Authors: Hailey Shanovich, Dr. Arthur Vieira Ribeiro, Dr. Robert Koch, Dr. Bill Hutchison, Annie Klodd, Natalie Hoidal
Research on the impact of Japanese beetle (JB) in Minnesota apple orchards is ongoing. A research team at UMN has been studying infestations of JB in apple orchards and whether JB prefer some apple varieties over others. This article discusses the current research findings and reviews the habits of JB in apple orchards.
JB is an invasive insect in the U.S. that has become a significant pest of turfgrass and ornamental, horticultural and agricultural plants in the eastern and midwestern states. JB is considered a “generalist” insect, meaning it can feed on many different plants, over 300 plant species. As larvae, JB feeds underground on turfgrass roots including lawns, athletic fields and golf courses. Adult JB mainly feed on leaves of plants, leaving a characteristic “skeletonized” or “lacey” appearance. Adult JB can also feed on flower petals, including roses, and on fruits in some cases, especially if the fruit is already damaged and opened by birds.
Currently, JB are established in 28 U.S. states and five Canadian provinces, with six other states partially infested (Fig.1).
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| Figure 1. from Shanovich et al. 2019. Map of the current distribution of the Japanese beetle in the United States and Canada. |
JB in the Upper Midwest
Throughout most of its range in the U.S., JB has an annual life cycle of one generation per year. In the Midwest, JB adults begin emerging from the soil in late June to early July and fly to host plants to begin feeding. JB adults can feed on many economically important crops in the Midwest including both row crops like corn and soybean, and specialty crops like grapes, berry crops and apples.
JB damage on fruit trees
JB primarily feeds on leaves of fruit trees, including apple, plum, peaches, apricots and cherries, leaving a characteristic “skeletonized” or “lacey” appearance (Fig. 2). However, the short-term and long-term effects of feeding by JB on leaves of fruit trees isn’t clearly understood and can vary widely depending on the severity of feeding.
Direct impacts of JB to flowers or fruits are unlikely. JB adults are not present in orchards during flowering of fruit trees in the Midwest, and they are not known to be able to damage fruits themselves unless previous damage exists; such as bird or deer damage (Fig. 3).
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| Figure 2. Characteristic “lacey” foliar feeding by Japanese beetle on an apple leaf; and Figure 3. Adult Japanese beetles feeding inside a damaged apple |
Japanese beetles in apple orchards
Recent research findings by Hailey Shanovich (graduate student), Dr. Arthur Vieira Ribeiro (postdoc) and Dr. Robert Koch (extension entomologist) at the University of Minnesota (UMN), have revealed that JB adults are present in Minnesota apple orchards from late June through early September, with peak JB abundance in late July. We sampled the season-long populations of JB at four different Minnesota apple orchards in summer 2017 and 2018.
Varietal preferences
We also found differences in the number of JB between two popular Minnesota apple varieties, Honeycrisp and Zestar!®. Overall, throughout the season, we found that the number of adult JB was, on average, almost 10 times higher on Honeycrisp than Zestar!®. The exact reason for JB’s preference for Honeycrisp over Zestar!® remains unknown, but we speculate that it may be related to the amount of sugar in the trees’ leaves. Sugars are major feeding stimulants for JB, and Honeycrisp trees commonly exhibit a leaf disorder called zonal chlorosis (Fig. 4), in which the leaves accumulate higher levels of sugar. However, Zestar!® trees may also be producing some feeding deterrent compounds, deterring JB from feeding, but this remains to be studied.
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| Figure 4. Honeycrisp leaves exhibiting zonal chlorosis or yellowing of the leaves in areas. |
Defoliation and management implications of defoliation
Despite the sometimes distressingly large numbers of JB in orchards, it is unclear if their feeding or defoliation of the trees should be of concern to growers. At first glance, defoliation by JB can look alarming. However, this defoliation is most often not as bad as it seems. For example, in the study mentioned above, we estimated JB defoliation to only be about 4% of the total tree canopy in Honeycrisp and less than 1% in Zestar!® per tree on average. JB tends to feed on the outer leaves of a tree canopy first, starting at the top of the tree or in full-sunlit areas and working their way down. JB is also known to congregate on a single plant and so can cause unsightly damage to isolated areas on a plant while still leaving much of the plant untouched.
