ARTICLE IN PRESS

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Crop Protection 26 (2007) 511–517

www.elsevier.com/locate/cropro

Effects of nitrogen and potassium in kikuyu grass on feeding by yellowsugarcane aphid

Susan C. Miyasakaa,�, James D. Hansenb,1, Ty G. McDonaldc,1, Glen K. Fukumotod

aDepartment of Tropical Plant and Soil Sciences, University of Hawaii, 875 Komohana Street, Hilo, HI 96720, USAbUSDA-ARS-Yakima Agricultural Research Laboratory, 5230 Konnowac Pass Road, Wapato, WA 98951, USA

cDepartment of Tropical Plant and Soil Sciences, University of Hawaii, Kona Extension Office, 79-7381 Mamalahoa Hwy., Kealakekua, HI 96750, USAdDepartment of Human Nutrition, Food, and Animal Sciences, University of Hawaii, Kona Extension Office, USA

Received 11 August 2005; received in revised form 3 April 2006; accepted 7 April 2006

Abstract

In Hawaii, infestations of yellow sugarcane aphid (YSA), Sipha flava (Forbes) (Homoptera: Aphididae) reduced growth of the forage

grass, kikuyu (Pennisetum clandestinum Hochst. ex Chiov). To determine the effects of nitrogen (N) and potassium (K) on tolerance of

kikuyu grass to YSA, cuttings of eight and five cultivars were grown in the greenhouse using nutrient solutions in two separate trials,

respectively. The first trial was conducted during the summer of 1991 and the second during the winter of 1993. In both trials, kikuyu was

grown at three N levels (0.05, 0.5 and 3.0mM) and four K levels (0.05, 0.5 1.0 and 3.0mM), with one cultivar comprising a block. Prior

to exposure to aphids, representative plants were harvested, and shoots were analyzed for foliar nutrients. Then, plants were confined

with aphids and rated visually for YSA injury. Dry matter yields and foliar N concentration increased significantly with increasing N

fertilization in both trials. In the first trial, there was a significant interaction between N and K levels, in which the greatest increases of

shoot and root dry matter with increasing N levels were found at the highest K level. In the second trial, K fertilization had no effect on

dry weight of shoots. In both trials, foliar K concentration increased significantly with increasing K levels. Damage due to YSA tended to

increase with increasing N levels, although it was unaffected by K fertilization in both trials. Thus, fertilization with increasing N resulted

in greater kikuyu dry matter production, but it also tended to increase the damage caused by YSA feeding.

r 2006 Elsevier Ltd. All rights reserved.

Keywords: Sipha flava; Pennisetum clandestinum; Insect resistance; Plant nutrition

1. Introduction

Kikuyu (Pennisetum clandestinum) grass is one ofHawaii’s most important forage grasses (Whitney, 1974).During the 1980s and 1990s, infestations of yellowsugarcane aphid (YSA), Sipha flava (Forbes) (Homoptera:Aphididae), resulted in major decreases in kikuyu forageproduction on the island of Hawaii (Fukumoto and Mau,1989). This problem continues sporadically through to thepresent (G.K. Fukumoto, unpublished data).

e front matter r 2006 Elsevier Ltd. All rights reserved.

opro.2006.04.023

ing author. Tel.: +1808 981 5180; fax: +1 808 981 5190.

ess: miyasaka@hawaii.edu (S.C. Miyasaka).

of the research, the second author was a Research

rtment of Entomology, University of Hawaii and the third

esearch Associate, Department of Agronomy and Soil

sity of Hawaii, USA.

