Weak Competition among Spartina Stem Borers, by Means of Murder

We examined co—occurrence among five species of insect larvae that bore the stems of salt marsh cordgrass, Spartina alterniflora, in northwest Florida. Statistical analysis of 5400 stems at three different sites revealed fewer empty stems, more stems containing one species, and fewer stems containing two or more species than one would expect if individuals were arranged randomly and independently. Particular interspecific co—occurrences at individual sites were significantly rarer than expected for 6 of the 10 possible species pairs. This deficiency in species co—occurrences is likely a result of direct interspecific aggression and murder of larvae within stems, as demonstrated in laboratory experiments. However, we detected no obvious decreases in population densities or alteration of stem section occupancy patterns due to competition. Thus, although interspecific competition occurs, it does not play a critical role in structuring this community.