Boxwood leafminer (Monarthropalpus flavus, family Cecidomyiidae) damage is already becoming apparent. However, the symptoms are highly variable and may be mistaken for winter injury, a “leaf blotch” disease, or something else. Birds seeking meat morsels in the form of leafminer larvae can produce an entirely different type of strange-looking damage.
The boxwood leafminer is a European import first reported in the U.S. in the early 1900s. It’s not known exactly when it arrived or how it made its way to North America. Regardless, the non-native midge fly is now common on its namesake host throughout Ohio.
I once believed there was a “standard” set of boxwood leafminer symptoms aiding an accurate diagnosis. However, observations over the years have convinced me that symptoms are wide-ranging, and they change over time. This season has brought a new set of symptoms I’ve never observed before with the following images showing dramatic and highly peculiar “bullseye” symptoms.
Boxwood leafminer damage may be mistaken for winter injury and vice versa. Both produce similar symptoms when viewed at a distance. I’ve learned there is no way to be certain if it’s one or the other without a close examination of the foliage. It’s why I don’t make a diagnosis from pictures unless there are close-ups. Adding to the challenge, it’s not uncommon to observe both leafminer damage and winter injury on the same plants, particularly at this time of the year.
I have no explanation for the leafminer symptom variability but suspect it may be related to differences in boxwood species, varieties, and cultivars. Winter injury is most certainly associated with different types of boxwoods as well as location and of course, environmental conditions. Unfortunately, few of the boxwoods that I’ve photographed have had signage providing an accurate identification.
A Major Miner
Quoting from the New York State IPM Program publication listed in “Additional Resources” below: “Boxwood Leafminer, Monarthropalpus falvus[sic], is the most destructive insect pest of boxwoods in landscapes and nurseries.” I’ve never seen the leafminer kill boxwoods; however, heavy leaf damage is no doubt a significant stress inducer. It’s possible that heavy infestations may make boxwoods more susceptible to stress-related issues such as Volutella Blight caused by the fungal pathogen Pseudonectria buxi (syn. Volutella buxi).
Boxwood leafminer females use their needle-like ovipositors to insert eggs between the upper and lower leaf surfaces. Oviposition only occurs on new growth. Each leaf may contain multiple oviposition sites with several eggs per site. These sites will become individual leafmines producing blister-like leaf symptoms.
On a side note, all leaf tissue is fair game. This includes leaves that have become cupped under the influence of Boxwood Psyllids (Psylla buxi). Of course, the leafminer has largely supplanted the psyllid as the most significant insect or mite pest of boxwoods in Ohio.
Eggs hatch in early summer and the resulting maggots spend the remainder of the season consuming interior leaf tissue as they develop through the 1st and 2nd instar stages. There is some evidence that the maggots pause in their feeding and development during the summer; a physiological condition known as aestivation (= estivation). They resume feeding in late summer to early fall.
Winter is usually spent as 3rd instar midge maggots inside the leafmines. The maggots resume feeding in the spring and develop through a 4th instar stage. Usually, much of the leaf damage occurs in early spring with the ravenous maggots rapidly delaminating the upper and lower leaf surfaces as they expand their leafmines.
The maggots use their hook-like mouthparts to scour the tissue near the lower leaf surface just before they pupate leaving a thin layer of epidermal cells to create a windowpane-like effect. This odd feature will become visible on boxwood leaves in southwest Ohio when most of the leafminer maggots have pupated. The pupae change colors as they mature from the light-yellow color of the maggots to orangish-yellow and finally reddish-orange.
Eventually, the pupae will wriggle through the “windowpanes” giving the delicate adults unencumbered access to the outside world. Adult emergence is heralded by pupal skins (exuvia) hanging out of the leaves.
The delicate adults superficially resemble miniature mosquitoes except for their bright orange abdomens. They may be seen swarming around their boxwood hosts and large numbers commonly collect in spider webs which is a disastrous outcome given that the adults only live for about a day.