Fertilization of kikuyu with N increased forage produc-tion in Africa, Australia, and Hawaii (Tamimi et al., 1968;Mears, 1970; Campbell et al., 1971; Whitney, 1974; Marais,2001; Hanna et al., 2004). Kikuyu production in responseto K fertilization was variable, depending on K status ofthe soil, removal of K in crop residues through mechanicalharvesting, and amount of N application (Tamimi et al.,1968; Mears, 1970; Hanna et al., 2004).Three mechanisms of host plant resistance are generally

recognized (Painter, 1951; Smith, 1989): antixenosis, ornon-preference; antibiosis, which reduces insect survivaland reproduction; and tolerance, where the plant compen-sates for insect feeding. In an earlier study (Miyasaka et al.,2007), we identified cultivars of kikuyu grass that differedin response to YSA feeding, based on either antixenosis ortolerance. Little is known about the effects of N and K

ARTICLE IN PRESSS.C. Miyasaka et al. / Crop Protection 26 (2007) 511–517512

fertilization on susceptibility of kikuyu grass to YSAdamage. If tolerance was a mechanism of resistance toYSA, then we expected that adding N and K could increaseplant vigor and further increase resistance. The objectivesof these two studies were to determine the effects of N andK fertilization on injury of kikuyu grass by YSA.

2. Materials and methods

2.1. Experimental design and analysis

Several kikuyu cultivars were grown using nutrientsolutions in a factorial combination of three N levels asNH4NO3 (0.05, 0.5, and 3.0mM) and four K levels asK2SO4 (0.05, 0.5, 1.0, and 3.0mM). These N and K levelsin solution were selected to supply deficient to sufficientamounts of N and K to kikuyu grass. The experimentfollowed a randomized complete block design with 12treatments (3N� 4K), and kikuyu cultivars treated asblocks. In Trial 1, eight cultivars (B-11, D-3, E-15, G-15,F-2, F-9, G-8, and G-15) were selected randomly; inTrial 2, three cultivars were selected as moderately resistant(B-13, C-1, and F-11) and two cultivars were selectedas more susceptible (D-17 and F-20) to YSA damage(Miyasaka et al., 2007).

Analysis of variance (ANOVA) was conducted usingPROC GLM in SASr software (SAS Institute Inc., Cary,NC). Main treatment effects of N, K, and block werecalculated along with the interaction of N�K for dryweights of plant parts, foliar concentrations of N and K,and YSA damage ratings. In Trial 2, a single degree offreedom contrasts were estimated for shoot dry weight andYSA damage ratings that compared moderately tolerantcultivars to more susceptible cultivars. A probability levelof 0.05 or less was considered to be statistically significant.

2.2. Trial one

Aphid-free, kikuyu cuttings were transplanted intoplastic cone-shaped containers (4 cm� 20 cm) (SC-10Super Cell, Stuewe and Sons, Inc., Corvallis, Oregon).2

The media consisted of 4:1 (v:v) mixture of absorbent tonon-absorbent rockwool (Grodan, AgroDynamics, Brook-lyn, New York).

The eight kikuyu cultivars were from hybrids developedby the late U. Urata in Hawaii during 1976–1983 (Urata,1981). They were rated as moderately resistant to YSA inthe kikuyu germplasm trials, and they did not differsignificantly from each other in YSA damage ratings(Miyasaka et al., 2007).

In addition to the treatment levels of N and K,basal macronutrient concentrations were, in mM: P as

2Mention of a trademark, proprietary product or vendor does not

constitute a guarantee or warranty of the product by the University of

Hawaii, and does not imply its approval to the exclusion of other products

or vendors that may be suitable also.

Na2HPO4, 0.1; calcium (Ca) as CaCl2, 1.0; Mg as MgSO4,0.4. Basal micronutrient concentrations were, in mM:manganese (Mn) as MnSO4, 2; boron (B) as H3BO3, 6; zinc(Zn) as ZnSO4, 1; copper (Cu) as CuSO4, 0.5; molybdenumas H2MoO4, 0.1; Fe as FeH-EDTA [N-(2-hydroxyethyl)-ethylenediaminetriacetate], 10. Uniformly sized kikuyucultivars were fertigated five times per week using a flow-through system in which excess solution was allowed toleach from containers.The trial began in June 1991 in a greenhouse at the

University of Hawaii’s Waiakea Research Station(191430N, 155140W) at 90m elevation near Hilo, Hawaii.Maximum and minimum temperatures ranged from 17to 41 1C.After seven weeks, four randomly selected blocks (cvs.

