Density-dependent ant attendance and its effects on the parasitism of a honeydew-producing scale insect, Ceroplastes rubens

The intensity of attendance by a honeydew-foraging ant, Lasuis niger, on the red wax scale insect, Ceroplastes rubens, was estimated at different manipulated densities in the field. The time that individual ants were present and the total attendance time (seconds x number of ants) of ants on scale-infested twigs significantly increased as the density of C. rubens increased, i.e. ant attendance was density dependent. To determine the effects of density dependence of ant attendance on parasitism of C. rubens by Anicetus beneficus, we measured parasitism rates in the field at different density levels of C. rubens both with ant attendance and with ants excluded. Parasitism rates were higher when ants were excluded, at each density level. Although the parasitism rate significantly deceased as scale density increased, whether or not ants attended, the difference in parasitism rate between density levels was strikingly less without ant attendance. Therefore, the density-dependent decrease of parasitism rate was more pronounced with ant attendance. Mortality not due to parasitism showed density dependence in both conditions and did not change when ants were excluded. These results indicate that attending ants reduce parasitism and that, as a consequence of the density dependence of ant attendance, the efficiency of reduction of parasitism by ants is enhanced at higher densities of C. rubens.