Abundance and Consumption Rate of Glassy-Winged Sharpshooter (Hemiptera: Cicadellidae) on Peaches and Plums

Peter C. Andersen, 2 Russell F. Mizell, Ill, Brent V. Brodbeck, Thomas G. Beckman,3 and Gerard Krewer4

University of Florida, North Florida Research and Education Center, Quincy Florida 32351 USA

J. Entomol. Sd. 43(4): 394-407 (October 2008) Abstract Homalodisca vitriperinis (Germar), the glassy-winged sharpshooter, is a primary vector of phony peach and plum leaf scald diseases caused by XyIe/Ia fastidiosa Wells et al. A survey of H. vifripennis on peach [Prunus persica (L.) Batsch] varieties established that leafhopper abundances varied from 0-13 per tree. Prunus persica cvs. Flordaking and June Gold and Prunus salicina Lindl. (cvs. Methley and Santa Rosa) were then budded on each of 3 P. persica rootstocks (cvs. Aldrighi, Lovell and Nemaguard). Leafhopper abundance was monitored on each of the two scions budded on each rootstock and on non budded rootstocks over a 2-yr period. The genotypes were container-grown in Year 1 and were planted in the field in Year 2. For both years leafhopper abundance was greatest during early June and on Methley and Santa Rosa cultivars compared with the peach genotypes. The feeding rates of leafhoppers were substantially higher on plum scions than on peach scions, and nocturnal feeding rates were often higher than daytime feeding rates. Mean leafhopper feeding rates were correlated with leafhopper abundance on Prunus genotypes from 3-8 June in a quadratic manner (F = 53.8; df= 2,12: R 2 = 0.90; P< 0.0001); the mean nighttime feeding rate was best correlated linearly to mean cumulative leafhopper abundance (F= 446.9; df= 1,13; R = 0.972; P< 0.0001).

Key Words abundance, consumption rate, glassy-winged sharpshooter, Homalodisca vitrip- cnn/a, leafhopper

The glassy-winged sharpshooter, Homalodisca vitriperinis (Germar) [=Homalo- disca coagulata (Say)] (Takiya et al. 2006), is endemic to the southeastern United States and northeastern Mexico (Redak et al. 2004, Turner and Pollard 1959). Homa- lodisca v/fr/penn/s is a primary vector of xylem-limited diseases caused by XyIeIIa fastidiosa Wells et al. (Hopkins and Purcell 2002, Wells et al. 1987). In the southeastern United States, H. vitripennis is one of the most important problems in the culture of plums (Prunus salicina Lindl.) (e.g., vectoring plum leaf scald) and peaches [Prunus persica (L.) Batsch] (e.g., vectoring phony peach disease) and precludes a grape industry based on Vitis vinifera L. Homalodisca vitripennis is an exotic pest in California (Sorensen and Gill 1996) where Pierces disease now threatens a multi-

Received 03 December 2007; accepted for publication 30 May 2008. 2Address inquires (email: pcand @ ufl.edu). USDA-ARS Fruit and Tree Nut Research Laboratory, Byron, GA 31008. 4 University of Georgia, Tifton, GA 31793.

