July has arrived. The weather is beautiful, it's warm, and you are enjoying the good weather in the shade of your favorite trees and shrubs. You admire their splendor, but suddenly, something unusual catches your attention. The leaves are riddled with holes, skeletonized, and it even seems they have been gnawed on by insects. You keep a watchful eye on the situation, but it worsens day by day. Then suddenly, you finally spot the culprits on a leaf, gathered in a small group. The dreaded Japanese beetles (Popillia japonica) have arrived in your yard. This species, which was introduced to the continent in 1916 through contaminated horticultural plants, is still here to haunt you 100 years later. But don't worry, there are several solutions, and your level of success will depend on your level of knowledge about this pest. So get ready for a 101 course on Japanese beetles. But be warned, we may offer you advice that you don't want to hear. Do you have the courage to change your habits?
What now? First of all, you need to take the time to thoroughly understand the life cycle of these insects because trying to intervene with a miracle solution on just one front would surely be a losing battle. That said, with a good understanding of their habits, needs, and preferences, you can easily implement several measures to make your gardens much less conducive to their development. And the more different measures you apply to combat the problem, the better your chances of success will be. But rest assured! Although some solutions may represent a sacrifice of the ideal garden for some, most of these solutions are simple to implement and inexpensive. I therefore invite you to take the time to read this text in its entirety to prepare your arsenal against these invaders. After all, if you are attentive enough to have noticed, identified, and researched this pest, you have probably already invested significant amounts of money, effort, and/or time in the growth of your plants. Taking a few more minutes to better understand this predator is simply time well invested in the well-being of your plants, trees, and shrubs.
Why you? You may notice that your family and friends seem little or not at all affected by this problem. What is it that makes your trees the prey of these pests? Sadly, it's probably because you and/or your neighbors have beautiful, well-maintained gardens.
Although Japanese beetles are a highly invasive species, it's a problem that almost exclusively affects trees and shrubs in urban areas. This is due to the presence of several contributing factors that are mainly found in cities and are necessary for their life cycle. This cycle has four stages: egg, larva, pupa, and adult. If your property and those of your neighbors provide all the essential elements to complete this cycle, the population of this insect will be greatly exacerbated and the damage to your trees will be guaranteed. Conversely, if these factors are absent, the insect will be much rarer and the damage almost undetectable.
Life cycle
The eggs
In August, adult female beetles lay about sixty eggs each, preferably in soils that offer the best food to support their offspring. And the best food is none other than a type of grass that seems absolutely inseparable from urban areas: the revered lawn. This largely explains the rarity of this insect in heavily wooded areas where lawns are almost entirely absent. No lawn, no Japanese beetles... or almost. But the purpose of this article is not to convince you to replace your lawn with white clover, which has the benefit of fixing nitrogen to fertilize your soil, or thyme, which perfumes the air and maintains a reasonable height without needing to be trimmed every week, or even a garden filled with a variety of plants beneficial to the rhizosphere. That said, if you were already considering varying the recipe for the perfect yard a bit, there's an opportunity to kill two birds with one stone by challenging the norms of landscape aesthetics.
The larvae
Once the eggs hatch, the larvae stay in the soil to feed on the roots of the lawn until around October. As the days get cooler, they will then migrate deeper into the soil (15 to 30 cm) to spend the winter well-protected. Once the warm weather returns, around early April, they come back up to feed on the roots at the surface until they transform into pupae.
The pupae
The larvae generally transform into pupae around June. This stage is essentially transitional, preparing them for their emergence as adults between late June and early July.
The adults (beetles) Late June unfortunately signals the beginning of visible infestation. If you're lucky, you'll only notice a few damaged leaves and have difficulty finding the pest responsible. But if you're among the most affected, your yard will look like an infestation that could have inspired the movie "The Mummy" circa 1999. While the larvae cause the most damage to the lawn, it's the gregarious adults that will wreak havoc on the leaves, flowers, and fruits.
The solutions
Physical removal
For this solution, a bucket of soapy water and a little patience in the morning are all you need. If you're an early riser, this is your chance to visit your gardens while the beetles are still sluggish from the cold night. They will be clumsy, and a few vigorous shakes of the branches they're resting on will be enough to make them fall into your trap. It won't solve the entire problem, but it's a good start for immediate control. If done very early, the beetles removed won't release pheromones that would otherwise attract more beetles.
Biological control
There are primarily two methods for biological control: nematodes (Neoaplectana carpocapsae and Heterorhabditis heliothidis) and the bacterium Bacillus thuringiensis var. kurstaki, better known as BTK. These two biological agents infect larvae, harming them sufficiently to drastically reduce the level of infestation locally. This means that applying these solutions at your home will not prevent larvae from neighboring yards from becoming beetles and jumping over your fence to infest your plants once the treatment is finished. Additionally, these two agents also have limited effectiveness over time because they are not organisms that naturally persist in your garden from year to year. They generally need to be applied over several years to see desired results.
WARNING: Even though these are biological agents, caution is necessary when using them. Furthermore, provincial laws and/or municipal regulations may restrict their use at the time you are reading these recommendations. It is your responsibility to ensure that you use them appropriately, safely, and in accordance with the law.
