What’s the deal with all the green berries on a holly tree?
Holly trees play a significant role in the beliefs and traditions of the season. To the Romans, hollies were the trees of the god Saturn and wreaths of holly were given as gifts during his holiday, Saturnalia. In Celtic legends, the evergreen hollies with their beautiful red berries announced the triumph of the Holly King during the winter season, over the Oak King who ruled the forest with his green leaves in the summer. For Christians, the pointed leaves of the holly are associated with the crown of thorns worn by Jesus and the bright red holly berries symbolize drops of his blood. For a bug-guy, this is the season to marvel at insects that make their living through an intimate association with holly. Last week we met the amazing native holly leafminer and learned of its travels between the epidermal surfaces of holly leaves. This week we meet another denizen dining on holly, the holly berry midge.
The small dusky winged holly berry midge leaves behind a brown papery pupal case as it emerges from the green holly berry.
The beautiful bright red berries of American holly attract not only insects, but also many fascinating feathered friends, such as mocking birds, blue jays, and cardinals, to a winter feast. The berries provide a nutritious meal and, in return, the birds distribute the holly to new places by depositing seeds in their droppings. While photographing the native holly leafminer, I spotted several holly berries that were distinctly green rather than scarlet red. Inside these fruits were tiny yellow maggots, the larvae of the holly berry midge. In the spring when hollies were in bloom, the adult holly berry midge, a small mosquito-like fly, deposited eggs into the developing fruit of the holly. These eggs hatched into larvae that fed within the berry. During the past growing season, the maggots consumed tissue of the fleshy fruit. In winter, larval development slows. When the warmth of spring returns, these maggots complete development and become pupae, from which emerge small midges that mate and deposit eggs into the developing berries, thereby completing the cycle of life.
Inside the holly berry, tiny holly berry midge larvae consume juicy plant cells. At less than 2 mm in length it’s hard to tell which end is which. In this video the head end is to the left and the rear end is to the right.
For a small maggot, life in a holly berry is precarious. Its fate is tied to a red berry that advertises, "eat me" to hungry birds and squirrels that happily oblige. These frugivores could eat the holly berry midge as they dine on luscious holly berries. However, the larva of the holly berry midge has a neat trick to lessen its risks of disappearing down the gullet of a bird along with its fruity home. Berries infested by the holly berry midge fail to turn red as do normal holly berries. The midge and an associated fungus prevent the formation of bright red pigments by the berry. Infested berries remain green all winter. Through a series of detailed observations, researches found that green holly berries were much less likely to be eaten by squirrels and birds than red berries on the same tree. By preventing the berry from turning red and thereby dodging the attention of fruit eating vertebrates, the holly berry midge enhances its chances for survival. So as you decorate your home with boughs of holly, should you spy a green holly berry, you will understand why green is good for the clever holly berry midge.
References
The interesting article “Selective Avoidance by Vertebrate Frugivores of Green Holly Berries Infested with a Cecidomyiid Fly (Diptera: Cecidomyiidae)” by Vera Krischik, Eric S. McCloud and John A. Davidson was used as a resource for this episode.