Variation in Susceptibility of Rice Lines to Infestation...

Variation in Susceptibility of Rice Lines to Infestation bythe Rice Water Weevil (Coleoptera: Curculionidae)1

Michael J. Stout and M. Rita Riggio

Department of Entomology, Louisiana Agricultural Experiment Station, Louisiana State UniversityAgricultural Center, 402 Life Sciences Building, LSU, Baton Rouge, Louisiana 70803

J. Agric. Urban Entomol. 19(4): 205–216 (October 2002)ABSTRACT The rice water weevil, Lissorhoptrus oryzophilus Kuschel, isan important insect pest of rice in the United States and Asia. Current man-agement programs for this pest rely heavily on insecticides. Host plant resis-tance is an underused strategy for management of this pest. In the experi-ments reported here, the susceptibilities of 19 rice lines to infestation by therice water weevil were evaluated in greenhouse experiments. The linesscreened in this study had exhibited either resistance or susceptibility (relativeto commercial varieties) in previous field screening experiments. Thus, thepurpose of these experiments was to confirm the presence of resistance/susceptibility in these lines and to assess variation in susceptibility relative totwo commercial varieties. Both choice and no-choice experiments were con-ducted. The experiments were designed primarily to detect antixenosis. Ricelines screened in these experiments exhibited significant variation in theirsusceptibilities to infestation by L. oryzophilus. Some of the lines consistentlysupported fewer weevil eggs and larvae than commercial varieties, whereasothers consistently supported more eggs and larvae than commercial varieties.The lines exhibiting resistance (PI 319512, PI 321310, PI 321264, and PI321278) had also exhibited resistance in at least four prior field experiments,and may be useful as germplasm in a breeding program. The identification oflines possessing both resistance and susceptibility may facilitate the charac-terization of biochemical or morphological mechanisms of resistance to the ricewater weevil in these lines.

KEY WORDS Rice, rice water weevil, Lissorhoptrus oryzophilus, antixeno-sis, host-plant resistance

The rice water weevil, Lissorhoptrus oryzophilus Kuschel (Coleoptera: Curcu-lionidae), is the most destructive insect pest of rice in the United States and hasrecently become a global threat to rice production through its introduction intomajor rice-producing regions of Asia (Heinrichs & Quisenberry 1999). Althoughboth larvae and adults of this insect feed on rice plants, it is the larval stage thatgenerally causes economic losses. Adult rice water weevils emerge from overwin-tering sites in early spring and migrate to rice fields, where they feed on theleaves of rice plants (Muda et al. 1981, Morgan et al. 1984). Oviposition does notcommence in full until rice fields are flooded (Everett & Trahan 1967). Eggs are

1Accepted for publication 3 March 2003.

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laid in leaf sheaths at or below the water line (Stout et al. 2002). Neonatesgenerally feed on or in leaves for a short time before moving to the roots to feed.Larvae pass through four instars on roots (Smith 1983). Root pruning by larvaeresults in reductions in vegetative growth, tillering, panicle density, grain weight,and, ultimately, yield (Stout, unpublished data). Yield losses as a result of ricewater weevil feeding on untreated plots or fields typically exceed 10% and can bemuch higher under heavy pressures (Smith 1983, Stout et al. 2001). The insect ismultivoltine in southern areas of its range (Smith 1983). Applications of insecti-cides remain the primary means of controlling this insect. However, several fac-tors have revived interest in alternative strategies for management of this insect,including difficulty in properly timing insecticide applications, inability to ad-equately manage heavy infestations with insecticides alone, and toxicity of insec-ticides to non-target organisms associated with rice paddies.