B-11, E-15, F-2, and G-8) were harvested, tops separatedfrom roots, and both plant parts were washed in deionizedwater, weighed, dried to constant weight at 70 1C in aforced-air oven, and reweighed. Shoots from two randomlyselected blocks were combined (B-11 plus E-15; F-2 plusG-8) and two samples from each N�K treatment weresent to the University of Hawaii’s Agricultural DiagnosticService Center for analysis of plant nutrients.Shoots were analyzed for total N by the micro-Kjeldahl

method (Isaac and Johnson, 1976; Nelson and Sommers,1972). They were ashed in a muffle furnace and analyzedfor K in 1M HCl using an inductively coupled plasma(ICP) emission spectrometer (Model 6500, Perkin-ElmerInc., Norwalk, Conn.) (Isaac and Jones, 1972). Concentra-tions were calculated on a dry weight basis. Total contentswere calculated by multiplying concentrations in shoots bydry weight of shoots.Then, five randomly selected blocks (cvs. B-11, D-3,

E-15, F-9, and G-15) were transferred to the University ofHawaii’s Kona Research Station (191320N 1551550W) at500m elevation near Kona, Hawaii, where they wereplaced in a YSA screening test. Due to an insufficientnumber of plants available within each cultivar except forcv. B-11, different ones were measured for dry weight andfor YSA damage. Maximum and minimum temperaturesranged from 16 to 30 1C.

2.3. Trial two

The same experimental procedures were conducted as inTrial 1, except for the following ones. Cuttings from fivekikuyu cultivars were placed in containers in January 1993and grown as described previously in a greenhouse at theKona Research Station. Minimum and maximum tem-peratures ranged from 16 to 25 1C.Cultivars B-13, C-1, and F-11 were selected as moder-

ately resistant to YSA, and their damage ratings did notdiffer significantly from the best or most resistant cultivar(Miyasaka et al., 2007). Cultivars D-17 and F-20 wereselected as more susceptible to YSA damage, although onlycv. F-20 was significantly different from the most resistantcultivar (Miyasaka et al., 2007).

ARTICLE IN PRESSS.C. Miyasaka et al. / Crop Protection 26 (2007) 511–517 513

Plants were fertigated two to three times per week withthe treatment nutrient solutions. After seven weeks, onlyshoots of plants from five blocks were harvested, washed,weighed, dried, and re-weighed. Shoots were combined intotwo samples (one composed of the moderately tolerantcultivars and the second one of the more susceptible ones),then analyzed as described for Trial 1. Plants from fiveblocks of each treatment were placed in a YSA screeningtrial.

2.4. Screening tests for YSA damage

Screening tests were carried out as described earlier(Miyasaka et al., 2007). A population of YSA was obtainedoriginally from common kikuyu grass in a commercialpasture located near Waikii, Hawaii (191510N, 1551390W)at 1510m. This YSA was reared on common kikuyu grassin other cages in the greenhouse at Kona.

Within each cage, cuttings from a kikuyu cultivar grownwith each of the 12 N�K fertilization treatments werearranged randomly. Each plant was placed over a cup filledwith the appropriate treatment solution. There was onekikuyu cultivar per cage and five cages per test.

Approximately 250 aphids (nymphs and adults) from thecolony were introduced into each cage. After 10–14 days,feeding damage was visually assessed by three or four

Table 1

Shoot and root dry weights and foliar N and K concentrations in Trial 1

N level (mM) K level (mM) Shoot dry weighta (g)

0.05 0.05 0.23(0.06)

0.05 0.5 0.68(0.26)

0.05 1.0 1.13(0.58)

0.05 3.0 0.26(0.02)

0.5 0.05 0.85(0.29)

0.5 0.5 1.27(0.50)

0.5 1.0 0.28(0.02)

0.5 3.0 0.60(0.19)

3.0 0.05 1.40(0.29)

3.0 0.5 0.90(0.29)

3.0 1.0 1.13(0.51)

3.0 3.0 2.38(1.12)

Tmt. means

0.05 0.57(0.17)

