Japanese Beetle Study Results


Japanese Beetles have had the run of many plants in Eastern Canada this year. These beetles have been around for decades, but numbers appears to be increasing in numbers in recent summers, particularly in 2023. CFIA has provided some insight into what attracts the beetles and why they might be worse in 2023 than previous summers (they prefer hot summer days with humidity above 60%, (CFIA, https://inspection.canada.ca/plant-health/invasive-species/insects/japanese-beetle/fact-sheet/eng/1328165101975/1328165185309, retreived Sep 1, 2023). But why are they choosing specific plants to feed on? That is the intent of this study. 

When a plant becomes stressed, which can result from limited nutritional resources, they send out pheromones (Hermary, H., (2020) Relationships with other organisms [Botany]. moodle@gaiacollege.ca. gaiacollege.ca/login.html). If the soil biology isn’t sufficient for the plant, it could exhibit signs of stress. Insects, including Japanese beetles, follow the pheromones sent out and find their next meal. In order for plants to thrive, there needs to be sufficient soil species and nutrients available. 

The theory being tested in this small study is that plants are struggling due to low fungal biomass or an improper ratio of fungal to bacterial biomass. 

Many of the plants attacked by Japanese Beetles are shrubs, bushes, vines, or deciduous trees. These species require a fungal to bacterial biomass ratio of at least 2.0 and up to 10. Additionally, these species should have a bacterial biomass minimum of 135 microg/g and a fungal biomass minimum of 270 microg/g (The Soil Foodweb School). This part of this study (done in September 2023) looked specifically at the bacterial and fungal biomass of the soil samples as well as the F:B ratio.

Of the 12 samples collected, four had minimal damage; five had medium damage; two had significant damage, and one didn’t have any damage. 

Sample collection and analysis

Samples were collected from home gardens from across Ottawa and Rockland. Two samples were taken from four yards; three samples from one yard; and one sample from one yard. Plants to sample were chosen based on the preferred food source of Japanese beetles; all but one plant exhibited some degree of damage. In some instances sunny vs shaded plants were chosen or plants with some, or no, ground litter providing fungal food.

Samples were analyzed within 24 hours of being collected. All analysis was completed based on the Soil Food Web protocol. 


Sample IDAmount of damageBacterial biomassFungal biomassF:B Ratio

Eleven samples had significantly low fungal biomass (significance was based on more than 50% less than expected). Four samples had bacterial biomass counts higher than the minimum of 135 ug/g; two samples were less than 50% below the minimum (between 67.5 ug/g and 135 ug/g); six samples had bacterial biomass levels more than 50% below expected (less than 67.5 ug/g). Only one sample had a higher than expected F:B ratio, however, both bacterial and fungal biomass were low for the sample. The higher than expected ratio was due to the incredibly low bacterial biomass.


A direct relationship between the fungal biomass or F:B ratio and the amount of damage to the plant was found. What was observed was that all samples were low in soil biology. This supports the theory that low soil biology is making the plant vulnerable. However, a second study will be carried out in the spring using high fungal compost to increase the soil biology.

Published by juliamdupuis

Green Angel Sustainability Consultant. Environmental Chemist. RRU MEM Graduate.

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