Giant Swallowtails.  2 year cycles in the Nort…

Giant Swallowtails.  2 year cycles in the Northeast?  Since August of 2012, Giant Swallowtails (a Southeastern swallowtail species) have appeared as far north as Central Vermont.  My observations indicate that these recent Northern incursions represent a permanent expansion of its range, and that it occurs in two year life cycles.  Thus they have appeared only in the even years, 2012; 2014; 2016 and 2018.  Having noted their arrival I researched their host food and found that their larva feed on Ruta Graveolens (their favorite food is orange trees; not a thing in the North).  I have had Ruta in my garden since 2014.  I first had a Giant Swallowtail visit the garden in August 2014.  Then in 2016 a female laid eggs on the Ruta and I had at least 8 caterpillars on the plants.  In 2017, despite this there was no sign of Giant Swallowtail adults.  Then in August 2018, two specimens appeared and remained in the gardens for over 2 weeks.  Generally a sign that they were hatched nearby.  Passing butterflies, as in 2014, stop for a while to feed and move on.  In 2018, it was also the first time I had more than one adult.  As in 2016, I noted a female laying eggs on the Ruta.  A 3rd adult showed up the last week in August, about 5 days after the last appearance of the original two adults.  This butterfly remained in the garden off and one for more than a week.  Again indicating a local hatchling (butterflies can be territorial when mating).  There are currently numerous caterpillars feeding on the Ruta, of varying size, some nearly ready for pupation others early in the larval instar. 

My guess is that I will not see adult Giant Swallowtails in the garden again until 2020, but I’m quite certain that I am building a growing recurrent population in my gardens.

The adults are particularly fond of Phlox, and annual Tithonia and Zinnias too.