The love-hate relationship with some caterpillars can be vexing even for a nature lover

Sue Pike seacoastonline.com Published August 18 2022

I find I have to constantly keep my knee-jerk reactions to certain insects in my garden in check.  I tend to have an us vs them mentality when it comes to insects that I find feeding upon the foliage of plants I hold near and dear to my heart.  I planted a hedgerow a number of years ago that is just starting to flourish.  In it grow a variety of native shrubs-spicebush, elderberry, chokecherry, serviceberry.  The idea is to create safe harbors and food sources for native insects and birds.  Last week two different caterpillars showed up on the serviceberries.  These two caterpillars perfectly illustrate my sometimes conflicted reaction to the insects that eat my plants-a tiger swallowtail caterpillar and some yellow-necked caterpillars (Datana ministra).

Tiger swallowtail caterpillars rest during the day in webby hammocks Sue Pike photo

Tiger swallowtail caterpillars are adorable: medium-sized plump green caterpillars with a swollen thorax (the segment right behind the head, which in this case covers the head) sporting two prominent false eye spots used to scare away would-be predators.  The flying adults are large yellow and black butterflies with the dangling tails, fairly common in this area.  There are two similar tiger swallowtails found in this area.  The range of the Eastern tiger swallowtail (Papilio glaucus) extends further south and the Canadian tiger swallowtail (Papilio canadensis) further north.  To make things more confusing, these two species hybridize in central New England-so this particular caterpillar might be a cross between the two species.  Since it is difficult to distinguish hybrids from Eastern and Canadian tiger swallowtails, I have referred to this caterpillar as a tiger swallowtail-that much I’m sure of. 

Yellow-necked caterpillars are cute in their own way-dark heads, a bright yellow segment just behind the head (hence the name), with sporty brown and yellow stripes running the length of their bodies.  But they lack those engaging cartoon eyes and, more importantly, they are very gregarious-I counted at least 50 on the serviceberry bush.  Groups of ten or more were clumped on the branch tips, actively stripping the serviceberry of its outermost leaves (unlike the low-key solitary tiger swallowtail). Later this summer they will burrow into the ground to overwinter before emerging next spring as small moths.  Instead of the delight I felt upon seeing the tiger swallowtail caterpillar I immediately began planning their demise.    

Yellow-necked caterpillars are not as ‘cute’ as tiger swallowtails and are more obvious defoliators. Large groups can quickly strip branches bare of leaves. Sue Pike photo taken at littleriverphotoworkshop

If you do a search on tiger swallowtail caterpillars you’ll find lots of interesting life-history, nothing about ‘control’ since these are not considered a pest species.  Yellow-necked caterpillars, on the other hand, are considered pests.  They can cause damage to planted fruit and shade trees, but in their natural environment, in a forest for example, they do not significantly damage their host trees.  

The purpose of my hedgerow is to provide food and shelter for native species.  Both the tiger swallowtail and the yellow-necked caterpillars are doing what comes naturally, feeding on the leaves of the trees and shrubs in my hedgerow.  Both of these species provide food for birds, predaceous bugs and parasitic flies. As I considered the fate of my yellow-necked caterpillars I realized that removing them meant removing a food source for others as well as contributing to the decline of a native insect.  I figure that if the yellow-necks feed according to plan they will defoliate just the outer branches of my serviceberry bush.   If I’m lucky some robins (robins evidently love yellow-necked caterpillars) will discover them and provide a little bit of control. But if not, my serviceberry should survive this natural pruning, and hopefully next year will support even more backyard diversity.

Hopefully my serviceberries are healthy enough to withstand these yellow-necked caterpillars! Sue Pike photo