An ecological housing crisis: Tissue switching in a specialist galling insect

Background/Question/Methods: Ecologists have long recognized the merit of using specialists as model systems for studies of coevolution, mutualisms and antagonisms, and tradeoffs. Being a specialist, however, can be risky. A specialist’s resource choices are limited, they rely exclusively on one partner, and they may be more likely to go extinct due to extreme environmental variation or changes in their host. Galling insects are specialists that oviposit their eggs inside host plant tissue, where their larvae then develop. Galling insects are generally highly specialized in that they often have only one or two host plants and can only attack one specific tissue type. For example, the specialist galler Schizomyia impatientis only attacks Impatiens capensis and I. pallida flower buds, while Lasioptera impatientifolia galls leaf midribs. During summer 2009, abnormal galls with the external morphology of S. impatientis were observed on leaves and meristems of I. capensis plants. This observation led to three questions: 1) Are the odd galls on leaves and meristems made by S. impatientis? 2) Why did S. impatientis switch host tissues? 3) Is tissue switching a viable strategy for specialist gallers? To answer these questions, I dissected galls, studied larval morphology, examined plant and gall phenology and weather for 2008-2009, and determined survival rates of larvae. 

Results/Conclusions: The odd galls found on leaves and meristems had the same internal and external morphology as S. impatientis and the larvae in the odd galls were positively identified as S. impatientis, suggesting this specialist bud-galler switched tissue types in 2009. The phenology of the specialist gall and its host plant revealed that, in 2008 I. capensis plants flowered significantly earlier and S. impatientis galls appeared significantly later than in 2009; in the latter year, galls began to appear before flower buds were available. This difference in phenologies is likely due to changes in weather between the years. June and July were significantly wetter and cooler in 2009 than 2008 or the average, conditions which delay flowering but which S. impatientis prefers. Faced with a housing crisis – not enough buds to gall – it appears that S. impatientis switched tissues to gall leaves and meristems. However, survival until emergence was very low for larvae on leaves (~10%) compared to flower buds (~85%), suggesting that this may not be a viable long-term strategy. Together, these observations provide an example of how climate change may alter specialized interactions via a phenological mismatch.