Flower predation by Zelleria maculata (Lepidoptera) on Peraxilla mistletoes: effects of latitude and fragmentation, and impact on fruit set

Flower predators (florivores) may affect plant reproduction directly through loss of pollen and ovules, or indirectly by deterring pollinators which avoid damaged flowers. Caterpillars of the widespread endemic moth Zelleria maculata feed inside flower buds of the endemic mistletoes Peraxilla tetrapetala and P. colensoi in New Zealand. We measured flower predation rates between 1995 and 2007 at 24 sites throughout New Zealand and assessed Zelleria feeding impact on fruit set. Zelleria predation showed a strong latitudinal gradient, being rare in the North Island but affecting 81% of flowers in 1996 at Waipori (Otago) and averaging > 38% at Waipori and Eglinton (Fiordland). The reasons for this pattern are unknown, but a wasp parasitoid of Zelleria, Campoplex sp. (Ichneumonidae), may be less common further south. Rates of Zelleria attack were higher in mistletoes growing several metres above ground and in less fragmented habitats (i.e. in shaded positions, and away from edges), and higher in P. tetrapetala than P. colensoi. An experiment following 1005 tagged P. tetrapetala flowers at two sites showed that Zelleria attack significantly reduces both pollinator-flower-opening rates (required for effective pollination) and fruit set rates. Zelleria attack increased the chance of a flower not being opened by pollinators from 11 to 37% at Ohau in 1996 (reducing fruit set from 28% to 7.9%) and flower non-opening from 6 to 30% at Craigieburn in 1995 (fruit set: 44.4% to 16.6%), representing reductions in fruit set of 72% and 63% at the two sites. Therefore, Zelleria reduces reproduction in Peraxilla spp. through both pollinator deterrence and direct loss of flowers at many sites. This could affect conservation of these mistletoes, which are seed-limited and in decline. However, Zelleria impact is reduced by its avoidance of edge habitat, as P. tetrapetala is more abundant and flowers more heavily on edges.