Demographic consequences to the specialist herbivore (Euphydryas phaeton) following the adoption of the weedy exotic host plant Plantago lanceolata

Background/Question/Methods

The adoption of alien host plants may strongly affect the life history, demographic structure, and dispersal of insect herbivores. The Baltimore Checkerspot (Euphydryas phaeton), historically a specialist on Chelone glabra, recently adopted the exotic Plantago lanceolata as a larval host. To understand the effects of this new host and its potential implications, we initiated a series of investigations to understand how P. lanceolata affects demography, dispersal, and population stability of E. phaeton. Prediapause larval growth was measured by inoculating 14 replicates of three host plant treatments (C. glabra, P. lanceolata growing in 30% soil moisture, and P. lanceolata growing in 12% soil moisture) with 50 neonate larvae and reared to diapause weighing 15 larvae weekly on a microbalance.  Post diapause survival was measured in the field by inoculating 48 screen enclosures deployed over either P. lanceolata on a soil moisture gradient or C. glabra with 3rd instar post-diapause larvae and counting adults produced.  At the primary field site in Harvard Massachusetts, adult movement patterns were measured via mark-recapture and fine-scale GPS tracking.  Finally, oviposition preference trials were conducted following the method of Singer (1982).

Results/Conclusions

C. glabra was strongly preferred by ovipositing females, but P. lanceolata was accepted. Post-diapause larvae fed P. lanceolata had lower survivorship (39%) compared to C. glabra (56%), but survival on P. lanceolata was equal to (55%) C. glabra where soil was wet (>23% soil moisture). Prediapause larval growth was highest on C. glabra (0.091± 0.012 mg at diapause); larvae feeding on P. lanceolata from wet and dry soil weighed 19% (p = 0.04) and 38% (p = 0.001) less.  GPS and mark-recapture experiments suggest that movement in habitats containing the respective hosts differs substantially. Interpreting these results collectively, we infer that P. lanceolata is an inferior but viable host where soil is perennially wet. Populations of P. lanceolata growing in such wet conditions are rare.  In climatically normal years, populations of P. lanceolata growing in typical upland conditions are likely sinks. Such upland populations, however, may be viable sites in wet years, allowing the establishment of transient populations of E. phaeton that increase metapopulation connectivity and colonization of new sites.  The adoption of a new, although marginal host, may be driving the increased abundance of the once uncommon Baltimore Checkerspot on the New England landscape.