Alstad, D.N., & Edmunds, G.F. 1987 Black pineleaf scale (Homoptera: Diaspididae) population density in relation to interdemic mating (Hemiptera: Diaspididae).. Annals of the Entomological Society of America 80(5): 652-654.

Notes: Abstract: Density of Nuculaspis californica, insects populating ponderosa pine, varies with distances separating sampled twigs from foliage of adjacent infested trees; scale densities on needles touching or resting within 10 cm of needles from a neighboring infested tree were significantly lower and more variable than densities on more distant twigs from the same pine; this result is consistent with the hypothesis that interdemic mating between scale insects adapted to the defensive patterns of different individual host trees introduces locally maladaptive traits; both reductions in scale density and sex ratio patterns reported previously suggest an outbreeding depression resulting from this interdemic gene flow.