Can a Green Roof Get Unwanted Bug Infestations?

by Oscar Warmerdam on Wednesday, April 27, 2022 updated Thursday, April 28, 2022

Pest infestations on Green Roofs?

In my 15 years, I have been on over 500 green roof visits. I have been asked many times to diagnose a failing green roof. Apart from the South, more or less, all these visits took place from LA to Vancouver, from Minneapolis to Chicago to Toronto, and from Boston to Washington DC. I have never seen a bug infestation on our roofs, but sometimes they do pop up, and if they do happen, it is because the green roof has been mistreated: too much/little water and/or too little nutrients, and the infestation could easily have been avoided. Weirdly enough I have never seen any ants nests on green roofs.

In the US and Canada, we also grow 200 acres of Sedum crop in three locations, and we have never had to react to any pest-related bugs in any way, shape, or form. Nonetheless, if we see a plant covered in aphids, for instance, which sometimes happens, we do not panic. We let the aphids be, as they only attack plants that are already weakened. Once done, the aphids simply disappear.

The Biodiversity of a Green Roof

In the fall, during the early morning hours at the farms, we see the dew collect on what appears to be hundreds of thousands of funnel-shaped spider webs. As these spiders can only survive if there is something for them to eat, I can only assume they catch something airborne because, on the plants, we simply do not see a lot of bugs.

We see these web funnels on green roofs as well, and of course, we see many birds landing looking for food. On my vegetated roof at my house, I see birds on the roof throughout the day, picking at something that I can only assume to be food in the form of insects or spiders.

At the nursery, we also have deer, wild pigs, rabbits, raccoons, and whatever else scrounges over the green roof plant fields at night, and somehow our Sedum crop must not taste well, so we don’t even have to worry about larger animals. Insects and animals tend to eat crops that grow in abundance. Plants that grow in adverse, highly stressed locations like deserts (cacti) or rock crevices on mountains (Sedums) tend to develop defense mechanisms that protect their slowly accumulated resources. Cacti do this with, for example, prickly needles, whereas Sedums do so by chemical defenses (alkaloids and polyphenols) and compensatory growth strategies as a response to herbivory1. Hence, we tend to see bugs but no pest explosions that would require insecticides.

The Link Between Water and Bugs

In forest-like moist environments, insects tend to be present in large numbers, where lots of organic material is degrading, supporting a rich chain of life. You see way more bugs in the jungle than in the desert and it’s a simple supply problem. If there is little to feed, there is little to harvest.

Similarly, a green roof is also a very harsh environment, as it tends to be on the drier side, especially in summer. There is very little organic material on a green roof, perhaps 2-3cm (1”) of dead leaf debris, but the moisture swings from wet to bone dry, making it hard for bugs to thrive. Furthermore, the pH of the roof soils tends to be on the high side (7.5-8.5), and this might be something that does not foster insect explosions.

Green Roof Plant Drought Tolerance

Sedums and many other green roof plants that can tolerate drought are survivors that tend to adapt or cope with adversity better than most. It is essential to understand that it has little to do with preference and everything to do with tolerance. When you observe plants in large drifts, these plants do not prefer to be there more than other plants. They grow in drifts because they can tolerate that particular condition better than others.

If you disturb soil (either due to a storm or artificial excavation), a wide selection of plants scrambles to settle themselves. As it turns out, that open soil wound spot might be flooded regularly, or perhaps it is bone-dry for six months, or it gets eaten by goats or rabbits, whatever extreme adversity you can think of. Over time there will be a handful of plants that can handle that regular abuse. Those plants will remain in large drifts because of their tolerance to accept the harsh conditions. It certainly is not a preference.

The same is true with the vegetated roof; the plants that do well on a green roof are the plants that have the most tolerance for heat, drought, coldness, wind scours, etc. Their leaves are tough, often leathery, and simply difficult to chew through, and they taste bad, so you can assume few bugs are going after your typical green roof plant foliage. You can grow other plants on a green roof too, but then you need to plan for frequent irrigation unless you live in a cool temperate climate with few drought periods and frequent small rain showers. You can check the optimal green roof in the online evapotranspiration/retention modeler.

There is One Insect of Concern – But Only if Your Green Roof is Already Sick

This is a real-life story of a grub-infestation-on-a-green-roof story in the Montgomery County School District. A story about a larva, a whitish immature form of a Japanese beetle called the grub.
Imagine mama Beetle wants to lay her eggs somewhere, and it is flying through this Montgomery neighborhood, and it can lay these eggs everywhere. It can do so on a lawn, a ditch, or a vegetated roof. In this case, the school had two vegetated roof areas (Green roofs A and B) roughly the same size, approximately the same conditions, same soil depth, and same plant genus.

As any mother would know, you want to place your offspring where their survival chances are in their favor, so mama Beetle decides to drop her eggs on Green roof B. Why B and not A? Was B closer, and Mama Beetle was simply lazy and didn’t want to fly the extra 50 yards?
No. She decided that B was showing all the signs of a perfect place for her offspring. Somehow these bugs can identify plants or drifts of plants that are sick, stressed beyond normal, or dying. They perhaps use color and UV signatures; maybe they can sense pheromones, who knows, but somehow mama Beetle knows precisely where to lay her eggs, and B was the perfect place.

A few months later, in August, the green roof had been ravaged. Plants are unrooted and even tossed over the roof edge laying on the ground a few stories down. We have a look and it truly looked as if someone had made a deliberate effort to uproot 80% of roof B.

