Counting Caterpillars

Twice a year, Dawn visits Up on the Hill Conservation Area in Charlestown, managed by Upper Valley Land Trust, to monitor the monarch population. The public is invited to help count caterpillars in July and tag adults in September. Counting caterpilars (and eggs) is one of the most fun projects because participants get to tromp through the fields checking milkweed for monarchs. Along the way, we end up noticing other organisms that use the milkweed plant as their main habitat as well. This year, we noticed milkweed bugs, milkweed beetles, yellow aphids, a grasshopper, and a nearby orb weaver spider.

SCCD has been monitoring the breeding population of monarchs at Up on the Hill for 5 years now in the same field. The first year there was an abundance of flowering plants in the field as well as milkweed and an abundance of monarchs in all stages of their life cycle - eggs, caterpillars, and adults. We didn't notice any pupa or chrysalises any year, becuase monarchs tend to move ot a studier plant for that stage of life. Over the years, UVLT has started mowing the field for monarchs. Xerces Society has a fact sheet on Mowing for Monarchs that helps landowners plan for monarch conservation. Even since doing this, the amount of monarchs we have seen breeding in this field has decreased. This year we only saw 2 caterpillars and no other life stages.

Why might this is happening? It's complicated as are most explanations for natural phenomenon. Dawn noticed that there weren't many flowering plants and many less milkweed than previous years. Monarchs need both milkweed and nectar sources to be attracted to a habitat. It's possible adult monarchs are not attracted to this field because there aren't enough flowers blooming.

Looking at the Journey North migration map and iNaturalist sightings of monarchs, it seems that the Atlantic flyway (east of the Appalachian Mountains) doesn't have an abundance of monarchs flying along it this year. The main flyway for monarchs is in between the Rockies and Appalachians. This area has plenty of sightings of monarch adults and caterpillars. So it could be an off year for the Atlantic flyway. It seems that every 5 or 6 years, there are an abundance of monarchs seen, then there are quite a few years in between where they are seen less. This probably has to do with weather patterns at specific times of the migration north or south.

Lastly, this summer has been wet, rainy, and quite cool. Monarchs are ectotherms (get their heat from their environment) and like most insects are not flying when it is cold or wet out. This may have suppressed the population or stopped monarchs from coming this far north in their migration this season.

The next time Dawn will be out at Up on the Hill is September 10 from 3-5pm for Monarch Tagging. This is an attempt to gather information about migrating monarchs, which are not the same individuals as the breeding population. Any monarchs you see this time of year are most likely migrating. If you want to learn more about monarchs, check out and join us.