394 ANDERSEN et al.: Abundance and Consumption Rate of H. vitripennis 395

billion dollar grape industry along with other commodities such as peach, plum and almond (Hopkins and Purcell 2002). Homalodisca vitripennis is highly polyphagous and may feed on hundreds of plant species belonging to many botanical families (Adlerz 1980, Redak et al. 2004, Turner and Pollard 1959), although the seasonal pattern of host plant usage is repeatable from year to year (Brodbeck et al. 1990, Mizell and French 1987). Polyphagy may be critical for H. vitripennis survival because xylem fluid contains the lowest concentration of nutrients of any plant tissue (Andersen and Brodbeck 1 989a,b, Andersen et al. 1995, Mattson 1980, Raven 1983), and nutrient levels in xylem fluid change with host plant phonology (Andersen et al. 2005, Brodbeck et al. 1990). Insects that subsist on xylem fluid consume 100-1000 times their body mass each day (Mittler 1967, Horse- field 1977, Andersen et al. 1989, 1992, Brodbeck et al. 1993, Crews et al. 1998, Ponder et al. 2002). All insects that feed on xylem fluid have well developed cibarial pumps that enable extraction against the negative pressure that exists in xylem vessels (Scholander et al. 1965, Raven 1983). Other characteristics of H. vitripennis that facilitate subsistence on xylem fluid include: (1) a seasonal and diurnal synchro- nization of feeding in response to optimum host nutritional status (Bi et al. 2005, Brodbeck et al. 1990); (2) a high assimilation efficiency of primary organic compounds (amino acids, organic acids and sugars) (Andersen et al. 1989, 1992, Brodbeck et al. 1993); and (3) excretion of ammonia as the primary nitrogenous waste product (Andersen et al. 1989, 1992, Brodbeck et al. 1993). The concentration of secondary compounds is exceedingly low in xylem fluid (Raven 1983). Xylem fluid chemistry of Prunus genotypes and the abundance of H. vitripennis were rootstock dependent and were correlated with concentrations of amino acids in xylem fluid (Brodbeck et al. 1990, Gould et al. 1991). It has been proposed that the amides serve as phagostimulants for adult H. vitripennis (Andersen et al. 1992, 2005, Brodbeck et al. 1993). The objectives of this study were to determine the preference (abundance) and consumption rate of H. vitripennis on 2 peach and 2 plum scions budded on 3 peach rootstocks. Seasonal abundance was determined over a 2-yr period, and consumption rate was measured diurnally during the second year. In addition, xylem fluid osmolality and xylem tension were recorded during the day and night. Leafhopper abundances also were monitored on a diversity of P. persica germ- plasm to assess the range of abundances typically encountered in the southeastern United States. Materials and Methods Prunus salicina L. cvs, Methley and Santa Rosa and P. persica cvs. Floridaking and June Gold" were budded onto Aldrighi (from Brazil, Gould et al. 1991), Lovell and Nemaguard rootstocks at the North Florida Research and Education Center at Monticello (NFREC-Monticello). Each tree was double budded (contained 2 scions) yielding the following combinations of scions: Flordaking/June Gold, Flordaking/ Methley, Flordaking/Santa Rosa, June Gold/Methley, June Gold/Santa Rosa, and Methley/Santa Rosa, each on 3 rootstocks. Control plants also were maintained without budding (Aldrighi on Aldrighi, Lovell on Lovell, Nemaguard on Nemaguard). Each combination was replicated 5 times. Because each scion was not influenced by the other adjacent scion, data were pooled for Flordaking, June Gold, Methley and Santa Rosa for each of three rootstocks. In Year 1, plants were grown in 10-L containers. Media consisted of 2 parts pine bark, 1 part Canadian sphagnum peat, and 1 part sand. The medium was amended

