Aerial Dispersal of Red Pine Scale, Matsucoccus resinosae (Homoptera: Margarodidae) 1

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Abstract
The aerial dispersal of red pine scale, Matsucoccus resinosae Bean and Godwin, was observed by trapping crawlers (1st instars) in the open at distances as far as 180 m from the edge of an infested 4-ha, 45-yr-old red pine ( Pinus resinosa Ait.) plantation. The ratio of the settling speed of crawlers in still air to the avg horizontal wind speed is crucial for predicting crawler dispersal. The measured settling speed of crawlers was 27±5 cm s −1 . In the laboratory no crawlers were removed from needles or twigs by air speeds of 5 m s −1 or less and complete removal of all crawlers required speeds of fully 17–22 m s −1 . Consistent with this difficult removal is our estimate that in the forest only ca. 5–25% of the crawlers available for dispersal at any time are actually released. Further, an unexpectedly large number of doublets (2 crawlers touching each other) found on traps under the infested trees suggested that newly-hatched crawlers were most susceptible to removal and that they constituted a large percentage of the total released. Numbers of crawlers captured per unit trap area decreased rapidly in a regular pattern away from the edge of the forest. Compared to the numbers captured underneath the trees, capture leeward was reduced 94% at 2 tree heights and ca. 99.5% at 10 tree heights. Model calculations predict the general pattern of crawler dispersal but overestimate slightly the numbers trapped within the 1st 3 tree heights from the forest edge. We calculate that, in a 5 m s −1 wind, the number of individual crawlers deposited per m 2 of land at a distance of 1.6 km from the forest will be ca. 0.075% of those deposited underneath the infested plantation. For the highest capture rates observed beneath the trees, 800 crawlers m −2 h −1 , deposition rate would be ca. 0.6 crawlers m −2 h −1 at 1.6 km distance. Thus, assuming that dispersal occurs at this rate on all favorable winds during the period of one wk, as many as 22 crawlers could reach each m 2 of canopy 1.6 km away. In a more moderate wind, 2 m s −1 , the numbers of voyagers are further reduced by a factor of ca. 30. It appears reasonable that the higher rate is sufficient to start a new infestation at this distance.