August 2002. Scale insects were found on sugarcane leaves of a nitrogen trial established on a ferruginous latosol in Fiji. The outer two rows were heavily infested and leaves were stunted and weak. Ant populations have increased. The question asked was what is the relationship between the ants and the scales.
It was assumed that the message was using the term 'scale' insects to include mealybugs. The information below would also apply to soft scale insects, margarodids and eriococcids, but not so much to armoured scale insects like Aulacaspis tegalensis (which have not been recorded from the South Pacific), because they do not excrete honeydew.
In a mixed infestation a pest problem is likely to occur if the ants attending some mealybugs end up deterring natural enemies, so allowing the armoured scales nearby to proliferate and cause damage. Ants are not predators of scales/mealbugs, they protect them in order to live off their waste product called honey-due (sugary secretions).
Controlling the ants is the first priority and should result in improved regulation of the scale/mealybug populations by natural enemies, e.g., the coccinellid Cryptolaemus montrouzieri and various hymenopteran parasitoids (provided appropriate natural enemies are present in the ecosystem). However, the scale/mealybug populations would decline over a period of several weeks, not in just a few days.
If it is no possible to wait more than a few days, then localised use of an insecticide to kill both scale/mealybugs and ants might be necessary. However, be aware that this would kill all the mealybug natural enemies in the vaccinity, placing nearby crops (and the uninfested rows of sugarcane) at risk of mealybug attack. Not all the mealybugs would be killed by the pesticide, so it would be advisable subsequently to keep the ant population under control to allow natural enemies to re-colonise the area and maintain the mealybugs at a low population density.
Information was also provided from work done elsewhere. There is a small unidentified scale not more than 2 mm causing streaking in the sugarcane leaf blades in a recent survey of eastern Indonesia. This damage looks very similar to chlorotic streak disease in sugarcane. Usually scale damage is associated with some plant stress: lack of fertiliser, dry weather, etc. The sugarcane scale, Aulacaspis tegalensis, attacks the stems and sometimes can be found on the leaf-sheaths (and rarely on the leaves). The best 'medication' for a scale problem is natural enemies, but white oil can also be effective. Fipronil was suggested as a possible control for the ants.
Sometime later, the 'scale insect' was identified as the sugarcane whitefly, Neomaskellia bergii.