396 J. Entomol. Sd. Vol. 43, No. 4 (2008) I with 1.48 kg dolomite, 2.97 kg superphosphate, 889 g Micromax (12S-0.1B-0.5Cu- l2Fe-2.5Mn-0.O5Mo-lZn) and 5.93 kg Osmocote 18N-2.64P-9.96K (18-6-12) per m3. After the first growing season, trees were planted in the field at a spacing of 4.6 m within rows and 6.1 m between rows. Soil type was a Dothan loamy sand. Drip irrigation was provided for container-grown and field-planted trees. Leafhopper abundance was monitored on the Prunus genotypes from 18 May to 12 August 1987 and from 25 May to 3 August 1988 at the NFREC-Monticello. Insects were counted every 2-10 don container-grown (1987) and field-grown (1988) trees by holding a clipboard on the far side of each scion allowing them to be enumerated. Insect abundance was monitored on the 4 scions on each rootstock (and from control scions). Leafhopper abundance data were subjected to a repeated measures analyses (SAS Institute 2003). Multivariate analyses were used to examine between-plant variation and to assess the effects of scion, rootstocks and interactions on leafhopper counts; univariate analyses were performed to examine within-plant variability in leafhopper counts over time to assess the significance of date and date x scion and date x rootstock interactions. Additionally, leafhoppers were counted on 30 peach genotypes on 16 and 26 June 1992 at the University of Georgia Attapulgus Research Center to assess abundance on a wide range of genotypes for comparative purposes. Trees were in the second leaf growth stage with a plant spacing of 3.9 m within rows and 6.1 m between rows. Means (±SE) are reported. For feeding experiments, leafhoppers were obtained from a block of crape myrtle, Lagerstroemia id/ca L., located at a local nursery. Leafhopper consumption rate was monitored from 2-8 June 1988 using a feeding assembly (Andersen et al. 1992). Adult H. vitripennis were caged in collection devices that allowed easy quantification and removal of the liquid excreta (the only excretory or digestive waste product). In brief, feeding assemblies were constructed by drilling a hole in the bottom portion of a 50-ml polypropylene centrifuge tube (Nalgene 3150-0050) and gluing a snap cap from a 15-ml tube (Falcon 2057) over the opening. Excreta was collected and measured by removal of a smaller tube inserted in the snap cap. The 50-ml tube was slit to allow easy enclosure of shoots. Leaves were removed from the enclosed shoot. A sponge stopper (2.5 cm diam) was slit on one side, and aluminum foil was wrapped around the assembly to exclude rain and to prevent overheating. Two female leafhoppers were placed in each feeding assembly. Feeding rate was presumed to be equal to excretion rate because evaporative losses under these conditions should be negli- gible. Feeding rate was monitored at various times of the day and overnight. Times of collection were 0800, 1200, 1400, 1600, 2000 h, and midpoints of feeding were 0200 (night), 1000, 1300, 1500 and 1800 h. Feeding rate was correlated with mean abundance for each of 4 scions on each rootstock plus a nonbudded control for each rootstock (n = 15). Mean daily feeding rates were compared with mean leafhopper abundance from: (1) 3-8 June; (2) 1-30 June, and; (3) for the entire year (1988) using the General Linear Models of the Statistical Analysis System (SAS Institute 2003). Xylem fluid was collected from shoot segments 15-25 cm long during the day and during the night using a pressure chamber apparatus (Scholander et al. 1965). The plants were those used for the evaluation for leafhopper abundance. Extraxylellary tissue was stripped at the cambium, and fluid was collected for 90 sec from the stem protruding from the chamber using a 0.5 MPa overpressure. Xylem fluid was frozen at -20°C for later analysis. Xylem fluid was thawed and total osmolarity was determined using a Wescor Model 5500 vapor pressure osmometer. PW