Grubs had eaten the roots of the Sedums because the plants had shown signs of stress. Next, the birds (usually crows), and sometimes raccoons, notice the tasty grubs and use their feet and/or beaks to uproot the plants, which is now easy because they are like a toupee layer without roots. There were 5-20 grubs per 0.1m2 (1 sqft), which basically is an all-you-can-eat buffet. More birds came, and within 24-48 hours, 370m2 (4000 sqft) of Sedum roof was uprooted and destroyed.

The video shows the grubs in closer detail

What Was the Cause of the Grub Infestation?

The green roof system provider for Green roof B was called in, and they said it was evident that the grubs destroyed the green roof and they were the cause. Later, we clearly stated that the issue was not the grubs. The grubs simply responded to a wide-open door. Let me clarify:
Green roof B was made up of vulnerable small plants in distress, and the grubs simply took advantage of it. The vegetation was sick because there was a drought combined with nutritional deficits. Soil samples taken by us revealed the soil was missing a few critical nutrients and this ‘toppled’ the Sedum crop into a very desirable beetle egg-laying location, as ten-thousand or more grubs were thriving on Green roof B.

In a contrast, at the exact same time, just 50 feet away, Green roof A was perfectly fine. How can that be? Green roof A was a different system that held onto water much better. As it held onto water much better, it, therefore, leached out fewer nutrients, and the plants had just a tad more water to bridge the drought and a tad extra nutrients which made for more robust bigger plants with much bigger reserves.
Mama Beetle flew over that roof too, and she decided that her offspring would have a hard time battling that healthy crop on Green roof A and preferred B instead.

Grubs are a symptom, not a cause. If you design a green roof to bridge regular periods of drought properly, you will avoid most grubs. If you make sure the vegetation is getting at least the bare minimum of nutrients, they will be able to grow into large plants that can build nutritional reserves allowing them to be OK with a few roots chewed off by grubs, and they have the resources to fight off pests naturally.
Please use the evapotranspiration/retention modeler to verify that your roof configuration is optimal for your climate.

How Did We Get Rid of the Grubs?

Green roofs have thin profiles and highly coarse soils that quickly dry out. Winter precipitation usually washes out much of the nutrients by spring, and even though Sedums need very little food, they do need some, especially in the 1st four to five years when they are getting established. After that, they require less because of the accumulation of organic material over time. We take soil samples in spring and fall, and we apply only the nutrients that are in short supply. We never add fertilizers that are not needed. Water can be stored in water-absorbing layers like needled mineral wool or cups in the drainage layers.

Within three days, we visited the remainder of the Montgomery County School District green roofs in a 48km (30-mile) radius, and of the 26 schools at the time, 19 had severe green roof problems due to grubs. These failed roofs were from two different green roof system suppliers, and they had one thing in common: excessively porous system design without much water holding capacity.

The other seven roofs were also of various brands (three suppliers), and as all schools were getting the exact same maintenance protocol, we could conclude the grub problem was not due to a lack of applied nutrients. The seven that looked perfectly fine all had one thing in common: they were trays as well but had solid walls, and larger water reservoirs to bridge the drought, and due to the systems, they lost fewer nutrients.

But the story gets a bit more complex. You might ask if only the needled mineral wool or water cup reservoir is the entire secret solution? No. That same week we inspected many more roofs in the area. None of them had grub problems, except for two others, located in Fairfax County, just 20 miles away. At both these roofs, we saw extensive grub problems. You could uproot a few thousand square feet of green roof vegetation without roots, frail-looking and stressed because they couldn’t tap into the water reserves. The foliage looks gray-ish or red-isch and plants look shrunken. These two green roof systems had the capacity to store enough water, so in this case, it wasn’t merely water-related, and yet there was indeed a massive grub infestation.

We took soil samples, and it was clear that this roof was utterly devoid of any nutrients. These plants had only water to feed themselves with, and they got weak. People were weeding the roof, but they were not feeding the roof. Plants, like us, need water and food. You give them both and the community of plants will live forever.

The Final Grub Solution

How to deal with grubs? There are ways to kill the grubs, but these are nasty chemicals that we prefer for you to avoid. The safest way is to let the damage continue for a while (by the time you notice it, it is too late already, and let these larvae develop into beetles, and they will fly away. Then treat the roof with Milky Spore, a natural product that affects the egg stage of the beetle, and this will be a natural defense system that will prevent 95% of the problem from reoccurring next year.

Milky Spore contains spores of the bacteria Paenibacillus popilliae, which work specifically against the grub stage of the Japanese Beetle (Popillia japonica). It poses no threat to people, animals, plants, or beneficial insects. When Milky Spore is introduced into the soil, it will lie dormant until grubs begin feeding on roots where the bacteria are present. Once ingested, the spore multiplies inside the grub and kills Japanese Beetle grubs about a week after infection. As the grub decomposes, the billions of spores it contains are released back into the soil to start the whole process again. Over time, Milky Spore fills out the soil creating a soil environment that Japanese beetles simply cannot survive in. Like other bacteria, it is highly survivable in drought conditions but suffers in temperatures of Zone five and colder.

You might say: I will treat my roof with Milky Spore before it’s a problem. I disagree. I think it is better to take care of your green roof. If you just pay a little more attention to your green roof, test your soil, and keep them fed, you will not have to worry about grubs or treat them for grubs, and your maintenance crew will thank you as well since healthier plants mean less open soil and less weeding.

Feel free to contact us if you have any feedback or questions!

1. Adamski, P., Margielewska, K. & Witkowski, Z. Compensation and induced defense in response to herbivory in Sedum maximum (Crassulaceae). Fragm. Florist. Geobot. 45, 193–202 (2000).