Abstract

By the use of belt transects through selected tree stands, in the Bogan–Macquarie Outbreak Area of Chortoicetes terminifera (Walker), it was possible to show quantitatively that the abundance of non-swarming locusts is greatly limited by the presence of trees. Thus, if the former community dominants were re-established in those outbreak centres which were originally densely timbered, the production of swarms in them would probably be prevented, and locust damage thereby reduced. It was found that the abundance of adult locusts with in tree stands is not related to tree density, as such, but to a "barrier effect" of trees in depth, acting in such a way as to diminish the chances that an individual will penetrate to a given depth in the stand as that depth is increased. Hoppers (predominantly 4th instar), the non-swarming residue of bands hatching outside tree stands, appear to be similarly affected. On the other hand, the population density of hoppers hatching within stands, both at the time of hatching and when the majority reach the 4th instar, is correlated with tree density as well as with distance within the stand, suggesting that adult female locusts are influenced hy tree density in their selection of oviposition sites. The effect of tree density, as such, on the density distribution of locusts apparently disappears gradually as locusts grow and become increasingly mobile, because after they reach the adult stage the relationship no longer exists. The evidence obtained suggests that the limiting effect of trees on locust abundance is largely due to the reduction of light intensity that they bring about, the insects being affected per medium of their positive phototactic response.