ANDERSEN et al.: Abundance and Consumption Rate of H. vitripennis 397

Results

Thirty peach cultivars were evaluated on 16 and 26 June 1992 in an effort to discern the range of preference of H. vitripennis on different peach genotypes (Table 1). There were substantial differences in mean leafhopper abundance ranging from 0-13.4 leafhoppers per tree. Six cultivars had at least a mean of 5 leafhoppers per tree, and 5 cultivars had 0 leafhoppers per tree. The mean (± SE) number of leafhoppers was 1.6 ± 0.5 on June Gold and 0.5 ± 0.3 on Flordaking, 2 important commercial cultivars used in the coastal plains peach industry. Leafhopper abundance was monitored on Prunus genotypes from 18 May to 12 August 1987 (Fig. 1). There was no effect of one scion on the adjacent scion on the same plant; therefore, data were combined by scion. The first leafhoppers appeared on 28 May. Leafhopper abundance on Prunus was maximal during early to mid-June with peak values recorded on 9 June. The abundance of H. vitripennis rapidly declined after mid-June. Homalodisca vitriperinis preferred plum scions more than peach scions. Insect abundance was highest on Methley scions (19.1-19.5 per scion) on all rootstocks, intermediate on Santa Rosa (6.3-9.7 per scion) scions, and lowest on Flordaking, Junegold scions and non budded peach rootstocks. After 26 June, the number of insects was <2 for all genotypes. A repeated measures univariate analysis (within subject effects) showed that date and date x scion interaction were highly significant during 1987 (Table 2, Fig. 1). In the multivariate analysis (between subject effects), only the scion factor was significant (Table 3). From 29 May through 12 August, scion effects were highly significant (P< 0.01) (Fig. 1). Rootstock effects were significant (P < 0.05) on 16 June, 1 July and 4 August (Fig. 1), and rootstock x scion effects were significant for 1 July and 4 August (data not shown). The seasonal pattern of leafhopper abundance during 1988 was similar to that during 1987, although the peak number of leaf hoppers during 1988 was lower (Fig. 2). Peak populations were encountered on 16-25 June. Higher numbers of H. vitripennis occurred later in the season during 1988 compared with 1987. For example, from 3-22 July there were 1-3 leafhoppers on Methley scions. The univariate repeated measures analysis (within subject effects) showed that date, date x rootstock, date x scion and date x scion x rootstock were all significant (Table 2). The overall repeated measures for between subject effects, both rootstock and scion were significant (Table 3). Scion effects were highly significant (P< 0.001)for all measurement dates after 1 June. Rootstock effects were significant (P < 0.05) on many days close to the period of peak leafhopper abundance (1, 8, 10, 17 June) (Fig. 2); similarly significant rootstock x scion occurred on 8, 10, 17 June (data not shown). Leafhopper consumption rates on Prunus genotypes were measured diurnally from 2-8 June 1988 (Table 4). Feeding rate data paralleled that of leafhopper abundance. Consumption rates were generally highest for Methley on the 3 rootstocks followed by Santa Rosa. The highest feeding rates on plum occurred at night or in the morning. The feeding rates on the peach scions and rootstocks were very low, often less than 20 uL h 1 . Cumulative average feeding rate per day varied from 119 uL d1 (Aldrighi rootstock) to 4,566 uL d 1 (Methley on Lovell rootstock). A quadratic relationship best described the correlation between mean feeding rate or feeding rate at night and leafhopper abundance from 1-8 June on Prunus genotypes (F = 53.8; df = 2,12; R 2 = 0.90; P< 0.0001) (Table 5). Night time or mean daily feeding rates were better correlated with insect abundance than was day time feeding. The best

398 J. Entomol. Sd. Vol. 43, No. 4 (2008)

Table 1. Mean (±SE) leafhopper abundance on cultivars of Prunus persica at University of Georgia Attapulgus Research Center (trees replicated 2-4 times)

Cultivar Mean leafhopper abundance SunFre 13.4 ± 3.4 LaPecher 5.5±1.5 Redskin 5.4 ± 1.9 Lafestival 5.3±1.6 LaFeliciana 5.0±1.1 Juneprince 5.0 ± 2.0 Loring 4.9 ± 0.7 LaWhite 3.8±1.4 Hawthorne 3.5±1.2 Empress 3.0 ± 0.8 Suncrest 2.3 ± 1.0 Texstar 2.3 ± 1.7 Dixieland 2.2 ± 1.3 Flordagold 2.1 ±0.7 Cary Mac 2.0 ± 1.2 Flordadawn 1.8 ± 1.4 Mayfire 1.6 ±0.6 June Gold 1.6 ± 0.5 Cherry Gold 1.5 ± 0.4 Flordaglobe 1.5 ± 0.6 Suwannee 1.3 ± 0.6 Rio Grande 1.0 ± 0.7 Goldprince 1.0 ± 0.8 Sungem 0.9 ± 0.3 Flordaking 0.5 ± 0.3 Sunbrite 0.5 ± 0.3 OHenry 0.5 ± 0.3 Earligal 0.3 ± 0.2 Flordacrest 0.0 ± 0.0 TexRoyal 0.0 ± 0.0 V

ANDERSEN et al.: Abundance and Consumption Rate of H. vifripennis 399 1987 25

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Fig. 1. Seasonal leafhopper abundance during 1987 on Aldrighi (A), Lovell (L) and Nemaguard (N) rootstocks and June Gold (JG), Flordaking (FK), Methley (MT) and Santa Rosa (SR) scions. Stars on the top of graph indicate that scion was significantly different (P < 0.01) by a repeated measures analysis. Circles indicate significant rootstock effects at P < 0.05.

