Induced defenses, those that are synthesized only upon perceived attack, are an important part of the defense arsenal against insect herbivores.  It is known that plants release volatile chemicals when they are attacked by herbivores, and some of these chemical cues attract insect parasitoids that act as plant defenders.  Less resolved is the concept of defense priming whereby plants can induce defenses in their neighbors through volatile release, though evidence for this phenomenon is increasing.  Here we present data from an experiment of a plant and novel herbivore association that suggests defense priming is triggered by neighboring plants defending against a coevolved herbivore.  The experiment, which ran from May 2009 through February 2010, separated two treatments by time.  In the first treatment we reared a newly-associated invasive herbivore (Cactoblastis cactorum) on native pricklypear hosts (Opuntia humifusa and Opuntia stricta).  Treatment two involved pricklypear fed upon by the native herbivore Melitara prodenialis intermingled with plants fed upon by C. cactorum.

Results/Conclusions

Within the first treatment, host plants displayed no obvious signs of defense against C. cactorum, and rapid larval development rates (40 days ± 4 from neonate to pupa) were congruent with findings from previous studies.  In the second treatment, host plants defended themselves against the native herbivore, with 44 of 47 plants exhibiting obvious mucilage and/or necrotic defenses, also similar to findings from previous research.  Moreover, plants defended against the invasive herbivore, a phenomenon never before documented in these host species.  Defenses against C. cactorum translated into an increase in the time to pupation from ~40 days in the first treatment to 113 days ± 19 in the second.  Proportion survivorship to adulthood of M. prodenialis (mean=0.20 ± 0.03se) was significantly lower than survivorship of C. cactorum from either treatment (no defense=0.69 ± 0.04se, defense=0.57 ± 0.05se).  The lengthened larval period for C. cactorum on defending plants, however, likely would increase mortality in nature due to extended exposure to predators, parasitoids, and environmental factors.  A second experiment is underway that separates three concurrent treatments spatially, with the primary goal of replication and verification that Opuntia defenses against the invasive C. cactorum can be triggered by growing near plants defending against the native M. prodenialis.  This second experiment isolates into separate rearing rooms plants fed upon by C. cactorum only, M. prodenialis only, and a combination of plants each with one or the other herbivore.