Cooling in a fungal- Cecidomyiid gall on the undererstory shrub Symplocos tinctoria

Background/Question/Methods

Although there has been debate in the literature about the role of fungi in galls caused by Cecidomyiid larvae, evidence that the relationship between larvae and fungi is mutualistic is mounting.   One of the earliest publications providing evidence of this mutualistic relationship addressed a plant in the genus Symplocos (Symplocaceae).  Plant-fungal-larval interactions have been described in several species of Symplocos.  Here, we describe galls produced by Cecidomyiid larvae and the fungus Exobasidium symploci  on the understory shrub Symplocos tinctoria.  Previously, the plant-fungal relationship has been described in the literature, with no mention of the larvae. We measured the temperature of 50 ungalled and galled leaves at various times of day and dissected the galls to examine internal and external architecture.  

Results/Conclusions

We present SEM images of the gall surface and internal structure, fungal hyphae and spores, and the larvae.  The most notable aspect of the plant-fungal-larval interaction on Symplocos tinctoria is the temperature of the gall surface, which ranges from 5 to 10 degrees C cooler than ungalled leaves.  The most obvious explanation for this marked temperature difference would be increased transpiration by the galls.  We believe this explanation may not be sufficient and present images of a dense epicuticular wax present on the gall surface that may slow water loss.  A review of the literature also reveals that most galls result in lowered transpiration rates or no change in transpiration, rather than increased transpiration.  We propose that this temperature differential may represent a defense mechanism by the plant, slowing larval development and increasing time of exposure to natural enemies of the Cecidomyiid larvae.