400 J. Entomol. Sd. Vol. 43, No. 4 (2008)

Table 2. Univariate repeated measures analysis of date, date x rootstock, date scion and date x rootstock x scion effects on within plant variability in leafhopper counts

1987 1988 Variable Fvalue Fvalue Date 39.50 0.0001 39.73 0.0001 Daterootstock 1.40 0.0735 1.47 0.0163 Datescion 15.81 0.0001 12.50 0.0001 Daterootstockscion 0.81 0.933 0.81 0.0418

Table 3. Multivariate analysis of between plant variability in leafhopper abundance as a function of rootstock, scion and rootstock x scion interaction

1987 1988 Variable Fvalue P< Fvalue P< 2.65 3.52Rootstock 0.0732 0.0321 Scion 38.01 0.0001 55.50 0.0001 Rootstockscion 0.84 0.573 1.76 0.0418

correlation of cumulative mean abundance for 1988 was with night time feeding rate (F= 419.3; df = 1,13; A 2 = 0.970; P< 0.0001) (Fig. 3). Xylem tension varied from 1.8-2.4 MPa during the day and 0.4-0.6 MPa at night (Table 6). A repeated measures analysis indicated that time of day (F= 2,249; df = 1,73; P < 0.0001) and rootsock x scion (F = 2.68; df = 4,73; P < 0.042) were significant. Xylem fluid osmolality varied between 14 and 22 mM, and the only significant differences occurred for the time x scion interaction (F= 2.78; df = 5,73; P < 0.027). Day time osmolality was higher than night time osmolality in 9 of the 12 comparisons; however, there was no significant difference in osmolality between day time or nighttime collection periods (F= 2.63, df = 1,73; P< 0.1113).

Discussion

Peak populations of H. vitripennis on Prunus genotypes occurred during early to mid-June. Leafhopper abundance during peak periods was more than an order of magnitude higher on plum than on peach scions. Leafhopper abundances on P. persica or P. salicina were typical for that species despite the attachment to a preferred or nonpreferred scion on the same plant. Ball (1979) found a consistent cycle of H. vitripennis trap catch in north Florida over a 3-yr period. Leafhoppers were first observed in low numbers in April, peaked during June-July and declined to low levels after midsummer. Leafhoppers caught during the spring probably represent overwin- ANDERSEN et al.: Abundance and Consumption Rate of H. vitripennis 401

1988

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Fig. 2. Seasonal leafhopper abundance during 1988 on Aldrighi (A), Lovell (L) and Nemaguard (N) rootstocks and June Gold (JG), Flordaking (FK), Methley (MT) and Santa Rosa (SR) scions. Stars on the top of graph indicate that scion was significantly different (P < 0.001) by a repeated measures analysis. Circles indicate significant rootstock effects at P < 0.05. 402 J. Entomol. Sd. Vol. 43, No. 4 (2008)

Table 4. Feeding rates of H. vitripennis from 2-8 June 1988 at various times of the day on scions budded on P. persica rootstocks

Feeding rate (ul h -1 ) Time

Rootstock Scion 0200 HR 1000 HR 1300 HR 1500 HR 1800 HR Aldrighi Aldrighi 5±3 8±3 0 6±2 5±2 Flordaking 4±1 15 ± 4 4±3 6±2 12 ± 6 June Gold 5±2 19 ± 3 6±4 10 ± 2 9±2 Methley 156 ± 25 106 ± 16 107.L 107 38.L 9 48.t 8 Santa Rosa 45 ± 12 86 ± 17 121 ± 100 38 ± 9 34 ± 8 Lovell Lovell 1±1 5±2 0 3±1 3±1 Flordaking 2±1 14 ± 5 14 ± 9 6±2 4±1 June Gold 5±2 20 ± 4 20 ± 13 15 ± 3 9±2 Methley 252 ± 29 157 ± 16 178 ± 72 70 ± 11 80 ± 15 Santa Rosa 99 ± 17 104 ± 13 81 ± 35 73 ± 13 60 ± 12 Nemaguard N emaguard 12 ± 3 40 ± 8 0 22 ± 5 18 ± 7 Flordaking 28 ± 14 33 ± 7 20 ± 15 21 ±6 20 ± 4 June Gold 3±1 50 ± 12 30 ± 27 14 ± 4 9±2 Methley 218 ± 29 254 ± 35 17 ± 8 70 ± 16 85 ± 15 Santa Rosa 139 ± 29 200 ± 31 31 ± 31 89 ± 19 63 ± 14

