If you’ve noticed your boxwoods looking worse for wear, there’s a good chance the culprit is the boxwood leafminer. Although it might initially be mistaken for winter injury or a leaf blotch disease, the damage caused by this pesky insect is becoming increasingly evident. Curious about the strange-looking symptoms it produces? Well, you’re in the right place. Let’s dive into the world of boxwood leafminer damage and explore its peculiar effects.
The Boxwood Leafminer Mystery
The boxwood leafminer, a European import, first appeared in the United States in the early 1900s. Its exact arrival and method of transportation to North America remain a mystery. Regardless, this non-native midge fly is now wreaking havoc on boxwoods across Ohio, its namesake host.
In the past, I believed there was a set of standard symptoms that could aid in diagnosing boxwood leafminer damage. However, my observations over the years have proven otherwise. Symptoms can vary greatly and change over time. This season, I’ve observed a new set of symptoms that I’ve never seen before – dramatic and highly peculiar “bullseye” patterns on the leaves.
These unusual patterns may be confused with winter injury or vice versa. At a distance, both can produce similar symptoms. However, a close examination of the foliage is necessary to determine the underlying cause. This time of year, it’s not uncommon to observe both leafminer damage and winter injury on the same plants.
The variability in leafminer symptoms remains unexplained, but it may be linked to differences in boxwood species, varieties, and cultivars. Winter injury, on the other hand, is undoubtedly associated with various types of boxwoods, their location, and environmental conditions. Unfortunately, most of the boxwoods I’ve encountered lacked proper identification signage, adding another challenge to the identification process.
The Mighty Boxwood Leafminer
According to the New York State IPM Program, the boxwood leafminer, scientifically known as Monarthropalpus falvus, is the most destructive insect pest for boxwoods in landscapes and nurseries. While I’ve never seen the leafminer kill boxwoods, it inflicts substantial leaf damage that undoubtedly stresses the plants. In fact, heavy infestations might make boxwoods more susceptible to stress-related issues such as Volutella Blight, caused by the fungal pathogen Pseudonectria buxi.
Boxwood leafminer females use their needle-like ovipositors to lay eggs between the upper and lower leaf surfaces. This process only occurs on new growth. Each leaf can contain multiple oviposition sites, with several eggs per site. These sites develop into individual leafmines, resulting in blister-like leaf symptoms.
Interestingly, the leafminer isn’t picky about leaf tissue. It even targets leaves that have been cupped under the influence of Boxwood Psyllids. However, the leafminer has largely replaced the psyllid as the most significant insect or mite pest affecting boxwoods in Ohio.
After hatching in early summer, the leafminer maggots spend the remaining season feasting on the interior leaf tissue as they progress through the first and second stages of their development. Interestingly, there’s evidence to suggest that the maggots temporarily pause their feeding and development during the summer in a condition known as aestivation. They resume their activities in late summer or early fall.
During winter, the third instar midge maggots typically stay inside the leafmines. In the spring, they resume feeding and enter the fourth instar stage. Most of the leaf damage occurs in early spring when the voracious maggots rapidly delaminate the upper and lower leaf surfaces, expanding their leafmines.
Using their hook-like mouthparts, the maggots scrape the tissue near the lower leaf surface just before pupating, leaving a thin layer of epidermal cells that creates a windowpane-like effect. This peculiar feature becomes visible on boxwood leaves in southwest Ohio when most of the leafminer maggots have entered the pupal stage. As the pupae mature, they change color from light yellow to orangish-yellow and finally reddish-orange.
Once mature, the pupae wiggle through the windowpanes, giving the delicate adults unrestricted access to the outside world. As the pupal skins (exuvia) hang out of the leaves, it signifies the emergence of the fully developed adults.
These delicate adults resemble miniature mosquitoes, except for their bright orange abdomens. You might spot them swarming around boxwood hosts, but don’t be fooled by their numbers. Their lifespan is incredibly short, lasting only about a day. Unfortunately, many adults end up trapped in spider webs, resulting in a disastrous outcome.
Two years ago, I noticed heavy bird predation of boxwood leafminer maggots or pupae. The damage remained visible throughout the spring, often surpassing the aesthetic injury caused by the leafminer itself. This year, I’m already witnessing some bird damage, and based on previous observations, I anticipate more to come over the next few months.
I’m not the first to observe bird-induced boxwood leaf damage. Gabriel John d’Eustachio’s M.S. Thesis noted that predatory birds, primarily titmice, have a significant effect on boxwood leafminer populations. These birds can cause serious damage to the plants while searching for mature larvae, often destroying most of the infested leaves. In fact, the damage caused by birds seems to outweigh that caused by insects.
Managing the Leafminer
When it comes to managing boxwood leafminer damage, the susceptibility of boxwoods can vary widely. The publications listed under “Additional Resources” provide valuable insights into host preferences and recommendations from experienced growers. Therefore, selecting resistant or less susceptible boxwoods is the best long-term solution, as it reduces the need for extensive management efforts.
However, if you’re dealing with older, highly susceptible boxwoods that cannot be easily replaced due to their size or landscape significance, insecticide applications remain an option. Timing is crucial for both foliar and systemic applications. Foliar applications made when adult leafminers are flying can be effective, but they come with the downside of potentially harming non-target arthropods like predaceous insects, mites, and spiders.
The Virginia Tech publication “Horticulture and Forest Crops, 2023 Pest Management Guide” highlights dinotefuran (e.g., Safari, Transtect), imidacloprid (e.g., Merit, Xytect), and cyromazine (Citation) as effective systemic insecticides against boxwood leafminer larvae. Dinotefuran and imidacloprid belong to the neonicotinoid group, while cyromazine is an insect growth regulator (IGR) from the aminotriazine family.
Neonicotinoids, including imidacloprid, have been found in flower nectar, which poses a potential indirect risk to pollinators. Thus, it’s crucial to apply these insecticides after the flowering period to minimize any harm to essential pollinators.
Another downside of imidacloprid is its potential to trigger secondary pest outbreaks. Research has established a link between imidacloprid application and an increase in the abundance of boxwood spider mites. These spider mites are now causing stippling damage on boxwoods, and the use of pyrethroid insecticides may also contribute to mite outbreaks by killing predacious mites.
On the other hand, cyromazine, as an IGR with systemic activity, has proven to be particularly effective against dipterous leafminers. However, it’s essential to apply it after boxwoods have finished flowering.
An alternative management option involves applying abamectin (e.g., Avid) during adult leafminer emergence. Unlike neonicotinoids, abamectin is not systemic but possesses translaminar activity. Its leaf penetration kills the first instar maggots and remains effective against spider mites, as well.
Dig deeper into the world of boxwood leafminer and explore additional resources:
- 1999, University of Maryland Master of Science Thesis, “Integrated Management of the Boxwood Leafminer,” Gabriel John d’Eustachio
- 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