It has been observed (4) that when the first attacks of Dendroctonus monticolae Hopkins and D. brevicomis LeConte were successfully established on ponderosa pines, the trees were subsequently invaded by a large number of beetles. This abrupt rise in the attack intensity has been termed the “mass attack”. In the absence of successful initial or “pioneer” attacks the mass invasion did not occur. The successful establishment of initial attacks and the occurrence of the subsequent mass attack were found to be correlated with low oleoresin exudation pressure (o.e.p.) of the trees observed.