successfully on sterile foodstuffs, but do better when fungi such as Saccharomyces, Mucor or Aspergillus are present. According to Pinniger (1984), packaging infested with booklice may be fumigated either with methyl bromide (150 to 300 mg h/litre depending on temperature) or phosphine (dose at 10°C is 16 days at 0.1 mg/litre). Not surprisingly, the food industry tends not to publish data on insect contamination problems. However, when in 1984 a survey was carried out in the U K under the auspices of the Society of Food Hygiene Technology (SOFHT), of the 43 firms who replied and who might have been considered to have the potential for booklouse problems, 31 reported a problem (Downing, 1984). Most of these were companies with an annual turnover in excess of £10 million. For the majority of these companies, the booklouse problems were in part or exclusively related to consumer complaints. Manufacture was the next most important problem area, Over one-third of the firms reported that the problem had increased during the past five years, and all these firms reported consumer complaints. Most complaints occurred during late summer and early autumn. One firm reported up to 1700 complaints per a n n u m , another firm up to 30 complaints per million units sold. Booklice gave rise to up to 52% of insect complaints, and up to 7% of all complaints. Most companies reported consumer complaints of booklice on flour products, sugar or packaging, and reported booklouse problems elsewhere in the distribution chain with these, and with milk powder. Turner (1987) classified 400 complaints of psocid infestation either extracted from the British Museum (Natural History) files since about 1955 or sent to his laboratory. Over 30% of infestations were on flour. The bulk of the remainder were from cereals, cereal products or sugar, and 5% were on paper and books. C o m m e n t i n g on the S O F H T survey, Downing (1984) postulated that possible reasons for the susceptibility of some commodities might be the presence of food dust on the outside of the package, or the presence of mould (originating at any stage from point of manufacture) on the package. C o n s u m e r complaints were ofbooklice both in the commodity and on the packaging, but at this stage it is impossible to be certain about the source of the insects. Where companies reported non-consumer problems, booklice were most frequently reported to be associated with Insects and packaging 179 the building fabric, pallets, packaging with product, and packaging alone. Given the numbers of companies reporting infestation of manufacturing premises and pallets, Downing expressed surprise at the lower numbers reporting infestation in intermediate storage, transport and shops. He considered it possible that cross-infestation from pallets was largely prevented by the outer bulk packaging which encloses foods when they are stacked on pallets at the factory and which is removed when goods reach the retailer. The genera of booklice identified by respondents to the survey were Liposcelis, Lepinotus and Trogium. The only clear relationship that emerged between problems and particular genera was that Liposcelis was always involved where only consumer complaints were reported. The relationship between genera ofbooklice and particular problems had been discussed by Lilley (1981) at an earlier SOFHT symposium. She reported that the company for which she worked experienced an increase in consumer complaints in the mid 1970s, due to booklice in packed semolina. The species found was Liposcelis bostrychophilus. However, this species was not found on the company premises~ although Lepinotus and Trogium were found on incoming goods and on pallet boards, and Liposcelis subfuscus was found in non-semolina areas. A report by the Pre-Packed Flour Association (Anon., 1982) came to similar conclusions. The numbers of consumer complaints ofpsocids in flour quadrupled between 1978 and 1981, and peaked in the autumn of each year. L. bostrychophilus accounted for the vast majority of complaints. The Association concluded however that there was no evidence that infestation by this species arose from their manufacturing and distribution systems. Data based on insects sent in for identification to Rentokil UK, the Danish Pest Infestation Laboratory and the Dutch Staatstoezicht op de Volksgezondheid show similar patterns of increase in psocid-related complaints over the 15-20 years up to 1984 (Turner, 1987). Again, the autumn peak in numbers was seen, but with a levelling-offor decrease in overall numbers over the last few years. UK companies responding in a recent small, and as yet unpublished, follow-up survey to the main SOFHT booklouse survey also reported a recent levelling-off or decline in numbers of consumer complaints, as did the Pre-Packed Flour Association. Whatever unknown factors are responsible for the variations in numbers of complaints ofbooklice, they appear to operate in at