April 2002. A message from American Samoa. The soft scale, Pulvinaria urbicola, is causing substantial damage to Pisonia grandis on a small, rarely visited atoll that’s a wildlife sanctuary. The infestation is, apparently, so
heavy that the trees, which serve as important bird roosts, may not survive. The collectors did not take note of any associated natural enemies or ants which could be disrupting biological controls. The questions were:
- Is there a fast-acting control method that will not harm the birds and which would not further disrupt any natural enemies that may be present?
- Does anyone know of a natural enemy species that could be considered for classical biocontrol of P. urbicola in such a sensitive situation?
There are several potential parasites of the Pulvinaria, but there may be some difficulties in getting them established on a remote (small) atoll – Is this Rose Atoll?
I believe that this problem has existed for sometime. It was noted of over twenty years ago. It is probable that environmental stress increases the scale infestation. This may well be “burning” of the foliage by the bird guano in dry weather conditions. So a simple solution would be to use a fine water spray to wash the guano off the leaves – if this is the problem. Soap could be added and this would kill the scales. [Be careful to do this spraying in the evening so that the treatment does not exacerbate the ‘burning’ of the leaves.]
If there is substantial rain, it may also solve the problem.
Dry conditions favour the development of large scale insect populations. Drought also makes the host-plants more susceptible to damage, due to stress and the accumulation of honeydew and sooty mould deposits on leaves (which impairs photosynthesis). Rain and wind keep scale populations low because both cause high mortality of the first instars.
Pulvinaria urbicola can cause severe defoliation and debilitation of trees; It has been identified Bird Island in the Seychelles, where it was feared to be a serious threat to the forest habitat used by the nesting birds.
Possibly, the ladybeetle, Cryptolaemus montrouzieri, would be worth considering – it is good at quickly reducing large scale insect populations. However, it often dies out on small islands and may need re-introducing periodically. A hymenopteran parasitoid would be better for maintaining low populations of the scale in the long term.