Aleurotrachelus trichoides, biocontrol kava, FSM Minimize

 
 

June 2008. At first, FSM thought that the photos showed a scale on kava, Piper methysticum. First there was a white fly problem, Aleurotrachelus trichoides and, more recently, scale insects. It is the coconut scale, Aspidiotus destructor, or a new species?

It was thought at first to be A destructor. However, later, a member suspected that the photos showed a fungus growing on the 'scales' producing orange/brown spots. The fungus would be killing the scale and producing enormous numbers of spores. A microscopic examination would reveal the spores.

The identification of the 'scale' was also questioned. It appears to have a black centre and this is atypical of A destructor - or, possibly, they have been parasitised by small parasitic wasps. Again, close examination of the scale insects under a x10 magnifying glass or dissecting microscope would determine the cause.

It may be that most of the 'scale' insects are dead either from parasites or the fungus.

Later still, and after re-examining the 'scale', FSM reported that the fungus was growing over white fly pupae, and it looks as if the fungus is controlling the infestation. It was not a scale after all. It was thought likely that the fungus was Aschersonia (possibly A aleyrodis).

If the fungus is Aschersonia aleyrodis, which is likely, it is specific to whiteflies and scale insects and certainly will not invade the leaf tissues. Of course, if it is covering the leaf surface that will reduce photosynthesis and yield because it will block light and air from the leaf tissues underneath.

Referring to Samson RA, Evans HC, Latge J-P 1988
Atlas of Entomopathogenic Fungi. Springer-Verlag, Berlin, Heidelberg, New York, London, Paris, Tokyo: xii, 187pp.  On p.143 they say:

Species of the genera Hypocrella (or their Aschersonia anamorphs), Torrubiella, Hirsutella and Verticillium are common on coccids and whiteflies colonising understorey shrubs and trees and can be collected in the litter layer on leaves fallen from upperstorey trees (Mains 1958; Evans 1974; Evans 1982). Invariably, the entire population is infected, the host being replaced by the typically bright-coloured fungal stromata. As a consequence, host identification in the majority of cases is rudimentary and thus the specific insect-fungal association has not been determined.

On p.145 it mentions that the fungus can be suppressed by use of fungicides, causing an increase in insect populations.

On p.151 it says that the mass infestation of homopteran colonies is due to the efficient dispersal of the mucous-coated slime spores in run-off and rain-splash during tropical downpours.

On p.167 it mentions that epizootic fungi of this sort were used to control homopteran infestations in citrus orchards in Florida in the early 1900s, using branches infested with diseased insects or spray applications of culture-grown spores. For Aschersonia, leaves with abundant Aschersonia pustules were washed and the filtrate was sprayed onto the citrus foliage. Subsequent experiments suggested that the fungi could not be successfully introduced unless the weather conditions were such that the fungus was spreading naturally in the groves.However, Aschersonia has been successfully used against whiteflies in the Black Sea region of Turkey and the USSR, causing as much as 90% larval mortality (Ferron 1978). A scan of Aschersonia aleyrodis from p. VIII was sent (left, lower).

The photos of the FSM whiteflies were sent to Harry Evans at CAB International, an expert on entomophagous fungi. These are his comments:

"The photos show heavy infection by a Hypocrella/Aschersonia sp., possibly H discoidea. The "bacterial masses" are, in fact the slime spores of the anamorph (asexual Aschersonia) (the sexual spores (Hypocrella) are wind-dispersed). The taxonomy has changed recently, some new genera created - see Chaverri et al. 2008 Neotropical Hypocrella...........in Studies in Mycology vol. 60 (CBS Netherlands publication which should be available on-line."

In short, therefore, a host-specific pathogen of whiteflies causing epizootics in tropical forests.

And... if the putative biocontrol is not suffiicent, a paraffin oil was suggested mixed with an insecticide. In Florida, this works well on guava (3-5 ml of oil and 1-3 ml of insecticide/litre of water). The treatments are best done when the scales are first instars, when they have soft bodies.