Effectiveness of Tortoise Beetle Larval Shields Against Different Predator Species

Larvae of the tortoise beetles Charidotella bicolor and Deloyala guttata carry shields formed from exuviae and feces over their bodies that are though to provide protection from natural enemies. We investigated the effectiveness of shields as a defense against three groups of invertebrate predators (mandibulate, piercing/sucking and chelicerate taxa) common in tortoise beetle habitats. Choice experiments, functional response studies, measurements of predator body size and mouthpart length, and a literature review indicated that predators with long piercing/sucking mouthparts (i.e., Heteroptera) were the most effective predators of tortoise beetles. For example, relatively large heteropterans with long, piercing/sucking mouthparts (e.g., Nabis americoferus and Podisus maculiventris) were not deterred by shields whereas small predators with short beaks (e.g., Geocoris punctipes) were. Likewise, coccinellid predators with short mandibles were deterred by the shield, while a carabid predator with long mandibles (Lebia fuscata) was effective at killing tortoise beetle larvae. No clear patterns of the effectiveness of shields against chelicerate predators were evident. Choice experiments also indicated that small tortoise beetle larvae were more susceptible to predator attack than were larger instars. Additionally, the effectiveness of the shields against different predators was quite similar between the two tortoise beetles. Field experiments indicated that predators are an important source of mortality for tortoise beetle larvae in natural settings, and high larval mortality is associated with elevated densities of predators with piercing/sucking mouthparts, such as heteropterans. Thus, shields provide some but not universal protection against the variety of predators present in tortoise beetle communities.