I posted a BYGL Alert on Boxwood Leafminer (Monarthropalpus flavus) back in March noting that heavy oviposition observed last season presaged heavy leaf damage this spring (click this hotlink: https://bygl.osu.edu/node/1461). However, I neglected to report a possible odd side effect: heavily infested boxwoods going snap, crackle, pop.
We first reported on this unusual auditory phenomenon in 2003 and it's been a recurring event over the years that's always associated with boxwoods that are heavily infested with the leafminer. During this week's Tuesday morning BYGL Zoom Inservice, Mimi Rose (Leader, OSU Pesticide Education Safety Program) reported that she had finally heard from her own boxwoods what she had "heard" from the BYGL. Her heavily infested boxwoods were crackling!
Marne A. Titchenell (Wildlife Program Specialist, OSU Extension, School of Environment and Natural Resources) e-mailed this exchange she had with her husband: Yesterday evening, my husband called me outside with an incredibly perplexed expression on his face. He led me to the bottom of our patio deck steps to where our only boxwood is planted, and instructed me to “lean in close and listen.” With one eyebrow raised, I leaned in, then looked up at him. NowIhad an incredibly perplexed look on my face.
I asked, “Why does our boxwood sound like a bowl of Rice Krispies?”
To which he replied, “I’m not sure I want to know.”
[Cue The X-Filestheme music]
The Truth is Out There (?)
I first heard noisy boxwoods in 2018 and must admit the sounds were a bit creepy. To my knowledge, no one has ever fully investigated the cause behind babbling boxwoods. However, heavy leafminer infestations have been consistently connected to the phenomenon. Here are two highly speculative explanations for the strange sounds that have been proposed over the years:
1.Wiggling Pupae: midge fly pupae wiggle around in the leafmines and will eventually pop through tiny window-like spots on the undersides of leaves produced by the larvae to aid adult emergence. Indeed, you will see pupal skins hanging from leaves when the adults appear. It's speculated that the crackling, snapping sounds arise as thousands of pupae encased in sclerotized skins wiggle through the exit holes in the leaves.
2.Drying Delaminated Leaf Tissue: Noises have been heard from boxwoods long before the maggots have pupated. So, another explanation is that cracking or popping sounds arise as the upper and lower leaf surfaces separate and dry out on heavily mined leaves.
The maggots spend the winter as 3rd instar larvae inside the leafmines. They resume feeding in the spring and molt to a 4th instar stage. These large larvae produce a lot of damage quickly with multiple maggots per leafmine. It would be easy to imagine noises arising from the leaves as the delaminated epidermal layers flex as they dry out. The sound may even be amplified by the dried tissue functioning like the head of a banjo.
Of course, the ultimate explanation could be all of the above. Whatever the cause, the bottom line is that reports from gardeners or landscapers who have heard boxwoods going snap, crackle, and pop should be taken seriously. The odd sounds are an indicator of a heavy boxwood leafminer infestation.