0.5 0.75(0.16)

3.0 1.45(0.32)

Tmt. means

0.05 0.83(0.19)

0.5 0.95(0.20)

1.0 0.85(0.26)

3.0 1.08(0.44)

P4F

N 0.0039

K 0.8200

N�K 0.0250

Block 0.0005

aMeans are followed by standard errors of mean in parentheses.

observers on a 10-point scale as follows: 0, no damage; 1,10% foliar chlorosis (yellowing); 2, 25% chlorosis; 3, 50%chlorosis; 4, 75% chlorosis and/or 10% necrosis of leavesand stems; 5, 100% chlorosis and/or 25% necrosis; 6, 50%necrosis; 7, 75% necrosis; 8, 90% necrosis; and 9,completely destroyed (Miyasaka et al., 2007). This ratingscale was based on previous work by Hansen (1986) andHansen et al. (1985). Ratings were averaged acrossobservers. Similar to the scale of Webster (1990), a meanrating from 0 to 3.0 is considered resistant, 3.01 to 6.0moderately resistant, and 6.01 to 9 susceptible.

3. Results

3.1. Plant dry weight in absence of aphid pressure

Shoot or root dry weight of kikuyu grass tended toincrease significantly with increasing N levels in the firsttrial; however, a significant interaction was found betweenN and K for shoot and root dry weights (Table 1). Thegreatest increase in shoot or root dry weight withincreasing N levels was found at the highest K level. Nochange in shoot dry weight or a decrease in root dry weightwas found with increasing N levels at 1.0mM K. In thesecond trial, shoot dry weight of kikuyu grass increasedsignificantly with increasing N levels (Table 2).

Root dry weight (g) Foliar N (g kg�1) Foliar K (g kg�1)

0.01(0.04) 6.6(0.2) 18.2(4.7)

0.41(0.19) 5.8(0.2) 22.6(1.2)

0.68(0.34) 8.2(1.4) 21.6(2.2)

0.09(0.04) 8.1(1.0) 25.4(0.5)

0.26(0.12) 8.2(1.4) 19.0(0.2)

0.44(0.20) 10.7(0.4) 23.0(2.7)

0.12(0.03) 10.3(0.6) 33.0(0.9)

0.16(0.07) 11.2(1.2) 33.2(5.6)

0.81(0.29) 21.6(1.0) 8.6(0.9)

0.43(0.23) 21.2(0.6) 20.6(1.6)

0.21(0.12) 21.3(2.5) 23.8(6.8)

1.23(0.68) 18.0(0.1) 38.1(2.4)

0.30(0.11) 7.1(0.5) 22.0(1.4)

0.24(0.06) 10.1(0.6) 27.1(2.6)

0.67(0.20) 20.5(0.7) 22.8(4.2)

0.36(0.14) 12.1(3.0) 15.3(2.4)

0.43(0.11) 12.5(2.9) 22.1(1.0)

0.33(0.13) 13.2(2.7) 26.1(2.9)

0.49(0.26) 12.4(1.9) 32.2(2.8)

0.018 0.0001 0.1100

0.810 0.6100 0.0005

0.014 0.1100 0.0900

0.001 0.2100 0.7200

ARTICLE IN PRESS

Table 2

Shoot dry weight and foliar N and K concentrations in Trial 2

N level (mM) K level (mM) Shoot dry weighta (g) Foliar N (g kg�1) Foliar K (g kg�1)

0.05 0.05 0.06(0.01) 11.6(2.8) 20.2 (0.0)

0.05 0.50 0.05(0.01) 8.6(0.6) 17.8 (0.8)

0.05 1.00 0.06(0.02) 7.5(0.9) 19.8 (4.6)

0.05 3.00 0.06(0.02) 9.4(0.8) 23.0 (1.2)

0.50 0.05 0.08(0.02) 9.6(0.1) 17.8 (0.8)

0.50 0.50 0.07(0.01) 11.8(0.8) 23.4 (0.2)

0.50 1.00 0.09(0.01) 10.6(0.8) 22.2 (2.0)

