Some eucalypts are much more susceptible than others to insect attack. About 20 to 30 years ago, botanists noticed that some specimens of candle-bark were resistant to Christmas beetles, some red gums to psyllid lerps and some red box to sawfly. The type and quantity of oils in the leaf provide at least part of the reason. This variation not only occurs between species but within species as well, so that one part of an individual organism may be susceptible but another part could be resistant. Biologists call this genetic variability within an individual, mosaicism. Researchers are now breeding replacement trees from these resistant lines.