Japanese beetles (Popillia japonica) have become a growing concern for apple growers in recent years. These voracious feeders primarily skeletonize leaves by feeding between the leaf veins, and they may also target flower buds, flowers, and fruit, depending on the host plant.

Japanese beetle adults feeding on apple fruit (Fig. 1) does not necessarily mean that they caused the initial damage, but rather that beetles were likely feeding on previously-damaged fruit. Indeed, research¹ showed that Japanese beetles were not able to feed on intact apple fruit of SweeTango but were able to feed on cut fruit. Though it is possible that this preference for feeding on fruit may vary amongst apple cultivars.

Varietal Preferences

Research has shown that Japanese beetles exhibit strong preferences for specific plant cultivars, and this phenomenon extends to apple trees as well. Studies have found varying defoliation rates among different apple cultivars. For example, a study in Arkansas tested 66 apple cultivars and found different defoliation rates on various varieties. Zestar had 18% defoliation, Granny Smith 26%, Golden Delicious 19%, Jonathan 15%, and Cortland only 6% defoliation. Moreover, the rootstock used can also influence defoliation rates. Gala planted on different rootstocks showed that M9s had higher defoliation rate (17-26%) compared to 9% on M27 rootstock. Research from the University of Minnesota revealed that the number of adult Japanese beetles on Honeycrisp apples was nearly ten times higher than on Zestar apples. This information suggests that specific apple cultivars may attract more Japanese beetles than others.

Defoliation Impact on Apple Trees

While Japanese beetles can gather in large numbers on a single tree, they are unlikely to cause significant damage to apple trees through defoliation alone. On average, Honeycrisp trees experience about 4% total canopy defoliation per tree, while Zestar apples only experience 1%. Japanese beetles tend to start feeding on the outer leaves of the tree canopy, starting at the top or in full-sunlit areas, and gradually working their way down. As a result, they may cause unsightly damage to isolated areas on a plant while leaving much of the plant untouched.

It is essential to take a step back and assess the entire tree foliage when estimating the percentage of defoliation, as damage to a single leaf or a group of leaves might appear alarming but may not have a significant impact on overall tree health. Research from the University of Minnesota reported cases of hundreds of Japanese beetles on a single tree throughout the season, yet total tree canopy defoliation was only between 5-10% for Honeycrisp trees.

While no specific research has yet linked defoliation caused by Japanese beetles to negative impacts on apple tree health, growth, or fruit production, it is generally believed that apple trees can tolerate 20-30% defoliation without significant harm. It is unlikely that apple trees in the Upper Midwest will experience such high levels of defoliation, but in the event that an outbreak of Japanese beetle is observed, young trees, preferred cultivars such as Honeycrisp, Lodi, and Pristine, and high-density orchards with small leaf canopies should be monitored and possibly protected.

Biology

Japanese beetles have only one generation per year (Fig. 2). Females mate multiple times and lay around 40-60 eggs. Females live for about 30-45 days and lay eggs ~10cm deep in the soil in small batches. Females prefer to lay eggs in moist soil in short turfgrass. Eggs will hatch 1-2 weeks later and larvae will come up to feed on grass roots and go through three instars. Then, the third instar will burrow down later in the season to 10-15cm deep and overwinter. The following spring, the move back up in the soil, feed, pupate, and emerge as adults in mid-late June. 

As adults, they are known to prefer sunlight but many feed in the shade as well. Feeding occurs at night as well even if it is less than during the day and peak feeding was reported between 10AM and 10PM. Temperature plays a role in their movement: when disrupted at <77F, they fall to the ground, at 77F about half fly away and half fall to the ground, and above 77F most fly away.

Phenology and Degree-Day Model

Understanding the phenology of Japanese beetles can aid in their management. These beetles typically first appear around June 20th and are present until around September 25th, with peak populations occurring from early July to the end of August.

Researchers at the University of Minnesota have developed a degree-day (DD) based phenology model for Japanese beetle emergence. This model uses a base temperature of 59°F and an upper threshold of 71°F. Early emergence occurs between 0-10% of degree-days, with peak emergence predicted to occur at 620 DDs. This model can be helpful in predicting beetle activity and planning management strategies accordingly. For real-time updates on Japanese beetle emergence and weekly forecasts, growers can follow the automated DD page for Japanese beetles on the University of Minnesota’s website https://vegedge.umn.edu/degree-days-midwest-insects/japanese-beetle

Cultural Control

In addition to monitoring and managing preferred cultivars and vulnerable orchards, there are cultural practices that can be employed to reduce Japanese beetle populations. These include:

Eliminating nearby attractive plants, such as grapes, crabapples, Japanese maples, and pin oaks, can help reduce the attractiveness of the orchard to Japanese beetles. Tilling the soil and using cover crops can disrupt egglaying in the orchard floor and reduce their populations. Applying mulch in the orchard can discourage egglaying by female beetles.

Keeping grass height above 3 inches can make the environment less conducive to Japanese beetle egglaying and withholding irrigation of grassy areas until mid-August can also reduce oviposition.

Chemical Control

Chemical control through insecticide spraying may not offer significant economic benefits and should be used judiciously, taking into account the potential impacts on beneficial insects and pollinators.

References

¹Evaldo Martins Pires and Robert L Koch. 2020. Japanese beetle feeding and survival on apple fruits. Bioscience Journal, Volume 36, Issue 4, p. 1327-1334. https://doi.org/10.14393/BJ-v36n4a2020-50364

This article was posted in Apples, Insects and tagged Apples, Christelle Guédot, insects, Japanese Beetles.