0.50 3.00 0.09(0.02) 10.7(0.1) 27.4 (2.4)

3.00 0.05 0.17(0.03) 23.2(0.6) 10.0 (0.1)

3.00 0.50 0.16(0.04) 22.4(0.6) 23.8 (1.2)

3.00 1.00 0.13(0.03) 20.0(1.4) 29.2 (2.0)

3.00 3.00 0.18(0.04) 19.6(1.9) 33.2 (1.4)

Tmt. means

0.05 0.06(0.01) 9.3(0.8) 20.2 (1.3)

0.5 0.08(0.01) 10.7(0.4) 22.7 (1.4)

3.0 0.16(0.02) 21.3(0.7) 24.0 (3.3)

Tmt. means

0.05 0.10(0.02) 14.8(2.8) 15.2(2.2)

0.5 0.09(0.02) 14.3(2.7) 21.6(1.3)

1.0 0.10(0.01) 12.7(2.4) 23.7(2.3)

3.0 0.11(0.02) 13.2(2.1) 27.8(2.0)

P4F

N 0.0001 0.0001 0.0390

K 0.4800 0.2000 0.0001

N�K 0.4000 0.3000 0.0050

Block 0.0001 0.5300 0.1100

Tol. vs. Sens. 0.0460 — —

aMeans are followed by standard errors of mean in parentheses.

S.C. Miyasaka et al. / Crop Protection 26 (2007) 511–517514

Potassium levels had no significant effects on shoot dryweights in both trials. Similarly, K levels had no significanteffects on root dry weight in Trial 1.

Significant differences in shoot dry weights were foundbetween blocks in both trials (Tables 1, 2). Similarly,significant differences between blocks were found for rootdry weight in Trial 1 (Table 1). In Trial 2, moderatelytolerant cultivars had a significantly lower mean shoot dryweight per cutting (0.0970.01 g) compared to that ofsusceptible cultivars (0.1170.02 g).3

3.2. Nutrient concentrations in shoots

Total N concentration in shoots significantly increasedwith increasing N levels in both trials (Tables 1, 2). Similarresults were found for total N contents of shoots in bothtrials (P ¼ 0.0001, 0.0001; data not shown).

Potassium concentration in shoots increased significantlywith increasing K levels in both trials (Tables 1, 2). Similarresults were found for total K contents of shoots in bothtrials (P ¼ 0.006, 0.0001; data not shown). In the firsttrial, foliar K concentration was not affected by N levels(Table 1); however, in the second trial, foliar K concentra-

3Means are followed by standard errors of mean.

tion (Table 2) and total K contents (P ¼ 0.0001; data notshown) increased significantly with increasing N levels. Nosignificant differences in foliar N or K concentrations werefound between blocks.

3.3. Injury by YSA

Leaves infested with aphids developed chlorosis ornecrosis within seven days after introduction. IncreasingYSA damage ratings were associated with increasing Nlevels in both trials (Fig. 1, N effect: P ¼ 0.005; Fig. 2, Neffect: P ¼ 0.0009).In the first trial, there was no significant effect of K

fertilization on kikuyu injury by YSA; however, asignificant interaction was found (N�K interaction:P ¼ 0.004). The greatest injury by YSA was found whena large imbalance in N and K levels existed, e.g. at0.05mMN plus 1.0mMK and at 3mMN plus 0.05mMK(Fig. 1). In the second trial, no interaction was foundbetween N and K treatments, and the highest level of YSAdamage occurred when both N and K levels were high (e.g.1.0mMK plus 3mMN) (Fig. 2).Significant differences between blocks were found for

YSA injury ratings in both trials (Fig. 1, Block effect:P ¼ 0.018; Fig. 2, Block effect: P ¼ 0.0001). In Trial 2,

ARTICLE IN PRESS

Fig. 1. Effects of increasing N and K levels on the average rating of

kikuyu damage by YSA in the first N�K study. Bars represent standard

errors of mean. Nitrogen and block main treatment effects, and N�K

interaction effects were significant (P ¼ 0.005; 0.018; 0.0004, respectively).