tering adults, and those present during the June-July period are a mixture of over- lapping first and second generations (Turner and Pollard 1959). Mizell and French (1987) segregated 19 host plants into 3 groups based on seasonal abundance: commonly accepted (plum), often accepted and occasionally accepted (peach). No more than 5% of H. vitripennis were observed on peach scions in the current study. Low leafhopper population densities on peach (Figs. 1 and 2), coupled with low titers of X. fastidiosa in peach (Gould et al. 1991, Wells et al. 1987) suggest that peach- to-peach transmission of X. fastidiosa would be minimal. However, leafhopper abundance was high on SunFresh, LaPecher, Redskin, LaFestival, LaFeliciana and June Prince (Table 1), and ostensibly under these conditions peach-to-peach transmission of X. fastidiosa can be significant. Leafhopper abundance varied by year and was counterintuitive with higher numbers in 1987 (smaller container-grown plants) compared with 1988 (larger field-grown plants). Feeding rates of leafhoppers were 2 orders of magnitude higher for Methley and Santa Rosa plum compared with June Gold and Flordaking peach and highest feeding rates usually occurred at night or in the morning (Table 2). Feeding rate on Prunus genotypes was highly correlated with leafhopper abundance from the specific time interval that feeding rate was monitored (2-8 June), for the entire month of peak leafhopper abundance (1-30 June) and for the entire year (Table 3). Night time feeding rate was best correlated to the 1 month peak abundance (F= 435.4; df = 1,13; ANDERSEN et al.: Abundance and Consumption Rate of H. vitripennis 403

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A2 = 0.970; P <0.0001) or yearly abundance during 1988 (F= 446.9; df = 1,13; A2 = 0.972; P < 0.0001). This suggests that leafhoppers may be responding to phagostimulants in xylem fluid at night and that host plant acceptance is mediated on that basis. The high correlations of consumption rates with abundance for H. vitripennis feeding on Prunus are noteworthy. Significant correlations have also been established for H. vitripennis feeding on V/f/s (Andersen et al. 2005), although correlation coefficients were not as strong in that study. In choice tests, the strength of the correlation of host selection with consumption rate varied with the leafhopper reproductive status and host plant species (Brodbeck et al. 2007). However, the strength of the correlations in the current study indicate that consumption rates may serve as a relatively easy bioassay for assessing host selection of H. vitripennis on Prunus for times of peak abundance or for the entire season. Effects of scion exceeded effects of rootstocks within our experimental framework, although both factors influenced leafhopper behavior during times of peak abundance. It is not possible to discern if primary effects may have been gustatory, visual, olfactory or tactile. Host plant acceptance by leafhoppers is considered predominantly mediated by gustatory sampling via precibarial sensilla (Backus 1985, Backus and McLean 1985). It has been proposed that the concentration of the amides in xylem fluid serve as a phagostimulant for H. vitripennis (Andersen et al. 1989, 1992, 2005, Brodbeck et al. 1993). Concentrations of asparagine, glutamine and arginine account for approx. 75% of the amino acids in xylem fluid of P. persica and P. sal/c/na

404 J. Entomol. Sci. Vol. 43, No. 4 (2008)

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7 ANDERSEN et al.: Abundance and Consumption Rate of H. vitripennis 405

Table 6. Xylem fluid osmolality of Prunus scions on three Prunus persica rootstocks during midday and at night

