The Biology and Behaviour of Ptinus Tectus Boie (Coleoptera, Ptinidae), a Pest of Stored Products V
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[ 152] THE BIOLOGY AND BEHAVIOUR OF PTINUS TECTUS BOIE (COLEOPTERA, PTINIDAE), A PEST OF STORED PRODUCTS V. HUMIDITY REACTIONS BY E. W. BENTLEY From the Zoology Department, University of Birmingham (Received 30 November 1943) (With Eight Text-figures) I. INTRODUCTION chamber; during this period they showed more This paper continues the series on Pttnus tectus activity than they did later. After this, maximal (Bentley, Gunn & Ewer, 1941; Gunn & Hopf, 1942; intensity of reaction occurred within 1 hr. of stirring. Ewer & Ewer, 1942; Gunn & Walshe, 1942). The Accordingly, in all experiments the first observation correlation to be observed between moisture condi- was made 3 \ hr. after the animals had been put into tions and the intensity of insect infestations of stored the choice chamber and subsequent observations at products needs no emphasis. For this reason it is hourly intervals. Using this method, eight readings desirable to analyse as far as possible the humidity were made in a day. For a number of reasons it was reactions of such a well-established pest as P. tectus not found possible to follow the usual practice of has become in this country. Although the species is repeating each experiment on the following day with not ideal for an attack on the outstanding problems the acid dishes and hygrometers reversed. of the humidity reactions of insects, the results The index used to express intensity of reaction obtained contribute to the growing body of know- may be represented as {100 (D— W)}/N, where D ledge in this field. represents the number of position records on the drier side, W the number of the wetter side and N the total number of observations (that is, including II. APPARATUS AND METHODS animals in the middle zone). The advantages of this The apparatus used in the majority of experiments index are that it gives equal prominence to 'wet' was the alternative chamber or choice chamber of and 'dry' reactions and it takes into account those Gunn & Kennedy (1936) as modified by Bentley animals which were in the middle zone at the time et al. (1941). This consists essentially of a shallow of observation. glass dish, 22 cm. in diameter, with a false floor of In the experiments on locomotory activity at perforated zinc and a glass lid sealed by vaseline. different constant humidities, ten animals were used Under the false floor are placed Pern dishes of the in each choice chamber. In recording, a distinction required strengths of sulphuric acid and the Edney was made between itinerant or active beetles and paper hygrometers. In a few experiments a ring beetles which were 'virtually inactive', that is, gradient, 90 cm. in diameter, was used (Gunn & showing movements which, if continued, would not Kennedy, 1936). In this, a circular track on per- carry them to another part of the chamber (Pielou forated zinc is available to the animals, the gradient & Gunn, 1940). Observations were made by the being controlled by granular calcium chloride and cross-section method (Bentley et al. 1941) in which water and measured in the same way as before. All the observer chose a random instant at which to the work was done at a constant temperature of note the behaviour of the insects. 2S±O5°C. and under constant light from a high All animals were bred at 25°C. in constant illu- ceiling lamp which gave an approximate intensity of mination on wholemeal flour to which 5 % of dried 20-30 m. candles at table level. baker's yeast had been added. A supply of water In the experiments on preferred humidity ten maintained a R.H. of about 70 % and also functioned animals were used in each choice chamber. A zone, as a drinking supply. Unless otherwise stated, i cm. wide, divided the gradient into a drier part beetles were removed from cultures on the day and a wetter part. After each reading the animals before an experiment and transferred to wet cotton- were moved by means of a pipe-cleaner into the wool from which they could make up any water middle of the dish, or so distributed that there were deficiency. After 15 min. a wad of dry cotton-wool equal numbers on each side of the middle zone. was introduced. The beetles were picked off this on A preliminary experiment showed that the maximum the following day and put in the experimental dishes. humidity reaction did not occur until 3 or 4 hr. after The dishes were set up on the previous afternoon the animals had been dropped into the choice and allowed to equilibrate overnight. Biology and behaviour of Ptinus tectus at present the only way to obtain large numbers siderable aggregation in the dry region. This