The relationship between psyllid leaf galls and redbay (Persea borbonia) fitness traits in sun and shade

The interactions between Persea borbonia (redbay), its psyllid leaf gallers, and its seed predators were examined in field conditions using natural differences in light levels in order to determine (a) the distribution of galls relative to light availability, (b) the relationship between galls and plant growth and reproductive traits and (c) the relationship between gallers and seed predators. Galls were more abundant in shade than sun across sites. This finding is consistent with the Sink Competition Hypothesis, which predicts higher gall loads on plants with longer internodes (as those in shade) and hence less competition among sinks. Galled leaves were smaller and senesced more rapidly, and galled shoots grew less. In the sun, galled branches produced fewer flowers, but there was ultimately no relationship between gall load and seed production in the year sampled. Gall load was not correlated with seed predation in sun or shade, suggesting no indirect interactions between herbivores. This work suggests that gall insects negatively affect plant growth and reproduction, but manipulative experiments must be conducted to confirm this. Additionally, this study suggests that relationships between leaf gall load and plant fitness traits may not be clearly regulated by the availability of sunlight resources to the plant.