The non-native boxwood leafminer (Monarthropalpus flavus) was accidentally introduced into North America from Europe in the early 1900s. It is now common on its namesake host throughout Ohio producing symptoms at this time of the year that may be mistaken for winter injury.
The leafminer is grouped with the so-called "gall midges" (family Cecidomyiidae, subfamily Cecidomyiinae). However, as the common name indicates, boxwood leafminer larvae (maggots) simply cause the upper and lower leaf surfaces to delaminate.
Some online resources describe the formation of gall-like structures within the leafmines. However, I consider the “structures” to be a response to wounding rather than representing directed plant growth through the production of phytohormones as seen with the “true gall-making” midge flies [see the BYGL Alert titled,“Hawthorn Pod Galls Arise,” by clicking this hotlink:https://bygl.osu.edu/index.php/node/1767].
Life in a Leaf
Boxwood leafminer females use their needle-like ovipositors to insert eggs between the upper and lower leaf surfaces of boxwood leaves. Ovipositiononly 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.
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. 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 change color as they pupate from light yellow to orangish-yellow with the pupae being a deep reddish orange. The maggots 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 is now visible on boxwood leaves in southwest Ohio and most of the leafminer maggots have pupated. Eventually, the pupae will wriggle through the “windowpanes” giving the delicate adults unencumbered access to the outside world.
New adults emerge at around the time red horsechestnuts (Aesculus × carnea) and doublefile viburnums (Viburnum plicatum) are in full bloom (440 GDD). Except for their bright orange abdomens, the delicate adults superficially resemble miniature mosquitoes. 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.
Rice Krispies Boxwoods
We’ve received numerous reliable reports over the years of snapping or crackling sounds emanating from heavily infested boxwoods in the spring. It has been suggested that the sounds are generated as the heavily sclerotized pupae push through the thin, dry tissue of the “windowpanes.”
However, noises have also been heard from boxwoods before the maggots pupate. This has led to speculation that the unusual sounds arise as the upper and lower leaf surfaces further delaminate in heavily mined leaves with the dried-out tissue acting like tiny resonators to amplifying the sound.
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.
For the Birds
This is the first season that I’ve observed heavy predation of boxwood leafminer maggots. At first, I thought the damage may have occurred last season after the emergence of last year’s leafminer adults. However, it became clear this was not the case after I invoked Question #6, “What exactly do you see?” in our OSU Fact Sheet titled, “20 Questions on Plant Diagnosis.”
First, the damage occurred on leaves that were produced last season. Remember that boxwood leafminer females only lay eggs on the new growth; the growth that was produced last season. Had the damage resulted from further leaf deterioration after adults emerged last season, it would have been on the older leaves.
Second, you can clearly see in the photos that several leaf blisters with “windowpanes” remain unmolested. These contain immature leafminersthat will emerge this season.
I’m speculating the damage was produced by heavy bird predation. Quoting from Gabriel John d'Eustachio’s Master of Science Thesis titled, “Integrated Management of the Boxwood Leafminer” (1999, Dept. of Entomology, University of Maryland): “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.”
Indeed, virtually all of the apparent damage to the boxwood leaves shown in the photo below was the result of bird predation. Few of the discolored leaves are the result of leafminer damage.
There is a wide range of susceptibility among boxwoods to the depredations of the boxwood leafminer as shown by research and credible observations by growers. Clearly, plant selection provides the best long-term solution.
The following online resources may be helpful with selecting boxwoods that require less attention and resources dedicated to managing boxwood leafminer. Hotlinks for each publication are provided.
2014, American Boxwood Society, "The Boxwood Bulletin"
2018, 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
A recent in-depth online discussion among entomologists throughout the U.S. has caused me to re-think my past boxwood leafminer insecticide recommendations. The discussion focused on concerns over the possible impacts of systemic neonicotinoids on pollinators.
On the upside, compared to topical insecticides sprayed over entire plants, systemic insecticides remove the direct risk to non-target organisms including beneficial insects. On the downside, systemics may pose an indirect risk to pollinators by exposure through the flowers.
Boxwood blooms are not particularly “showy” and they are relatively ephemeral; they don’t last long. Also, there is considerable variability in bloom production among boxwood species, varieties, and cultivars as well as among different aged boxwoods.
Based on reports from others and my own observations, there may also be a range of attractiveness among pollinators to boxwood blooms. I have observed boxwoods in heavy bloom with few pollinators as well as boxwoods literally buzzing with pollinators including western honey bees, bumble bees, carpenter bees, hover (syrphid) flies, and other flies, and small wasps that may have been parasitoid or gall-making wasps.
Of course, I cannot say whether or not my observations were made under identical environmental conditions. I also don’t know if the blooms were in their full attractive glory to pollinators or beginning to senesce. Frankly, the distinction is hard for me to discern with my non-pollinator eyes.
However, it’s important to consider possible negative impacts from systemic insecticides even if boxwoods are not consistently attractive to pollinators. It’s clear to me that they are sometimes highly attractive.
Another point to consider is the possible unintended consequence of secondary pest outbreaks posed by certain systemic insecticides. 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.
Efficacy studies published by Mike Raupp (University of Maryland, Entomology, Professor Emeritus) showed good boxwood leafminer control with abamectin (e.g. Avid) applied during adult activity. Abamectin is not a systemic like the neonicotinoids; however, it does have translaminar activity and the leaf penetration presumably kills the first instar maggots. This natural fermentation product also has good activity against spider mites.
Adult boxwood leafminer flies are typically on the wing long after boxwood flowers have senesced so there is no risk of directly killing pollinators with the topical application of abamectin. The downside is that as noted above, there remains a risk the topical application will suppress natural biocontrol agents such as insect predators and parasitoids important for keeping this and other pests in check.