By now most Ontario hop growers have noticed those funky spiked caterpillars feeding rather voraciously on the foliage in their yards. Most growers, both here and elsewhere, have identified them as the larval stage of the eastern comma butterfly, Polygonia comma. This is a logical deduction, as this insect is well known on hops, and in fact its other common name is the hop merchant. Why? The story goes that farmers used to predict the price of hops based on the pattern of silver and gold spots on the pupal stage of this critter, hence the name hop merchant. But is this identification correct?
The Eastern Comma has some closely related kin – the Question Mark butterfly, Polygonia interrogatonis – and they can be quite similar in appearance. Although the hop merchant (a.k.a. eastern comma) is most commonly reported in hop yards, both species will feed on hops. Larvae of both species also have those gnarly spikes which are so distinctive. Hop Merchant (Eastern Comma) caterpillars are most often reported as being either greenish white or greenish brown, with rows of yellow to white spines.
The caterpillars I have most commonly been seeing in Ontario hop yards are grey and black with whitish or reddish brown spikes – colours that are more commonly associated with larvae of the Question Mark.
However caterpillar colouring is notoriously variable and a much more effective way to distinguish these two species is at the adult stage – by the silver markings underneath the hind wings of the adult butterflies. On the Eastern Comma (aka the Hop Merchant), this marking takes the shape of a silver comma, with a swollen club at each end. On the Question Mark, the mark is broken into two distinct parts, a curved line and a dot.
In the interest of scientific accuracy, we collected some of these caterpillars from hops and gave them to my summer student to babysit through to adulthood. Our resulting butterfly is pictured below The wings are a little tattered (typical when a butterfly emerges in a closed container), but take a look at the silver pattern of the hind wings – a distinctive, question mark shape.
This suggests that, in Ontario at least, hops are also being damaged by Question Mark caterpillars.
In the case of the Question Mark and Eastern Comma caterpillars, the damage and management practices are likely to be similar, so specific ID is probably less important. However it illustrates the point that it’s easy to misidentify a pest if you just assume its identity based on what is most commonly mentioned on your crop and don’t look at it closely. For other pests, management practices can be quite different so proper ID will be important.