least three European countries! Turner (1986) suggests that in the mid-1960s, when complaints started to increase, houses were being equipped with 'fitted' kitchens, and central heating was becoming more common. Food was being stored not in cold 180 J. Newton larders, but in kitchen units at kitchen temperatures. Draught-proofing and double glazing have increased condensation problems. Psocids generally grow and reproduce more rapidly at higher temperatures and humidities (Dodd, 1981; Pinniger, 1984). However, Turner qualifies his suggestion by remarking that it does not tie in well with the autumn peak in complaints; central heating might be expected to cause a spring peak following winter population growth. In order to gather further information on the incidence of booklice in domestic larders and factors which might encourage infestations, Turner circulated members of staff of King's College, London, and personal contacts with questionnaires and five booklouse 'pads' each (Turner & Maude-Roxby, 1987). These 'pads' were small harbourages containing yeast, and participants were asked to place the pads in their kitchens amongst their dry foodstuffs for the month of October 1986. The returned pads were examined for booklice, or incubated if apparently booklousefree. About 30% (541) of those households circulated returned questionnaires and pads. Most of the arthropods found were identified as either non-predatory mites (15.3% of households), or the booklouse L. bostrychophilus (14.4%). Very few booklice of other species were found. Analysis of the replies to questionnaires showed no significant correlation between the presence of booklice and the type of kitchen (fitted or non-fitted), household conditions (condensation, central heating), methods of storage of dry foods (original packaging or different container), intervals at which various dried foods were purchased, retail outlet, or brand of flour. Turner concluded that there was no compelling evidence that differences in conditions in houses or kitchens affected their suitability as booklouse habitats. The results of the studies by the food manufacturers imply that L. bostrychophilus is a common pest in domestic kitchens, and adventitiously breeds on suitable packaged foods stored in kitchens. Turner (1986) states that his survey does not support this hypothesis, and argues that the lack of correlations in his data suggest that booklice appear in houses on a random basis, such as might be expected if they were introduced into houses in food products. However, the detection ofL. bostrychophilus in 14-4% of responding households suggests that this species is a common household pest. PHYSICAL DAMAGE BY PENETRATING INSECTS The second category of damage is penetration of food packaging by insects, generally stored-product pests, gaining access to or emigrating Insects and packaging 181 from foodstuffs. This can occur at any point from packet closure to c o n s u m e r kitchen storage. It is obviously desirable to deny insects access to 'pest-free' packaged foodstuffs. If insects can be contained within an infested package, spread to other packages will be prevented. An insect which has found its way between the outer and inner packaging of an uninfested product will still be cause for a c o n s u m e r complaint. Ifa stored-product insect is simply placed on a piece of packaging film, it will almost certainly fail to penetrate the film. Most insects cannot cling to a smooth packaging film with sufficient strength for t h e m a n d i b l e s to grip a n d damage the film. However, insects can bore into and out of sealed packets by bracing themselves between the surface to be chewed and any other nearby surface. Standard tests of the resistance of packaging films to insect penetration take this into account, and provide a tapered crevice between the packaging film and the frame in which the film is held. A typical test set-up consists of an array of rings, bolted together around the test film, which is exposed as a disc about 50 m m in diameter (Fig. 1). The tapered crevice ensures that different species can select the width of crevice ideal for their purposes (Wohlgemuth, 1979). Some workers use a device with a series of flour-filled notches next to the test film (Highland & Wilson, 1981). The other main film-testing method employs small bags of test material with foodstuff sealed inside. The bags are then exposed to insects, either in a container or in an infested room. In a variant of this technique insects are sealed inside the test bags, with or without a small a m o u n t of foodstuff (Cline, 1978b). One disadvantage of the bag technique is the long test period. It is also possible that some insects may enter or leave through imperfect seams. The presence or absence of food ~ Insect rr~etsaihnnig \ . . . . . B°sgp,°t° Fig. 1. Diagrammatic section of apparatus for testing resistance of film to insect penetration. 