became . tectus of the same sex is extremely laborious, more pronounced and then suffered little change for the insects used were not chosen for sex. As far as 5-6 days. The beetles forming the aggregation re- possible beetles of approximately the same age were mained in a dense clump from which single animals employed, ranging from 2 to 5 weeks old. would detach themselves temporarily, only to return The experiments on the location of the receptors after a few minutes. Towards the end of a week the involved amputation of varying numbers of antennal insects began to disperse and to migrate up the segments. It was found unnecessary to anaesthe- gradient. After 8 days from the start of the experi- tize. With practice it became possible to remove ment 94% of the beetles were in the immediate the required number of segments with a pair of vicinity of the water. It was noticeable that the iridectomy scissors after turning the beetle on its animals no longer clumped but formed much looser back and teasing with a fine paint brush until it gave aggregations and were restless. At this stage one insect was dead. The remainder were removed from Animals introduced here at 15.00. G.M.T. 23.10.'39 the gradient and separated into five groups. Each I group was weighed, allowed to drink, dried and re- 23.i.'39 weighed. The gain in weight was reasonably con- I5.3D.C M.T. sistent, varying from 17 to 29 % and averaging 22 % of the desiccated weight. Moat of the beetles were in air of about 16% R.H. for the greater part of the experiment, and the average rate of loss of water 90 (approx. 2^ % per day) indicates considerable powers X 70- of resistance to desiccation (cf. 4% per day at 25° C 25.I.-39 and o % R.H. in the case of Tenebrio molitor (Pielou & Gunn, 1940). In another similar experiment it '30- was noticed that after the desiccated animals had I been watered and put back in the gradient they usually, but not invariably, went immediately to the region of the calcium chloride as at the beginning of the experiment. 2. The effect of humidity level and desiccation on locomotory activity The previous experiment indicated that the humi- dity preference of P. tectus depends largely on the degree to which the insect has lost water. Before I 12 24 36 proceeding to a more detailed analysis of the be- Divisions of gradient haviour by means of choice-chamber tests, two sets of experiments were carried out to determine the Fig. 1. Results from an experiment in the ring gradient. Fifty Ptinus tectus were watered and introduced as shown. effect of desiccation on locomotory activity in the After about a week the aggregation near the calcium absence of humidity gradients. The two sets yielded chloride was replaced by a preference for wet air. The similar results, but the second only will be discussed. gradient remained steady throughout Twelve lots of ten animals were watered and kept overnight at 70% R.H. The following day they were a shamming reaction. Individuals which bled much transferred to experimental chambers in which a were rejected, but they were very few in number. constant humidity was maintained. The humidity In these experiments the animals, together with their levels chosen were o, 30, 80 and 95 %, and there controls (which were subjected to approximately the were three experiments at each level. Eight hourly same amount of handling), were kept in a damp observations were made each day from 0930 to 1630 atmosphere for 2 days before experiment. G M.T. Fig. 2 shows the mean activity (i.e. excluding virtual inactivity) for the three experiments at each III. RESULTS humidity. Each point represents an average for two 1. Experiments with the ring gradient consecutive days (sixteen observations). For the first To gain a rough idea of the nature of the humidity 12 days they are based therefore on 480 records. preference of P. tectus, fifty insects were allowed to After this period some of the insects were dead. The make up any water deficiency and introduced into total number remaining alive (out of thirty) at the ring gradient mentioned above. The apparatus each humidity is shown on the graph. gave a range extending from 14-5 to 98-0% R.H. Fig. 3 shows the 2-day averages for the individual representing a gradient of o-6 % R.H. per cm. It experiments at o % R.H., and indicates a fair degree remained very constant throughout the experiment. of correspondence in the results obtained. It is clear Fig. 1 shows that within 30 min. there was a con- that humidity level has a well-defined effect on loco- 154 E. W. BENTLEY motory activity. For the first week there is no of both sets of experiments stress the significant difference between the activity at o and humidity tests of choosing animals in which that at 30% R.H.