Host plant resistance has the potential to form an important component of themanagement program for the rice water weevil, as it does in the managementprograms for several other important pests of rice (Heinrichs 1994). Considerableeffort has been extended to identify rice lines resistant to the rice water weevil(Heinrichs & Quisenberry 1999). In Louisiana, a collaborative screening programconducted by USDA and Louisiana State University personnel over the past 35years has identified a number of cultivars and plant introductions that possesslow levels of resistance to adult feeding and larval infestation (Oliver et al. 1970,Smith & Robinson 1982, Cave et al. 1984, Pantoja et al. 1986, N’Guessan et al.1994, Rice et al. 1994). In addition, Stout et al. (2001) recently documented sig-nificant variation in the susceptibilities of modern commercial varieties to infes-tation by weevil larvae. Despite these studies, very little effort has been made toincorporate weevil resistance into commercial varieties or to integrate plant re-sistance into management programs for this insect.

Field evaluations of weevil resistance, in which population densities of larvaeare estimated using a soil-root core sampler, are often plagued by inconsistentresults from experiment to experiment. These inconsistencies probably occur be-cause levels of resistance in lines are low and infestation pressures are sporadicand often very high. Thus, there is a need to confirm the results of field evalua-tions under more controlled conditions. Here, we report the results of greenhouseexperiments in which the susceptibilities of 19 lines to infestation by the ricewater weevil were evaluated and compared with the susceptibility of two moderncommercial varieties. Some of the 19 lines evaluated in these studies had shownresistance to weevil infestation in several previous field evaluations, including afield experiment conducted in 2001 using single-row plots (Stout, unpublisheddata). Other lines had exhibited greater susceptibility to weevil infestation in the2001 single-row experiment. The purpose of these studies was to confirm thepresence of resistance and susceptibility in these lines, and to assess variation insusceptibility relative to the commercial varieties.

Materials and Methods

Experimental narrative. A total of 19 rice accessions (lines) were chosen forstudy based on their performance in previous experiments (Table 1). Lines wereof diverse geographic origin and included landraces, breeding lines, and, culti-vated lines. Two modern U.S. commercial varieties, ‘Cocodrie’ and ‘Jefferson’,

206 J. Agric. Urban Entomol. Vol. 19, No. 4 (2002)

served as standards in all experiments. ‘Jefferson’ had shown greater resistanceto L. oryzophilus than ‘Cocodrie’ in previous experiments (Stout et al. 2001). Therelative resistance of lines was evaluated in a greenhouse by exposing lines toovipositing weevils under both choice and no-choice conditions, and subsequentlycomparing the densities of eggs and (in most experiments) larvae associated withleaf sheaths and roots, respectively, of the different lines.

For initial, preliminary, screening experiments, the 19 accessions were arbi-trarily divided into three groups for infestation under choice conditions. Only eggdensities were determined in these preliminary screening experiments. Six lineswere identified as putative resistant lines for further study based on the resultsof these preliminary screens. Both choice and no-choice experiments were con-ducted on these resistant lines; ‘Cocodrie’ and ‘Jefferson’, along with a susceptibleline (PI 160555) as an additional check, were also included in these choice andno-choice studies. The final experiment was a choice experiment using four lines(and ‘Cocodrie’ and ‘Jefferson’) identified as susceptible from the preliminaryscreens.

Plant and insect culture. Rice seeds were generously provided by the Na-tional Small Grains Collection (National Small Grains Research Facility, Aber-deen, Idaho) in most cases. When seeds were not available from this source, seedharvested from the field experiment in 2001 were used. Experiments were con-ducted during the summer months of 2002 in a greenhouse maintained between25 and 35°C with ambient lighting. Rice seeds were sown either in 11.4-cm squarepots (700 ml capacity, used in the choice experiments) or in 10 cm diameter roundpots (500-ml capacity, used in the no-choice experiment). Plants were grown in a8:4:4:1 mixture of topsoil, sand, peatmoss, and vermiculite and were fertilized atplanting with approximately 0.7 g of 23:12:12 N:P:K mixed in with the soil. Plantswere maintained and experiments were conducted in large wooden basins linedwith heavy black plastic liner to contain flood waters when necessary. Plantswere thinned to a density of one or three plants per pot approximately 7 days afterplanting. Although there were slight variations in the growth rates of the linesused in these experiments, plants used for experiments always possessed no lessthan three and no more than four fully-expanded leaves when experiments werestarted.

