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A study on the flight of the Douglas-fir beetle Dendroctonus pseudotsugae Hopk. (Scolytidae). Atkins, Michael Donald

Abstract

This paper presents a study of three phases of the flight of the Douglas-fir beetle, viz: flight preparation and response, flight attitude and movements, and flight capacity. A room equipped with temperature and humidity controls was used for some of the experiments. The wing movements were studied with a stroboscope and flash photography. Flight mills were used in the studies involving flight duration and velocity. The only effect of nemic and mite associates was reduction of the duration of the initial flight caused by internal nematodes. Temperature played an important role in all phases of flight studied. Temperatures from 72° to 89° F. were optimum for spontaneous flight, while 68° F. seemed to be the lower limit of flight in the absence of additional stimuli. In sunlight, spontaneous flights occurred at 63° F. Increasing temperature caused a rapid increase in the wing-beat frequency up to the threshold for spontaneous flight, then increased only slightly to the level of heat prostration. Lower temperatures greatly reduced the duration of the initial flight and caused a broken flight pattern, but failed to influence the over-all flight duration. The effects of relative humidity followed a similar pattern to those of temperature inasmuch as evaporational cooling of the insect occurred at low relative humidities not at high relative humidities. Increased light intensity increased the speed of the flight response and caused slight increases in the wing-beat frequency; both results being attributed to increased nervous stimulation. Studies on the effect of the ehange in the beetles’ activity from dispersal to gallery construction to re-emergence, on the response to flight stimulation, showed that once gallery construction had begun the females became flight refractory, some regaining their inclination to fly after 15 days. On the other hand, the males could be expected to be either flight positive or refractory throughout the duration of gallery construction. The number of individuals responding positively to flight stimulation by tossing increased greatly following the first toss, then diminished until no further change occurred after six tosses. Wing mutilation and loading produced changes in the wing-beat frequency similar to those found by other workers, and indicated that the changes were probably due to altering the inertia of the oscillating system. Fatigue caused a gradual reduction in the wing-beat frequency over a four hour period although the final per cent decrease attributable to fatigue was much lower than that reported for other insects. The flight velocity was of the order of 90 to 115 metres per minute or 3.3 to 4.2 miles per hour, changes in the flight velocity seemingly being related directly to changes in wing-beat frequency. It is concluded from these studies that the Douglas-fir beetle is a strong uniform flier despite the complexity of components affecting several phases of flight. The various thresholds for spontaneous flight were consistently above the levels required for continuation of the activity. The flight of the Douglas-fir beetle can be expected to vary greatly from year to year and from season to season depending on the environmental factors. Of the flight movements it is the wing-beat frequency which is affected the most by environmental changes, thus varying the strength, velocity and magnitude of the flight. The Douglas-fir beetle is capable of an average flight of up to 10 miles immediately following take-off and from between 15 to 20 miles a day for several days; favourable air currents would increase these figures substantially.

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