MothBusters: Testing a Common Myth About a Small Butterfly
By Ryan C. Gott, Ph.D., Peter Coffey, and Michael Bechtel
Myths abound in the gardening world. From plant care to garden pests, solutions to problems and easy explanations are highly desired. One widely circulated bit of garden advice is about brassica crops and their ubiquitous pest, the imported cabbageworm (Pieris rapae), also sometimes known as the cabbage white. The story goes that adult P. rapae are territorial and will avoid each other. Therefore, white items that mimic the butterfly deter female P. rapae looking to lay eggs, protecting crops from hungry caterpillars. Everything from plastic butterflies to bread bag clips are suggested as mimics all across garden blogs.
Increasingly, gardeners want to avoid pesticides, and this idea provides an attractive, cheap, and easy alternative. But is there any actual biological basis to this method? Does or can it even work?
The P. rapae deterrent myth is rooted in insect territoriality, but is P. rapae even territorial? What interaction would drive P. rapae territoriality? Are males avoiding other males’ territory? Do females avoid laying eggs on plants occupied by other females? Most insect territoriality is described in males as guarding females or sites where females may visit or oviposit (Fitzpatrick and Wellington 1983, Kemp and Wiklund 2001). A few cases of female territoriality are reported from Gerridae and Tephritidae guarding food sources or oviposition sites, respectively (Vepsalainen and Nummelin 1985, Shelly 1999). No description of territoriality in P. rapae males or females could be found.
Recognition of individuals of the same species (conspecifics) also plays a large role in this myth. The butterflies must recognize a deterring item placed in the garden as another P. rapae to initiate a territorial response, if there is one. So, the item must mimic P. rapae recognition cues. Research suggests that P. rapae rapae, the British subspecies found in Europe and North America, uses visual cues on wings to recognize other P. rapae rapae. But these butterflies have very few differences between sexes in both visible-spectrum and UV-reflecting wing patterns. Most sex discrimination occurs behaviorally as the male approaches a potential mate (Obara and Majerus 2000). So, perhaps P. rapae could be fooled into thinking a good mimic is a fellow butterfly based solely on vision.
We decided to do a simple test of this myth using small plots in Pittsburgh, Pennsylvania, and Westminster, Maryland. Collard (Brassica oleracea ‘Tiger’) plugs grown from seed from Johnny’s Selected Seed were transplanted into the ground into six rows of nine plants. The middle plant of each set of three received a white flag, a white plastic butterfly mimic marked to resemble a female P. rapae, or no item. Plants in Pittsburgh went in the ground at Phipps Conservatory and Botanical Gardens on July 13, 2017. Ryan counted eggs and larvae on each middle plant every two weeks until October 5. In Westminster, plants went in the ground on August 31, and Peter counted eggs and larvae each week from September 21 to October 12, with a final count on November 2. ANOVA analysis of egg and larvae counts revealed no block effect of the planting rows in either plot. There was also no effect of the deterring items in either plot. Plants with and without items had the same numbers of eggs and larvae.
With no previous literature evidence of territoriality in P. rapae and no differences in the number of eggs or larvae on plants in our trial, this myth is looking busted. Female P. rapae were even observed during the trial laying eggs on leaves right next to butterfly mimics. These results seem to match with the few follow-up observations from gardeners after trying this technique.
Of course, it’s possible we need a better butterfly mimic for this to work. The mimic might need to resemble a male instead of a female. Maybe we need a higher density of mimics. Perhaps the mimics need to “flutter.” It’s nearly impossible to prove there are no situations in which this tactic could work, but the outlook doesn’t look good for this myth.
What other entomology myths do you know? Maybe we can work on busting them next!
Ryan C. Gott, Ph.D., is an entomologist interested in ecotoxicology, pesticide resistance, and pest management. He is currently the Integrated Pest Management Specialist at Phipps Conservatory and Botanical Gardens in Pittsburgh. Follow him on Twitter @Entemnein and Instagram @ryangott.
Peter Coffey is an Agriculture Extension Agent with the University of Maryland focusing on small farms and alternative agriculture. His work focuses on agricultural ecology and sustainable pest management. Follow him on Twitter @petercoffey, Instagram @peterlcoffey, or the web at petercoffey.com.
Michael Bechtel is a Display Horticulturist at Phipps Conservatory and Botanical Gardens. He specializes in edible plants and oversees Phipps’ Edible Garden, Green Wall, and hydroponics systems.