Can conspicuous leaf miner damage decrease other types of herbivory?

Background/Question/Methods

Plant defenses against herbivores may take the form of variegated tissues that mimic leaf damage.  Damage by leaf miners may also resemble conspicuous leaf variegation, deterring other types of herbivore damage. Thus far, there have been few studies to manipulate leaf miner damage in order to see its affects on other herbivores.  We used the plant Ageratina altissima to ask and answer the following questions: 1) Does non leaf-miner damage decrease fitness in A. altissima, 2) Is leaf miner damage associated with reduced amounts of other types of herbivory, and 3) Can masking leaf miner damage increase other types of damage relative to leaves with unmasked miner damage?

To answer the first question, we surveyed four areas in the Denison University Bioreserve (Ohio) for leaf damage and flowers produced per season.  We also surveyed two areas within the Bioreserve for the relationship between leaf miner damage on a leaf and other types of herbivory on the leaf.  For the third question, we estimated the percent leaf damage increase in leaves with exposed mines and those covered with green ink.  Leaves with exposed mines were sham-treated with the ink.

Results/Conclusions

We found that leaf herbivore damage was associated with decreased flower production across all four sites in the Bioreserve (F_1,172 = 9.558, P = 0.0023, R² = 0.05).  We found that leaves with mines received significantly less non-miner damage than leaves without mines at the same node (S  = 221.5, P  = 0.007).  Most telling, we found that plants with covered mines suffered 25% more non-mined damage than those leaves with exposed mines and treated with a sham ink treatment (F_2,67 = 4.77, P = 0.0114). 

These results suggest that conspicuous mines may deter other types of herbivory.  The putative signal to other herbivores is, in part, due to the visual aspect of the mines, as plants with mines covered with green ink suffered more damage than plants with exposed mines.  Although reduced herbivore damage may benefit leaf miners, we do not know whether the plant may benefit from having leaf miner damage versus non-leaf miner damage.  We also do not know if leaf miner damage reduces leaf quality for later herbivores.  Future experiments could include the application of sham mines to undamaged leaves to determine if the visual aspect alone is sufficient to deter further herbivory.