Tyroglyphid mites in stored products. Ecological studies

Abstract

The ecology of Tyroglyphids in stored products, particularly wheat grain, was studied by examining samples taken from the material. The moisture content and the numbers of mites in each sample were determined and in many cases extensive temperature data were available.Manitoba grain stored in bulk in one granary was repeatedly examined over a period of 15 months. Examinations of infested grain in other granaries provided corroborative data.The chief mite pest of grain is Tyroglyphus farinae (L.). In imported grain it usually attacks only those parts of a bulk which have taken up extra moisture, e.g. from damp walls or from the atmosphere. Glycyphagus destructor (Schr.) is commonly found throughout the surface layer of a bulk of grain; it does not damage the grains. Differences were found in the distribution and population density of Tyroglyphus, Glycyphagus, and the predatory mite Cheyletus eruditus Schr., in relation to the moisture content of the grain. Tyroglyphus was abundant in winter and spring, but was often reduced to very small numbers during summer and autumn. This was due to the attacks of Cheyletus, which was nearly always associated with Tyroglyphus, and maintained a moderate population density throughout the year. Tyroglyphus is known to multiply less rapidly as the moisture decreases; the drier conditions in the grain during summer and autumn enabled Cheyletus to control it. A subsidiary factor was the migration of Tyroglyphus into somewhat drier grain after dense populations had exhausted the available food (wheat germ). Extremely high moisture occasionally appeared to cause similar migrations. The seasonal temperature difference may also have been a factor affecting the predator/prey cycle, but if so its effect was masked by variations due to other factors. Cheyletus was able to survive in grain too dry for Tyroglyphus. The predator was also associated with Glycyphagus, but was unable to reduce its numbers to a comparable extent. Hence Glycyphagus assisted the distribution and maintenance of Clieyletus populations. The predator is also known to attack various insects.Grain stored in bags has a much larger area exposed to atmospheric moisture, so that infestations tend to be more widespread than in bulk grain. Concrete floors at ground level tend to transmit extra moisture to the bottom of a stack of grain, and to any loose grain on the floor, thereby providing conditions favourable to the growth of dense populations of Tyroglyphus. Hence it is important to keep the bags away from the floor by the use of dunnage. Cheyletus is less common in bagged grain and in flour.Infestations of flour in bags were also studied. The conclusions drawn were similar to those relating to bagged grain. Mites tended to concentrate in the more superficial flour in a bag, very few penetrating more than a few cm. into the flour. This would facilitate control by fumigation. It seems possible that Tyroglyphids, or at least their eggs, may survive the processes of milling.Dried fruits seemed to have become infested by Carpoglyphus lactis (L.) as a result of an increase in moisture.The most effective means of preventing the infestation of stored products by mites is to keep them moderately dry, so that Tyroglyphids cannot live in them. Failing this, steps should be taken to prevent local increases in moisture, which increase the potential rate of multiplication of Tyroglyphids. It might be useful in certain circumstances to introduce Cheyletus, perhaps together with Glycyphagus as a source of food, into stored products, to prevent the possibility of Tyroglyphus being introduced without its predators, e.g. after a fumigation.