Arboriculture & Urban Forestry 32(3): May 2006 111 Table 2. Mean (±SE) percentage leaf damage, based on missing leaf tissue, by Japanese beetle, Popillia japonica (Newman), adults for each rose genotype in the 2002 and 2003 multiple-choice experiments with 40 replications per rose genotype. Rose genotype Nearly Wild Cabbage n 40 57.2 ± 5.69 a* 40 52.7 ± 4.61 a Crimson Bouquet 40 34.6 ± 3.19 b Love and Peace 40 34.8 ± 3.67 b Knock Out 40 40.2 ± 3.72 b 2002 (mean ± SE) 2003 (mean ± SE) 52.4 ± 5.89 a 64.4 ± 4.78 a 47.8 ± 4.69 a 52.1 ± 5.59 a 51.0 ± 4.49 a *Means within a column not followed by a common letter are significantly different (P 0.05) as determined by a Tukey’s mean separation test. percentage of mean leaf damage exceeded 50% for most of the rose genotypes. The Cabbage and Nearly Wild roses, chosen for comparison, appeared to be more susceptible than the three new rose genotypes (Knock Out, Crimson Bouquet, and Love and Peace) based on overall mean percentage feed- ing damage ratings (Tables 1 and 2). In the 2002 no-choice experiment, Knock Out experienced significantly less beetle damage, with an average rating of 15%, whereas all the other rose genotypes tested had percent- age feeding damage ratings of more than 50%. Of the ten replications for the 2002 no-choice experiment, five were not fed upon, whereas the other five leaflets were fed upon ex- tensively. It was determined that a number of Japanese beetle adults used in the Knock Out treatment were either lethargic or dead after 24 hr. However, this occurred only in this one instance. The 2003 no-choice experiment better represented the susceptibility of Knock Out, in which the percentage feeding damage rating (69.8%) was similar to the other rose genotypes. In addition, the percentage feeding damage ratings for Knock Out in the 2002 and 2003 multiple-choice experi- ments were 40.2% and 51.0%, respectively. Knock Out rose has been reported to be resistant to plant-feeding insects (McCoy 2004). However, in our study it was just as suscep- tible to Japanese beetle adult feeding as the other rose geno- types tested. Field studies need to be conducted to substan- tiate this claim and determine whether laboratory effects in- fluence susceptibility. The no-choice experiments evaluated how susceptible the rose genotypes were when exposed to five adult Japanese beetles under laboratory conditions. Based on the results from both experiments, all of the rose genotypes experienced feed- ing damage ratings between 50% and 78%. This indicates that all five are highly susceptible to adult Japanese beetle leaf feeding when no other food source is available. In the multiple-choice experiment, the rose genotypes Knock Out, Crimson Bouquet, and Love and Peace experi- enced less feeding damage than the others (Table 2). How- ever, there was still considerable feeding damage, ranging from 34% to 65%. Crimson Bouquet had the least amount of feeding damage, followed by Love and Peace and then Knock Out (Table 2). Although all five rose genotypes are suscep- tible to Japanese beetle feeding, the three new rose genotypes experienced less feeding damage. It is possible that the use of 25 adult beetles per chamber may have created crowded con- ditions that affected feeding preference. However, under natural infestations, Japanese beetle adults tend to congregate in large masses (Smith and Hadley 1926; Hawley and Metzger 1940), which increases the amount of plant damage. In addition, the 24 hr test period, which is typically used to conduct feeding preference studies, may have compromised the ability of the study to detect real differences in suscepti- bility because Japanese beetle adults are mobile and likely to leave a nonfavorable food source under field conditions. The use of observers to evaluate insect feeding damage is not uncommon: similar research assessing Japanese beetle adult feeding damage has been conducted using observer es- timates (Ranney and Walgenbach 1992; Fulcher et al. 1998; Miller and Ware 1999). In our study, ten observers were used per leaf sample and the results were consistent across both the no-choice and multiple-choice experiments in the degree of variability, indicating there was no observer effect. Observer assessments, which are an accessible and simple measure- ment method, may be useful to home gardeners or landscape professionals who have an interest in measuring plant injury but would not have access to tools such as leaf area meters. This laboratory study suggests that the five cultivated roses tested are highly susceptible to Japanese beetle adult feeding. Our study is in agreement with Potter et al. (1998) in that most roses are going to be susceptible to Japanese beetle feeding to some degree. Potter et al. (1998) field-screened 53 rose genotypes during a summer when Japanese beetle popu- lations were high and found that all were heavily defoliated. The study was repeated the following summer when Japanese beetle populations were low, and Potter et al. (1998) found considerable variation in damage. Although some rose geno- types may exhibit some degree of resistance or tolerance, when Japanese beetle populations are high, most roses are likely to experience considerable plant damage, which will substantially reduce aesthetic quality. Other factors that can influence the level of susceptibility to Japanese beetle adult feeding include the amount of sun- light received and flower color (Held and Potter 2004). It was predicted that the Nearly Wild rose would not be favored by adult Japanese beetles in the study because of the low level of feeding damage observed in the field, which seemed to indi- cate possible resistance or tolerance in the Crasberg Rose Garden (T. Tiddens, pers. obs.). The Crasberg Rose Garden planting of Nearly Wild roses is in a densely shaded area underneath large trees. Japanese beetles prefer to feed on roses located in full sun (Johnson and Lyon 1991), whereas roses located in wooded areas are seldom attacked (Daar et al. 1996). However, in both the no-choice and multiple-choice ©2006 International Society of Arboriculture
May 2006
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