For the Birds
Two years ago, I observed heavy bird predation of boxwood leafminer maggots or pupae. The damage remained visible throughout the spring and commonly exceeded the aesthetic injury produced by the leafminer. I’m already seeing some bird damage this year and based on previous year’s observations; more damage may occur over the next two months.
I’m not the first to observe boxwood leaf damage caused by birds preying on the boxwood leafminer meat morsels. Quoting from Gabriel John d'Eustachio’s M.S. Thesis (see “Additional Resources” below): “The only predators that seem to have a significant effect on boxwood leafminer populations are predatory birds, primarily titmice (family Paridae). As noted earlier, they do serious damage to the plants to locate the mature larvae, and can destroy most of the infested leaves on a given plant. Damage done by birds appears more significant than that done by insects.”
There is a wide range of susceptibility among boxwoods to the depredations of the boxwood leafminer. Summaries of host preference trials as well as credible grower observations are provided in the publications listed under “Addition Resources” below. Hotlinks for each publication are provided.
Clearly, plant selection provides the best long-term solution. Choosing resistant or less susceptible boxwoods means less attention and resources must be dedicated to managing boxwood leafminer.
Of course, insecticide applications remain an option to manage boxwood leafminer on older more susceptible boxwoods. This is particularly important if the boxwoods cannot be easily replaced owing to their size and importance in landscape designs.
Timing is critical for both foliar and systemic applications. While foliar applications made at the time adult leafminers are flying can be effective, the downside is that non-target arthropods may be killed including predaceous insects and mites as well as spiders.
The Virginia Tech publication, “Horticulture and Forest Crops, 2023 Pest Management Guide” (see Additional Resources below), lists the systemic insecticides dinotefuran (e.g., Safari, Transtect, etc.), imidacloprid (e.g., Merit, Xytect, etc.), and cyromazine (Citation) as being effective against boxwood leafminer larvae. Dinotefuran and imidacloprid are neonicotinoids and cyromazine is an aminotriazine insect growth regulator (IGR).
Systemics reduce the risk to non-targets; however, they are not risk-free. Neonicotinoids may be found in flower nectar meaning that applications should be made after flowering to reduce the possible indirect risk to pollinators.
Another downside is specific to imidacloprid with the possible unintended consequence of stimulating a secondary pest outbreak. Research has shown a direct link between the application of imidacloprid and a rise in the abundance of boxwood spider mites (Eurytetranychus buxi) on its namesake host. I once considered this spider mite to be nothing more than an oddity. However, in recent years, stippling damage by the mite has become increasingly common. Of course, topical applications of pyrethroid insecticides may also support mite outbreaks by killing predacious mites.
Cyromazine is an IGR with systemic activity. Equally important, it has proven particularly effective against dipterous leafminers. Still, applications should be made after boxwoods have flowered.
A 2001 publication showed that abamectin (e.g., Avid) applied during adult emergence provides another management option (D Eustachio and Raupp). Abamectin is not systemic like neonicotinoids; however, it does have translaminar activity and the leaf penetration presumably kills the first instar maggots. This natural fermentation product is also effective against spider mites.
1999, University of Maryland Master of Science Thesis, “Integrated Management of the Boxwood Leafminer,” Gabriel John d'Eustachio (Mike Raupp advisor)
D Eustachio, G. and Raupp, M.J., 2001. Application of systemic insecticides in relation to boxwood leafminer's life history. Journal of Arboriculture, 27(5), pp.255-262.
2014, American Boxwood Society, “The Boxwood Bulletin”
2019, New York State Integrated Pest Management Program, Cornell Cooperative Extension, “Disease and Insect Resistant Ornamental Plants, Buxus – Boxwood”
2020, Saunders Brothers, Boxwood Guide 6th Edition
2023, Virginia Tech, Horticulture and Forest Crops, 2023 Pest Management Guide