Fig. 2. Effects of increasing N and K levels on the average rating of

kikuyu damage by YSA in second N�K trial. Bars represent standard

errors of mean. Nitrogen and block main treatment effects were significant

(P ¼ 0.0009; 0.0001, respectively).

S.C. Miyasaka et al. / Crop Protection 26 (2007) 511–517 515

single degree of freedom contrasts showed that moderatelyresistant cultivars had significantly lower mean YSAdamage ratings (4.2570.15) compared to more susceptiblecultivars (4.7970.19).

4. Discussion

The large, positive growth response of kikuyu to Napplication in our greenhouse trials is similar to fieldstudies on kikuyu (Mears, 1970; Whitney, 1974; Marais,2001; Hanna et al., 2004). Application of N at 336 kg ha�1

accompanied by P and K more than doubled kikuyu drymatter yield compared to that of control plots withoutfertilizer at two sites on the island of Hawaii (Campbellet al., 1971; Tamimi et al., 1968).

The large increases in shoot and root dry matter withincreasing N application at the highest K level observed inthe first trial are similar to those in an earlier field study(Tamimi et al., 1968). In that study, a positive yieldresponse of kikuyu forage was found as K rates increasedwhen applied together with N and P (Tamimi et al., 1968).However, in our second trial, no significant effect of Kfertilization on kikuyu shoot production was found,

perhaps due to growth limitations imposed by the coolertemperatures and lower light intensities occurring duringthe winter months. Regrowth of kikuyu was slower duringwinter months than summer months, and slower at a highelevation site (945m) than a lower elevation site (660m) onthe island of Maui, Hawaii (Whitney, 1974). An alternateexplanation is that cuttings in the second trial receivedfertigation only two to three times per week, perhapsresulting in declining levels of K between applications.Nitrogen concentrations of kikuyu shoots grown at the

lowest N level were below the foliar N reported for kikuyupastures of 13.6–41.1 g kg�1 (Marais, 2001). Foliar Nconcentrations of kikuyu grown at the highest N levelwere within this range. Several kikuyu plants grown at thelowest K level had foliar K concentrations that fell withinthe critical K concentration range of 6.4–10.0 g kg�1 foundto be associated with K deficiency symptoms in kikuyu(Mears, 1970). Thus, solution N and K levels in our studyresulted in foliar N and K concentrations that ranged fromdeficient to sufficient.Feeding damage to kikuyu by YSA significantly

increased with increasing N application in both studies.Such increased damage could be due to one or both of the

ARTICLE IN PRESSS.C. Miyasaka et al. / Crop Protection 26 (2007) 511–517516

following factors: (a) feeding preference by the existingYSA population to kikuyu grown at high N; and(b) increased reproduction by YSA on kikuyu with highfoliar N.

In both trials, increasing N application was found tosignificantly increase total N concentration in kikuyushoots, probably increasing levels of protein and aminoacids. Increasing N application to bermuda grass, (Cyno-

don dactylon (L.) Pers., significantly increased concentra-tions of 12 amino acids (Coto et al., 1990); and amino acidsare found in large concentrations in the phloem, thefeeding site of aphids.

Nitrogen is a limiting component in the diet ofhomopterans (Mattson, 1980); these insects may consumedaily up to 100 times their own body weight in orderto obtain sufficient N. Thus, adding N may benefitinsects feeding on fertilized plants. Pfeiffer and Burts(1983) found that the pear psylla, Psylla pyricola

Foerster (Homoptera: Psyllidae) caused increased pearfruit damage with increased N fertilization. Densityand development of the spirea aphid, Aphis spiraecola

Patch (Homoptera: Aphididae), increased at a fasterrate on young apple trees receiving high rates of Napplication (Kaakeh et al., 1992). Similarly, the populationgrowth rate of the green peach aphid, Myzus persicae

(Sulzer) (Homoptera: Aphididae), on potato increased withincreased N fertilization (Jansson and Smilowitz, 1986).Blua and Toscano (1994) found that the whitefly, Bemisia

argentifolii Bellows and Perring (Homoptera: Aleyrodi-dae), developed faster with increased N fertilization ofcotton plants.