182 J. Newton can affect penetration; not surprisingly, starved insects may penetrate a film which they fail to penetrate when fed. For example, larvae of Tribolium castaneum (rust red flour beetle) penetrated cellophane and polyethylene bags when sealed in without food, but not with food (Cline, 1978b). Such penetration may not result simply from a search for food; larvae of many stored-product insects pupate preferentially amongst edible or inedible debris, and chewing at a film creates debris. Despite their limitations, the various bag methods are useful for quality control of mass-produced packets, since they simulate the type of challenge to which packets are exposed and test the entire packet (including strength of seams and closures). Highland 0984) has categorised films of the thickness commonly used in food packaging in accordance with their resistance to insect penetration (Table 1). According to Schmidt (1979) the insect resistance of films is likely to be related to their smoothness, rigidity and thickness. Some species of insect are better penetrators than others. Rhizopertha dominica (lesser grain borer), Tenebroides mauretanicus (cadelle beetle), Lasioderma serricorne (tobacco beetle), Trogoderma variable (warehouse beetle) and larvae of Corcyra cephalonica (rice moth) are particularly efficient penetrators (Highland, 1984). However, Tribolium species (flour beetles) and Oryzaephilus surinamensis (sawtoothed grain beetle) tend to enter through existing openings in packages. Schmidt (1979) considered that the gnawing ability of the insects might be influenced by the size and shape of the mouthparts, and the position of the mandibles relative to the axis of the body. Folds and seams provide the major routes through which insects enter packages, either by boring through or finding defects in seams. The tendency to bore through along a fold or seam presumably results from roughening of the packaging film by mechanical or thermal distortion at this point, and the insect being better able to brace itself against an adjoining surface when it is chewing. Cline (1978b) found, when testing the resistance to penetration of films by confining stored-product larvae in bags, that most beetle larvae made their penetrations in the bottom half of bags, within 1 cm of a fold. The moth larvae, however, were able to climb to the top of the pouch to penetrate. In previous work (Cline, 1978a), he examined the ability of last-instar larvae of stored-product insects to cling to and climb packaging films and glass at various angles to the horizontal. Larvae of the three species of moth tested (Ephestia cautella, C. cephalonica, Plodia interpunctella) were able to climb all surfaces at 90 °. Three of the species of beetle (Oryzaephilus mercator, Insects and packaging 183 TABLE 1 The Relative Resistance to Insect Penetration of Commonly Used Food Packaging Films (after Highland, 1984) Packaging material a Excellent Good Fair Poor Polyca rbonate Polyester Polyurethane Cellulose diacetate Nylon Polyethylene (ca. 250/~m) Polypropylene (biaxially oriented) Polyvinyl chloride (unplasticised) Acrylic Ethylene-tetra fluoroethylene Fluorinated ethylene-propylene Polyethylene (ca. 125/am) Cellophane Cellulose propionate Corrugated paperboard Ethylene-vinyl acetate Ehtylene-vinyl acetate/polyethylene (coextrusion) Ionomer Kraft paper Paper/foil/polyethylene laminate (pouch material) Polyethylene (ca. 25, 50, 75, 100~tm) Polyvinyl chloride (plasticised) Saran Spunbonded polymers X X X X X X X X X X X X X X X X X X X X X "Where no film thickness is given, the resistance rating is based on thickness commonly used in food packaging. Cathartus quadricollis, Dermestes maculatus) were able to cling to all surfaces at 90 °. Larvae o f beetle species were able to climb o n l y paper at 90 ° . S o m e species c o u l d not climb s o m e materials at 5 ° . Paper and glass were generally easiest to cling to, a n d p o l y p r o p y l e n e m o s t difficult. The order o f difficulty for clinging was not the same as that for climbing. For example, glass was o n e o f the more difficult materials to climb, although o n e o f the easiest to cling to. Referring to an earlier paper, Cline (1978a) observed that the stored-product beetles investigated were generally better climbers as adults than as larvae, but larvae tended to be better than adults at clinging. It is possible that by tightly wrapping c o m m o d i t i e s in materials such as p o l y p r o p y l e n e w h i c h are difficult to 184 J. Newton climb or cling to, infestation by invading insects might be reduced or delayed. It is often possible by close examination of holes made by insects to determine whether the holes were made by insects entering a package from outside (and therefore arguably not the manufacturer's responsibility), or by insects escaping from within (Wohlgemuth, 1979; Highland, 1984). The hole on the exit side of the penetrated material generally has a smaller diameter than on