January 2010. A request from Niue: can anyone provide information on how to control the black sooty mould causing problems to fruit trees, breadfruit (which is fruiting), vanilla and support trees, and others. The effects have been seen for about 2 months now. Insects seen on the trees are mealybugs, aphids, ants, etc.
In reply it was said:
Both mealybugs and ants produce copious sugary honeydew that falls onto surfaces below them, providing a medium for sooty mould growth. Ants attend these insects because they eat the honeydew. The ants often drive away predators and parasitoids that might otherwise control the sap-sucking insects.
If possible, control of the ants ??? if they nest on the ground, use Stickum to band tree trunks to exclude them from the trees. However, if the ants nest in the foliage that will not help.
It is probably best not to use insecticides, as they are more likely to kill the parasitoids than the sap-sucking insects. If pesticides must be used, oil emulsions are best as they are less toxic to the natural enemies than other chemicals.
In the longer term, release of a predator such as a coccinellid beetle (e.g., Cryptolaemus montrouzieri, the mealybug destroyer) would help reduce the sap-sucking insect populations to an acceptable level. Even if this ladybird has been introduced before it may have died out ??? this happens on small islands.
In addition to the above, it was also suggested that any low-lying branches should be trimmed to prevent ants colonising the trees from the ground. Ant baits could also be used to kill them. This could be a sugar solution, mixed with equal amounts of a toxicant such a fipronil. The toxicant should be at a low dilution so that the worker ants can carry it back to the nest before dying. However, if the primary food source is protein then you should probably use ground up dry dog food. The dry bait can be scattered around nests, and the wet solution could be placed in a small container in the fork of the tree.