An account is given of observations made in two rooms of a warehouse during 1945 and 1946. In 1945 fortified flour was stored in a single tier of upright sacks, whilst in 1946 the rooms were empty except for 50 small experimental bags of wheatfeed.

Before the flour arrived in 1945, no insects were found except a few larval Ptinids in a bag of sweepings. Adult Ptinus tectus soon appeared on the sacks of flour and, in one room, the numbers on the sacks increased steadily to 6,000 by the time the flour was removed in November. In the other room, where the ecological conditions were obviously variable, the increase was erratic, but there were 4,500 adult Ptinus on the sacks by the time the flour was removed. The numbers on the experimental bags in 1946 rose from zero to 5,000 by the end of September. The weekly figures varied with temperature, falls in numbers coinciding with colder weather. In both years a steady 30–40 per cent, of the total Ptinus on the sacks or bags were fcund at the bottom except for the early months of 1946 when 80 per cent, were at the base of the bags until April.

In empty premises it was obvious that many adult beetles hid in the fabric of the building, a rough estimate of 150,000 being obtained following the use of traps and marked beetles. There were indications that the main hiding place in this building was at the junction of walls and floor. After the sacks of flour had been removed in 1945, it was estimated that the flour covering the floor of Room 70 contained about 7,000 larvae. These would normally be removed by sweeping and vacuum cleaning, but this treatment left behind about a further 10,000 larvae in the floor cracks of each room. No estimate of larvae inside the sacks was possible, but in eight months in 1946, about 1,500 eggs were laid in each small bag of wheatfeed. An average of at least 300 eggs per day were laid in the room in 1946.

Ptinus tectus has two complete generations a year. All stages may survive the winter so that overlap of generations is complete.

In a stack of sacks of flour, adults are most abundant on the sacks at the edge of the stack especially on those near the corners. Draughts, wetness and sunshine caused local changes of distribution. There was some suggestion that Ptinus preferred to rest upon cotton sacks rather than on jute. Larvae are normally present only in the outer layers of flour in a sack and spin cocoons on the inside surface of the sacking. The adults emerge through the sacking to the outside.

The activity of Ptinus in a warehouse is greatest during darkness. Hence it is fairly continuous in dark premises and periodic in light places. In the latter, beetles emerge from their hiding places at dark and return at dawn. If dark crevices are provided artificially they act as traps, for some of the beetles will use them as daytime hiding places. Activity is reduced by low temperatures but does not stop altogether until it is as low as 2°C.

Two methods are suggested for actual estimation of insect numbers without too much labour. One for estimating the numbers on the sacks is based on the fact that most of the insects are found on outside sacks, and involves the counting of the insects on several representative groups of contiguous sacks. Over half of the estimates made by applying this method to the data from one of the rooms in 1945 were within 20 per cent, of the correct values as indicated by total counts of the population. The combination of traps with the release of marked beetles is suggested for estimating the resident population of empty premises.