the entry side, and has a clean-cut perimeter. The perimeter on the entrance side is tapered or terraced, scratched or roughened, and with plastics is usually upturned. However, even if the direction of penetration may be unambiguous in a particular case, the interpretation need not be. Only if it is certain that insects could not have entered through minute defects in the packaging before despatch could the manufacturer use the presence of entry holes and absence of exit holes as a reasonable defence against liability. Unfortunately, adult stored-product beetles can squeeze through surprisingly narrow openings. By using stacks of sieves with circular apertures, Cline & Highland (1981) determined that O. mercator (merchant grain beetle) adults could pass through holes only 0.93 m m in diameter. Adults of T. confusum (confused flour beetle) could pass through apertures 1.35 m m in diameter. The limiting factor in preventing entry by newly-hatched larvae of stored-product insects is likely to be the size of the head capsule. The head capsule diameter of a newly-hatched larva is about 260 p m for O. surinamensis~ and 125pm for R. dominica (Khan, 1983a), so they can probably enter packages through correspondingly small holes, down to 100pm for some species (Wohlgemuth, 1979). Last-instar Tribolium larvae can easily pass through holes of less than 1 mm diameter (unpublished work at Rentokil). K h a n (1983b) concluded that a r h o d a m i n e test of packaging integrity was not a reliable substitute for an insect bioassay. If a packaging film is creased or crumpled it is more likely to be penetrated by insects. Noack (1982) used a specially designed apparatus to crease packaging films, which were then exposed to adults of T. confusum. Cellophane became less resistant when creased several times, but multilayer foils of polyester, polypropylene and polyamide with aluminium, and biaxially stretched polyamide, remained resistant even after being strongly creased. Ironically, the integrity of a packaging film may be intentionally damaged in a variety of ways. Embossing the date of manufacture or a similar code onto a packet will distort and weaken the film, providing an Insects and packaging 185 uneven area more readily accessible to insect mandibles. One solution is simply to print the date instead. The perforations around the thumb tag in a cardboard packet are another example of intentional damage. Wohlgemuth (1979) cites an example of a manufacturer of tinned instant chocolate drink who had frequent complaints of infestation by P. interpunctella. The tin, the aluminium foil seal and the seal seam all appeared to be resistant to larval entry. But the manufacturer was punching two small holes in the foil to allow for pressure equalisation in fluctuating temperatures. Even when these holes were made as small as possible (about 0.25 mm), newly-hatched larvae could still enter. Quite simple measures can improve the resistance of packaging to insects. Highland (1984) reported that cases of raisins could be made more insect-resistant by inverting prior to stacking, so that the weight of the raisins pressed down on the folded closures of the polyethylene liners, holding them tightly shut. Incorporation of insecticides and repellents into packaging, generally by surface application, has been used for many years to give increased protection against insects. As this is an extensive subject in its own right, it is here touched on only briefly. Egyptian papyrus scrolls dating from about 1600 BC record the use of cat and bird fat applied to grain containers to repel granary pests (Levinson & Levinson, 1985). Nigam et al. (1969) cited investigations into the protection of packaging by use of a variety of pesticides from silica-gel dust to carbaryl, and techniques ranging from the application of DDT to jute sacking, to the use on cardboard cartons of a varnish containing synergised pyrethrins. The many recent studies on pesticides and packaging have included work on chlorpyrifos-methyl and permethrin on jute and woven polypropylene bags (Barakat et al., 1987); the same pesticides on film overwraps (Highland et al., 1986); and etrimfos and malathion on jute bags (Rai et al., 1985). In the USA, the only pesticide currently approved by the Environmental Protection Agency for use in insect-resistant packaging is synergised pyrethrins, which may be used on cotton bags, multiwall paper bags, or in the adhesive between the layers of a cellophane/polyethylene laminate for dried-fruit packages (Highland, 1984). It is important that packaging incorporating pesticides is designed to prevent migration of significant quantities of pesticide into the foodstuff in contact with the packaging. In conclusion, it can be said that without doubt insects will continue to cause problems for the food packaging business, but the risk of contamination by insects can be minimised by manufacturing, handling 186 J. Newton a n d storing food