Osmolality (mM) Xylem fluid tension (MPa) Day Night Day Night Aidrighi Flordaking 19.0 ± 0 15.5+2.5 2.10 ± 0.10 0.50 ± 0.00 JuneGold 18.6 ± 1.5 20.8 ± 2.2 2.34 ± 0.11 0.48 ± 0.04 Methley 20.5.t 1.5 14.0 ± 0.0 2.20 ± 0.20 0.50 ± 0.05 Santa Rosa 19.0+1.2 15.0 ± 0.9 1.81 ± 0.13 0.44 ± 0.06 Lovell Flordaking 21.2 ± 2.5 16.2±1.0 2.38 ± 0.11 0.55 ± 0.22 JuneGold 16.3 ± 1.6 16.4 ± 1.5 2.14 ± 0.10 0.52 ± 0.01 Methley 16.8 ± 1.2 14.8 ± 0.5 2.29 ± 0.05 0.49 ± 0.04 Santa Rosa 22.3±1.31 14.2 ± 0.7 1.98 ± 0.08 0.46 ± 0.02 Nemaguard Flordaking 20.5.t 3.3 17.0 ± 1.9 2.38 ± 0.09 0.59 ± 0.01 June Gold 18.1 ± 1.1 16.3 ± 0.8 2.40 ± 0.09 0.54 ± 0.02 Methley 18.8 ± 0.6 17.0 ± 1.6 2.26 ± 0.12 0.50 ± 0.06 Santa Rosa 14.7+0.7 19.7 ± 3.4 1.93 ± 0.16 0.40 ± 0.03

(Andersen et al. 1989). Homa/odisca vitripennis consumption rates can be higher during midday on most host species when xylem fluid is most nutritive (Andersen et al. 1989, 1992, Brodbeck et al. 1993). However, feeding rates were higher on Ca- tharanthus roseus L. at night when host plant nutrient status was also maximal (Brod- beck et al. 1993). In the present study, total osmolality was not significantly different between day and night (range = 14-22 mM) nor was there a rootstock or scion effect; therefore, the effect of xylem chemistry on abundance and consumption cannot be ascertained in this study. Xylem fluid chemistry of Prunus genotypes is dependent on rootstock and scion (Brodbeck et al. 1990). Peach and plum scions were budded on 3 peach rootstocks and one may speculate that under conditions of low transpiration (night), the chemistry is influenced more by the scion than under conditions of high transpiration (day). During the day, xylem fluid moves rapidly from roots to shoots, and root xylem fluid chemistry of the scions may be impacted by the rootstock. By contrast, at night a relatively stationary xylem fluid may be more influenced by metabolic processes in the scion. Xylem fluid-feeding insects have a well-developed cibarial pump to extract fluid against high xylem tensions (Andersen et al. 1992, Backus 1985, Horsefield 1977, Raven 1983). Nocturnal feeding may also be facilitated by much lower xylem tensions at night (0.4-0.6 MPa) than during midday (1.8-2.4 MPa), thereby creating more fa- vorable energy balance and a higher potential energy gain of ingested nutrients (Andersen et al. 1992). Consistent effects of rootstocks on host preference by xylophagous leafhoppers could potentially suggest a role for the selection of rootstock as a management tool: however, the strong effect of scion implies that there is a limit to these rootstock 406 J. Entomol. Sci. Vol. 43, No. 4 (2008)

effects. It appears unlikely that one will be able to make a nonpreferred host from a preferred host simply by the selection of a rootstock within a given species. However, Gould et al. (1991) found that the concentrations of amino acids in xylem fluid of Flordaking on 1-1 plum rootstock were at least 2-fold higher than on the 2 peach rootstocks. Similarly, leafhopper abundances on Flordaking scions on 1-1 plum rootstocks were up to 3x higher than Nemaguard and up to 1 O higher than Aidrighi. In the current study, abundances of H. vitripennis on scions of P. salicina were similar to what would occur on P. sal/dna despite being grafted on a less preferred rootstock (Brodbeck et al. 1990, Mizell and French 1987). In conclusion, leafhopper density on Prunus spp. was highest during early to mid-June. Leafhopper abundance was often an order of magnitude higher on Methley and Santa Rosa plum compared with June Gold and Flordaking peach. Leafhopper abundance varied substantially with the 30 peach genotypes tested. Feeding rate data and leafhopper abundance were correlated, and night time feeding rate was best correlated to seasonal leafhopper abundance (R2 = 0.972). There does not appear to be sufficient variability within P. persica rootstocks tested to greatly override prefer- ences (leafhopper abundances or feeding rates) determined by the scion.

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