Chemical control
This article wouldn't be honest if it didn't mention the "p" word. This word, which no one wants to utter for fear of being publicly chastised or labeled as an enemy of nature, nevertheless has its place in many applications and for the benefit of nature. That said, few Japanese beetle infestations justify its use because the negative ecological impact of these products and the health risks associated with improper application often outweigh the aesthetic benefits that eliminating the pest may offer. Unfortunately, these products often attract users who rely entirely on them to control pests, always seeking a more potent product and ignoring all other control methods available to them. These products are not inherently bad for the environment, but their unreasonable use in urban environments certainly is.
Now that you understand our position on the subject a little better, allow me to discuss pesticides (yes, I said it) with honesty and transparency. There are spray application pesticides that are relatively effective in eliminating beetles but can also harm many other beneficial insects (those that control other pests) on the foliage of your trees and shrubs, thereby posing a risk of imbalance that could lead to an infestation of different pests. Additionally, they are also very risky for your health and the health of other vulnerable organisms that are not necessarily harming your plants. For these reasons, most of them are not approved for use in residential ornamental culture in Quebec. However, treating the lawn against larvae is generally effective in significantly reducing the population of this pest. Since the beetle typically has only one cycle per year, lawn treatments at the right time can be very effective in "breaking" this cycle. Such treatment will not eliminate adult beetles if you suddenly become aware of the problem in the middle of July, but it can help reduce the adult population the following year.
Attention: Note that this kind of treatment should be secondary to other available biological treatments and should always be used in combination with other methods if necessary. Additionally, caution is necessary when using them. Provincial laws and/or municipal regulations may restrict their use at the time you are reading these recommendations. It is your responsibility to ensure that you use them appropriately, safely, and in accordance with the law.
Cultivation control
Japanese beetles do not infest all plant species, trees, and shrubs. If you observe that despite your efforts, the problem persists from year to year, it may be advisable to remove the plants that attract them the most and replace them with plants they dislike. If their favorite foods are no longer available, the population will relocate to areas where food seems more suitable. You will find here a list of plants that Japanese beetles love, as well as a list of plants that they dislike. It would probably be unreasonable to completely overhaul your landscaping with only plants they dislike, but if you are planning to add or change plants anyway, it's wise to make an informed choice.
Additionally, if you experience damage in a fruit and/or vegetable garden, fine mesh nets available at horticultural centers can be an excellent solution.
Finally, it's better to let the grass grow a bit longer (10 to 15 cm) during the egg-laying period (August). This will discourage females from laying eggs there and help preserve the necessary soil moisture to assist your plants in recovering from defoliation by Japanese beetles, especially during the droughts typical of this time of year.
Environmental control
Let's discuss outdoor lighting and ambiance here. While a beautifully lit evening in a well-landscaped garden on a warm summer night is an ideal setting for gathering with friends, Japanese beetles share this affection with you. They too take advantage of these occasions to gather with friends outdoors, under the glow of your ambient lights, and colonize your plants. It's not our intention to suggest canceling your festive events or meeting in complete darkness, but whenever possible, lights should be turned off at night, especially when they serve no one, particularly from mid-June to late August. Your neighbors, local wildlife, and even your plants will thank you. Indeed, light pollution is a real problem for fauna and flora, and it can negatively affect your plants by disrupting their circadian rhythm. (Leave a comment if you'd like me to publish an article on this topic.)
Dangerous solutions
Pheromone traps
Although these traps are very effective at catching Japanese beetles, they do not necessarily decrease the population in your garden. The trap uses odors that attract the insect, but it generally attracts more beetles than it catches. Installing these traps can therefore invite neighboring colonies to settle in your garden, where pheromone odors are strong and the risk of being caught is relatively low. In other words, it might be better to place these traps at the end of the street rather than in your garden.
Soapy water application
The application of soapy water can sometimes be beneficial, but in most cases, the effort is not worth it. Many plants, trees, and shrubs do not tolerate soap application. Moreover, applying soap would be desirable when Japanese beetles are most visible (on a hot and sunny days), but these conditions are also when most woody plants close their stomata (pores under the leaves). They close their stomata to avoid gas exchange and preserve their water resources until the weather conditions are less arid. Spraying anything on foliage in these conditions risks altering the plants' perception, causing stomata to open and thus leading to leaf desiccation. It is therefore better to avoid wetting plants under these conditions, as the potential damage could outweigh that caused by Japanese beetles.
Complete lawn treatments
While we have mentioned lawn treatments for larvae in the form of insecticides, it is important to be cautious if you hire a lawn care company for this treatment. Many comprehensive plans also include herbicide treatments such as Dicamba (known by several other trademarked names). These compounds are often phytotoxic to most dicotyledonous plants (which germinate with two leaves). Thus, the lawn, being a monocotyledon, is spared, while most of your trees and shrubs may be poisoned. Moreover, these products are sometimes used in other countries to poison trees and shrubs under power lines to prevent future interference. In summary, during a treatment plan, it is best to be cautious and stick to what is essential. If weeds are also a problem, manual removal should be prioritized well before resorting to herbicides.
The final blow And since it's highly likely that your neighbors are unconsciously contributing to the problem, subtly inform them of your newfound knowledge or share this publication with them. When it comes to implementing measures against Japanese beetles damaging your trees, especially when working together with your neighbors, Aristotle's maxim holds true... 'The whole is greater than the sum of its parts.'
Thanks to John D. from Chelsea, QC for seeking our recommendations on this subject. It inspired us to share this information in this article for the benefit of all. If you believe another topic deserves a place on our blog, feel free to let us know in the comments.
Vincent Gendron Rossignol
Propriétaire et Consultant
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