Ant-scale mutualism increases scale infestation, decreases folivory, and disrupts biological control in restored tropical forests

Ant-hemipteran mutualisms can have positive and negative effects on host
plants depending on the level of hemipteran infestation and plant
protection conferred by ants against folivory. Differential effects of
such mutualisms on plant survival are well documented in undisturbed and
ant-invaded systems, but few have explored how anthropogenic disturbance
affects interactions between hemipterans and native ant species and what
the consequences may be for recovering ecosystems. Within a fragmented
landscape in Costa Rica, restored tropical forests harbor a mutualism
between the native ant Wasmannia auropunctata and the scale insect
Alecanochiton marquesi on the abundant, early-successional tree Conostegia
xalapensis. I added A. marquesi scales to C. xalapensis seedlings and
either allowed or excluded W. auropunctata to investigate if this
mutualism leads to increased scale infestation, decreased scale mortality,
and decreased folivory. I also examined whether these effects are mediated
by the percentage of remnant forest cover in the landscape. I found that
seedlings with ants excluded had fewer scale insects and higher herbivory
than plants with ants present. Combined scale mortality due to fungi and
parasitism was also higher on ant-excluded vs. ant-allowed plants but only
at sites with high surrounding landscape forest cover. Together, these
results suggest that mutualisms between scale insects and native ants can
promote scale infestation, reduce folivory on native plant species, and
disrupt biological control of scale insects in recovering tropical
forests. Further, my experiment underscores the importance of remnant
tropical forests as sources of biological control in
anthropogenically-disturbed landscapes.