p a c k a g i n g with the s a m e care a c c o r d e d to foodstuffs. Careful selection o f p a c k a g i n g materials a n d choice o f package design c a n m a x i m i s e the resistance o f packages to insect penetration. REFERENCES Anon. (1982). Problems of Infestation of Prepacked Flour by Psocids. Pre-Packed Flour Association, London. Barakat, A. A., Khan, P. & Karim, A. M. A. (1987). The persistence and activity of permethrin and chlorpyrifos-methyl sprays on jute and woven polypropylene bags. Journal of Stored Products Research, 23, 85-90. Cline, L. D. (1978a). Clinging and climbing ability of larvae of eleven species of stored-product insects on nine flexible packaging materials and glass. Journal of Economic Entomology, 71,689-91. Cline, L. D. (1978b). Penetration of seven common flexible packaging materials by larvae and adults of eleven species of stored-product insects. Journal of Economic Entomology, 71, 726-9. Cline, L. D. & Highland, H. A. (1981). Minimum size of holes allowing passage of adults of stored-product Coleoptera. Journal of the Georgia Entomological Society, 16, 525-31. Davis, R. (1986). Managing pests of food. InAdvances in Urban Pest Management, ed. G. W. Bennett & J. M. Owens. Van Nostrand Reinhold Co. Inc., New York, pp. 287-310. Dodd, G. D. (1981). The biology of psocids. In Psocids (Book Lice) A New Pest Problem in the Food Industry (Proceedings of symposium of Society of Food Hygiene Technology, March 1981) Society of Food Hygiene Technology, Hertfordshire, UK, pp. 1-10. Downing, F. S. (1984). The SOFHT survey of the booklouse problem. In Aspects of Pest Control in the Food Industry (Proceedings of symposium of Society of Food Hygiene Technology, September 1984). Society of Food Hygiene Technology, Hertfordshire, UK, pp. 43-56. Highland, H. A. (1984). Insect infestation of packages. In Insect Management for Food Storage and Processing, ed. F. J. Baur. American Association of Cereal Chemists, St Paul, Minnesota, pp. 311-20. Highland, H. A. & Wilson, R. (1981). Resistance of polymer films to penetration by lesser grain borer and description of a device for measuring resistance. Journal of Economic Entomology, 74, 67-70. Highland, H. A., Cline, L. D. & Simonaitis, R. A. (1986). Insect resistance of chlorpyrifos-methyl and permethrin-treated film overwraps on cases of treated food. Journal of Economic Entomology, 79, 775-8. Khan, M. A. (1983a). Untersuchungen ~ber die Invasion von Eilarven von vorratssch/~dlichen Insekten durch verschieden grosse Poren des Verpackungsmaterials. Anzeiger j~r Schi~dlingskunde, Pflanzenschutz. Umweltschutz 56, 65-7. Khan, M. A. (1983b). Invasion von Vorratssch~idlingen dutch Verschl~isse. Anzeiger J~r Schi~dlingskunde. Pflanzenschutz. Umweltschutz, 56, 91-4. Levinson, H. Z. & Levinson, A. R. (1985). Storage and insect species of stored Insects and packaging 187 grain and tombs in ancient Egypt. Zeitschrijqfur angewandte Entomologie 100, 321-39. Lilley, D. E. (1981). P s o c i d s - the food manufacturers' problems. In Psocids (Book Lice) A New Pest Problem in the Food Industry (Proceedings of symposium of Society of Food Hygiene Technology, March 1981) Society of Food Hygiene Technology, Hertfordshire, UK, pp. 11-17. Nigam, B. S., Perti, S. L. & Agarwal, P. N. (1969). Susceptibility of packaging materials to insects and rodents. Labdev Journal of Science and Technology, 7-B, 89-98. Noack, S. (1982). Widerstandsf~ihigkeit von Kunststoff-Folien gegen~iber Tribolium confusum nach mechanischer Beanspruchung in einem Knitterger~it.AnzeigerJ~r Schi~dlingskunde, Pflanzenschutz, Umweltschutz, 55, 19-23. Pinniger, D. B. (1984). Recent research on psocids. InAspects of Pest Control in the Food Industry (Proceedings of symposium of Society of Food Hygiene Technology, September 1984) Society of Food Hygiene Technology, Hertfordshire, UK, pp. 57-65. Rai, R. S., Lal, P. & Srivastava, P. K. (1985). Impregnation of jute bag with insecticides for protecting stored food grains: I. Preliminary studies on the efficacy of some safe organophosphate insecticides. Pesticides, 19, 16-18. Schmidt, H. U. (1979). Die mechanische Widerstandsf~ihigkeit von Packstoffen (Folien) gegen den Getreidekapuziner (Rhizopertha dominica F.), den Brotk~ifer (Stegobium panieeum L.), den Rotbraunen Reismehlk~ifer (Tribolium castaneum Hbst.) und die Larven der DOrrobstmotte (Plodia interpunctella Hbn.). Material und Organismen, 14, 241-58. Turner, B. D. (1986). What's moving in the muesli? New Scientist, 110 ( 1513), 43-5. Turner, B. D. (1987). Forming a clearer view ofL. bostrychophilus. Environmental Health, 95, 9, 11, 13. Turner, B. D. & Maude-Roxby, H. (1987). A survey of the incidence of booklice in the domestic kitchen environment. Poster display at British Crop Protection Council symposium Stored Products Pest Control, March 1987. Wohlgemuth, R. (1979). Protection of stored foodstuffs against insect infestation by packaging. Chemistry and Industry (19 May 1979), 330-4.