Defoliation is a reduction in leaf area, or photosynthetic tissue, and therefore is measured as the percentage of leaf tissue eaten. However, defoliation is difficult for most people to estimate without training. For example, try to guess the percent leaf area eaten in the photos of JB defoliation on grape leaves before reading the caption (Figure 5). Furthermore, a tree may have between 40-50% of its leaves with some defoliation from JB but only about 5-10% of the total tree’s leaf area could be defoliated. We saw cases of hundreds of JB occurring on a single tree throughout the season, but total tree canopy defoliation only being between 5-10% for Honeycrisp trees. JB feeding occurs on the outer tree canopy leaves, usually towards the crown, and it is easy to overestimate defoliation when not looking at the inner and lower canopy.
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| Figure 5. Examples of JB defoliation on grape leaves. From left to right, defoliation is 2%, 6%, and 10%. Percent defoliation was confirmed with computer software. Photo credit: Dominique Ebbenga |
Does Japanese beetle feeding actually impact apple production?
There has been no research on the short or long-term effects of JB defoliation to apple or other fruit trees. In fact, little is known on the effect of any defoliation on fruit and seed production in trees. However, studies from Ohio State University and Cornell University found that fruiting spur defoliation at bloom and in the month following bloom has been found to decrease flowering and fruit set for that same year. In Minnesota, apple trees bloom some time before adult JB begin emerging and feeding; bloom occurs in mid-late May (https://apples.extension.org/timing-of-apple-tree-bloom/) and adult JB typically begin emerging in late June. Therefore, it is unlikely that JB populations in Minnesota orchards could cause a substantial amount of defoliation before the end of June, which would affect the amount of flower buds and fruit set for that year. However, the effects of defoliation on yield for following years remains to be studied.
Furthermore, it has been shown that JB is not able to directly puncture SweeTango® apple fruits (Pires and Koch 2020) so they most likely not be directly impacting marketability of the apples. JB observed feeding on apples in the field are most likely able to do so via pre-existing damage that softens or punctures the apple skin such as bird damage.
Managing Japanese beetles in apple orchards:
Since the levels of defoliation we observed by relatively high numbers of JB in Minnesota orchards were low and JB is unable to directly feed on the fruit of SweeTango® apples(Pires and Koch 2020), based on the varieties studied so far, we urge growers to reconsider whether or not they need to spray. There is unlikely to be an economic benefit to spraying for JB on yield in apple. Additionally, bio-pesticides or biocontrol agents such as milky spore disease and entomopathogenic nematodes are not advised for soil application by growers as they have a high application cost per acre and are only recommended for controlling the white grubs; adult JB can simply fly in from neighboring orchards or farms with other crops (Shanovich et al. 2019; Hahn and Weisenhorn 2018). Furthermore, current spray regimes for apple pests in the Midwest (e.g., apple maggot, codling moth, plum curculio) probably already offer some control against JB adults. See Purdue’s Midwest Fruit Pest Management Guide for the most current spray recommendations for JB and all other apple pests.
However, it is clear from our study that JB prefer certain apple cultivars over others, like Honeycrisp over Zestar!®. Therefore, growers could focus scouting and management on more preferred, and high-value apple, such as Honeycrisp in orchards, if they are concerned about JB defoliation levels impacting yield or tree health. Additionally, newly planted, young trees or trees planted in high-density have much smaller canopies and less leaf area compared to mature or traditionally planted trees and should be monitored for defoliation levels during JB outbreaks.
Literature cited:
Evaldo 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
Hailey N Shanovich, Ashley N Dean, Robert L Koch, Erin W Hodgson. 2019. Biology and Management of Japanese Beetle (Coleoptera: Scarabaeidae) in Corn and Soybean, Journal of Integrated Pest Management, Volume 10, Issue 1. https://doi.org/10.1093/jipm/pmz009
Jeff Hahn and Julie Weisenhorn. 2018. Japanese beetles in yards and gardens. University of Minnesota - Extension. https://extension.umn.edu/yard-and-garden-insects/japanese-beetles#biological-insecticides-1592861
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