In addition, feeding by aphids has been found previouslyto increase the concentrations of amino acids, particularlyessential ones, in the phloem. Sandstrom et al. (2000)measured the level and composition of amino acidsexuding from a single wheat (Triticum aestivum L.) leafeither uninfested or infested with greenbug, (Schizaphis

graminum Rondani) (Homoptera: Aphididae), and foundsignificantly greater concentrations of total amino acids,particularly essential ones, in the infested leaf compared tothe uninfested one. It is thought that aphids inducesenescence-like changes in leaves, and benefit from theincreased translocation of products from the breakdown ofproteins (Dorschner et al., 1987). Previous infestation ofmature leaves of Johnson grass with YSA increased therelative growth rate of the aphid, providing support for thishypothesis (Gonzales and Gianoli, 2003). Certainly, ahigher N application to kikuyu resulted in higher total Nconcentrations in the shoots, and probably higher proteinlevels that could be metabolized during induced senescenceby YSA feeding.

In the first trial, treatments with the greatest imbalancein N and K fertilization (e.g. lowest K plus highest N level)resulted in the greatest kikuyu damage by YSA. Slightlydifferent results were found in the second trial, where thegreatest kikuyu injury by YSA occurred at the secondhighest K level and the highest N level. The reasons for

these differences between the two trials are not known, butthey could be related to concentrations of soluble Ncompounds in kikuyu. Interestingly, K-deficient plants arereported to accumulate soluble N compounds (e.g. nitrates,amino acids, and amides), because K is required for proteinsynthesis (Marschner, 2002, p. 300).Comparisons of YSA damage ratings between the two

trials show similarities in the curvature of the relationshipbetween ratings and N and K levels. However, themagnitude of the ratings was much lower in the secondtrial compared to the first one, with all mean ratings inTrial 2 falling within the moderately resistant category.Perhaps, the cooler, winter temperatures during Trial 2slowed the growth and reproduction of YSA, and hencetheir damage to kikuyu.The more susceptible cultivars in Trial 2 had significantly

greater mean YSA injury ratings than moderately resistantcultivars, confirming results obtained in the earlier study ofkikuyu germplasm response to YSA (Miyasaka et al.,2007). The more susceptible kikuyu cultivars had signifi-cantly greater shoot biomass than the moderately resistantcultivars, perhaps indicating that they provided a bettersource of carbohydrates and nutrients to YSA. Futurestudies need to separate out antixenosis from antibiosismechanisms within the kikuyu germplasm by measuringchanges in the YSA population. For example, a kikuyucultivar that grew more slowly, and provided lowercarbohydrates and nutrients for YSA growth and repro-duction, might not be an ideal cultivar for grazing ofanimals due to lower production of forage.This study demonstrates that the YSA screening

method developed earlier (Miyasaka et al., 2007) couldbe adapted easily to investigate the effects of nutrientregimes on aphid injury to grasses. Future studies need tofocus on the mechanisms involved in greater damage byYSA to kikuyu grass due to increasing N fertilization,perhaps analyzing kikuyu shoots for soluble N compounds(e.g. amino acids), as well as measuring changes in YSApopulation.Based on these research results, it is recommended

that N fertilization of pastures be avoided during themonths when YSA populations are high. At a ranchin Kona, Hawaii, it was observed that urea applicationalone appeared to aggravate YSA damage on kikuyu(G.K. Fukumoto, unpublished data).

Acknowledgements

This research was funded in part by the Governor’sAgricultural Coordinating Committee Contract #91-21.The authors would like to acknowledge the assistance of:C.M. Webster, former Research Associate, for her help indata entry; D.T. Matsuyama, Research Associate, for hishelp in data entry; and L.S. Kodani, AgriculturalTechnician, for his assistance in propagation of kikuyuplants for the first study.

ARTICLE IN PRESSS.C. Miyasaka et al. / Crop Protection 26 (2007) 511–517 517

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