Relative Susceptibility of Woody Landscape Plants to Japanese Beetle (Coleoptera: Scarabaeidae)

DESCRIPTION OF DATA PRESENTATION

Data are presented in table form, alphabetically by family, then scientific name. Tables 1 and 4 use a rating to indicate susceptibility. This rating is an adaptation of the system used by Fleming (1972). When a plant is designated “resistant,” it means observed plants were either never fed on or rarely fed on by Japanese beetles. “*” and “**” indicate plants on which feeding has been observed but is either occasional or light, respectively. “***” and “****” indicate plants that are commonly fed on by Japanese beetle, resulting in either moderate or extensive feeding damage, respectively. Plants with the latter two ratings will likely sustain considerable feeding damage or be completely defoliated if Japanese beetles are present.

Qualitative ratings for Prunus serrulata, P. serotina, and P. virginiana came from Fleming (1972), whereas all others were adapted from field defoliation data (Ranney and Walgenbach 1992). In the only field study with birch (Betula) species and cultivars, defoliation was 1% or less for all nine taxa, except for Himalayan birch (B. jacquemontii), which was 16% (Ranney and Walgenbach 1992). Based on these data, most birch are not preferred hosts (Table 1), except for Himalayan, European white, and gray, of which the latter two were ranked as more susceptible by Fleming (1972).

Tables 2 and 3 summarize resistance among cultivars of crapemyrtles and crabapples from field and laboratory experiments. Susceptibility ratings for crapemyrtle varieties are adapted from susceptibility rankings assigned by Pettis et al. (2004). The qualitative ratings assigned to crabapple cultivars (Table 3) were derived from three evaluations conducted in North Carolina (Ranney and Walgenbach 1992) and Kentucky (Spicer et al 1995; Potter et al. 1998). Relative susceptibility of the 26 cultivars common to both sites was similar (Potter et al. 1998).

Crapemyrtle and crabapple species or cultivars are listed under headings indicating their relative susceptibility. As before, “resistant” indicates that observed plants were rarely fed on. For crabapples, only those with less than 10% defoliation in field studies were assigned to this rating. “Moderately resistant” means that beetle feeding was observed but light. Crabapples ranked as moderately resistant generally sustained 20% to 45% defoliation. Plants designated “moderately susceptible” will have noticeable damage by Japanese beetle corresponding to 50% to 70% defoliation for crabapple varieties. All plants considered “susceptible” will be extensively damaged or completely defoliated by Japanese beetle, equivalent to about 75% to 100% defoliation in the crabapple field studies (Ranney and Walgenbach 1992; Spicer et al 1995; Potter et al. 1998).

Ratings for linden taxa (Table 4) were taken from observations in Fleming (1972), a 3-year field study of eight Tilia spp. in Kentucky (Potter et al. 1998), and Miller and Ware (1999), which combined laboratory feeding assays with leaves or leaf discs, with field defoliation data of 16 genotypes in Illinois. Ratings of linden were determined based on both studies; however, field defoliation data were used over laboratory results if there was any inconsistency between the relative rankings of the same variety.

DISCUSSION

Susceptibility of plants to Japanese beetle should be one factor, among many, considered when selecting plants, particularly long-lived woody plants, for residential and commercial landscapes. Resistance of one plant species to Japanese beetle does not necessarily imply resistance to other plant-feeding insects or plant pathogens (Smitley and Peterson 1993; Pettis et al. 2004). For example, the crapemyrtle varieties ‘Tonto’ and ‘Tuscarora’ are moderately susceptible to Japanese beetle, but the same varieties are resistant to metallic flea beetles (Altica spp.), an important pest of crapemyrtle in production (Pettis et al. 2004).

Resistance of woody host plants to Japanese beetle is probably mediated by the presence or absence of deterrent compounds found in the foliage (Keathley et al. 1999; Potter and Held 2002). Control products containing certain plant extracts, such as neem (azadirachtin), can effectively deter feeding in laboratory choice tests (Ladd et al. 1978; Held et al. 2001) but often fail to provide similar protection when tested on whole plants in the field (Harper and Potter 1994; Witt et al. 1999). Abundant field populations, however, will reduce the efficacy of both conventional and botanical insecticides because of additional adults re-infesting treated plants.

Elms and lindens are considered preferred hosts for the Japanese beetle. Among elms, only U. lancefolia and U. prunifolia were slightly less susceptible than the other species (Miller et al. 2001). Although no lindens are resistant, varieties such as ‘Parade’, ‘Legend’, and ‘Sterling’ appear to be less susceptible (Potter et al. 1998). Moderate to dense leaf pubescence may be an important factor in susceptibility of linden and elm to Japanese beetle. For example, foliage of T. platyphyllos ‘Parade’, T. tomentosa ‘Sterling’, and U. lamellose have heavy pubescence and is less preferred by Japanese beetle (Potter et al. 1998; Miller and Ware 1999; Miller et al. 2001). Conversely, certain plants, such as species of Ilex and Rhododendron, with waxy or glossy foliage are also resistant to Japanese beetle (Fleming 1972; Keathley et al. 1999).

Plants with purplish or deep red foliage (e.g., ‘Crimson King’ Norway maple) are often observed to sustain more damage by Japanese beetle than green-leaved cultivars (Rowe et al. 2002). Foliage color alone, however, does not account for these differences. When two artificial ficus trees with foliage painted either green or purple are placed side by side in the field, significantly more beetles land on the green-leaved plants (Rowe et al. 2002). Flower color, however, does influence susceptibility of flowering plants to Japanese beetle. Rose varieties with yellow or white flowers are more likely to be attacked than those with darker-colored blooms (Held and Potter 2004).

Resistance of certain plants to Japanese beetle also depends on the production of attractive volatile compounds following damage by Japanese beetle or other plant-feeding insects (Loughrin et al. 1995). Japanese beetles use a wide range of floral and fruitlike compounds to locate a host plant (Fleming 1972; Loughrin et al. 1995, 1996). Laboratory tests show that Japanese beetle often cannot discriminate among foliage of plants that differ in susceptibility in the field (Loughrin et al. 1995, 1996). This finding indicates that Japanese beetles are attracted to plants regardless of their status as a host (Potter and Held 2002). However, if susceptible plants suffer feeding damage, they produce an array of attractive volatiles that serve as aggregation stimulants for Japanese beetle (Loughrin et al. 1995, 1996). Therefore, a susceptible plant in the field may not be inherently more attractive, but, if damaged, these plants produce the volatiles that recruit Japanese beetles much like sharks attracted to a blood trail in the water.

Host plant resistance is the most sustainable means of managing feeding damage or plant losses resulting from Japanese beetle adults. Landscape designers in states on the front of this insect’s range expansion should consider incorporating resistant plants into residential, commercial, and municipal landscapes as well as any other long-term plantings. This approach can reduce the economic and environmental costs associated with the repeated use of insecticides to prevent or reduce damage to urban landscapes in the future.