Laboratory Studies on the Behaviour of the Douglas-fir Beetle, Dendroctonus pseudotsugae Hopkins

The behaviour of Dendroctonus pseudotsugae Hopk. in different physiological states was studied in relation to temperature, light, and host.Newly emerged beetles preferred temperatures near the threshold for flight. When conditioned in dry air at room temperature for 16 hours they preferred cooler temperatures.At room temperature, most newly emerged beetles settled in the brightest zone of a light gradient (55 foot-candles), although some settled in dimmer zones. If the beetles' emergence was delayed or if they were stored after emergence, more remained in the zone between 0.5 and 10 foot-candles.In light-dark tests at room temperature most newly emerged beetles were photopositive, but some were indifferent and a few were photonegative. Increasing the temperature and humidity increased the proportion of photonegative individuals. Newly emerged females oriented more directly to a single source of light than males, bur after conditioning or aging both reacted similarly.Females were separated into three behavioural types based on their reactions. Photopositive individuals flew more readily than indifferent or photonegative beetles; some photonegative females showed no inclination to fly.Photopositive females were usually host negative but photonegative females were host positive. The proportion of these behavioural types within different samples varied. A group that emerged early contained a higher incidence of photopositive host-negative individuals than a group that emerged late, or was stored for 10 days after emergence. But host-negative beetles would attack logs after starvation, flight exercise, or exposure to more attractive hosts.Host-positive females contained less fat than host-negative siblings. The decline in lipids during aging, starvation or flight thus may induce behavioural changes.There is a co-ordination of successive activities in the Douglas-fir beetle similar to that reported for aphids. The initial urge to disperse outweighs responses to host stimuli, but this balance changes as the beetles fly or as host stimuli increase. The balance between thresholds for dispersal and attack also varies in relation to the conditions under which the emerging beetles developed. Consequently, each individual behaves differently when it emerges. The behaviour of a population changes as the season progresses and from year to year owing to changes in the state of individuals and in the attractiveness of hosts.