Adult weevils were collected from rice fields in Crowley, Acadia Parish, Loui-siana, 1 or 2 days before their use in experiments. Weevils were maintained untiluse in glass jars containing moistened paper towels and excised rice leaves as afood source. The sex ratio of field-collected weevils was heavily biased towardfemales. To insure the presence of males in choice experiments, at least threemating pairs of weevils were captured in copula from jars and placed in eachinfestation cage (see below); the remaining weevils in each cage for choice experi-ments were unsexed. For no-choice experiments, equal numbers of males andfemales (captured as mating pairs or sexed under a dissecting microscope [Ever-ett & Newsom 1964]) were placed in each cage.

Choice experiments: general procedures. For choice experiments, ricewater weevil adults were allowed free access to different rice lines by placingplants and weevils together in infestation cages. Infestation cages consisted ofcylindrical wire frames (46 cm in diameter and 61 cm in height) covered withmesh screening. One pot of each rice line to be evaluated was placed in each

207STOUT & RIGGIO: Susceptibility of Rice to the Rice Water Weevil

infestation cage. In preliminary screening experiments, pots contained only oneplant; in later experiments, in which egg, first-instar, and late-instar densitieswere examined, pots contained three plants. In the first and third preliminaryscreens, weevils were placed in infestation cages at a density of two weevils perplant; in the second preliminary screen, the weevil density was 2.5 weevilsper plant. For subsequent choice studies with resistant and susceptible lines,weevils were placed in cages at a density of one weevil per plant. After placingplants and weevils in cages, cages were covered and the basins in which cageswere set were flooded to a depth of ≈ 22 cm. Weevils were allowed to feed, mate,and oviposit on plants in cages for 5 days. Plants were then removed from cagesand any weevils found on plants were destroyed. If three plants were present ineach pot, one plant was immediately removed for egg counts and a second plantwas immediately removed for counts of first instars. The third plant in each potwas retained in basins under flooded conditions for 20–22 days, then evaluatedfor density of late instars. If only one plant was present in each pot (preliminaryscreens), the plant was removed from the soil at the end of the infestation periodand examined for eggs.

No-choice experiment: general procedures. For the no-choice experi-ment, egg and larval densities were determined from plants on which weeviladults had been confined without access to other rice lines. Pots containing threeplants of a given line were placed into basins and basins were flooded to a depthof ≈ 22 cm. Immediately after flooding, weevils were confined to plants using clearplastic cylinders (8.5 cm diameter × 23 cm height) with one end forced into the soiland the top end covered with a mesh-screen lid. The cylinder cages also had twomesh-lined holes to allow circulation of water through the cage. Three male andthree female weevils were placed in each cage to achieve a density of two weevilsper plant. Weevils were allowed to feed, mate, and oviposit for 5 d, at which timecages and weevils were removed from plants. One plant from each pot was re-moved for determination of egg density and another was removed for determina-tion of first instar density. The final plant in each pot was retained in the green-house for another 20 days, then used for a root wash to determine densities of lateinstars.

Estimation of egg and larval densities. Methods used to assess densitiesof eggs and larvae were adapted from Heinrichs et al. (1985). For determinationof egg densities, plants were removed from pots at the termination of the infes-tation period, labeled, washed free of soil, and placed in 95% alcohol untilbleached. Numbers of eggs in plants were determined by examining leaf sheathsof plants under a dissecting microscope (Stout et al. 2002).

To determine the densities of first instars associated with different rice lines,plants were removed from pots at the termination of the infestation period,washed free of soil, and suspended individually in water in clean test tubes. Testtubes were labeled, arranged in a rack, and placed in a growth chamber (30°C16:8 L:D). Weevils infesting plants treated in this way eclose from eggs, emergefrom leaf sheaths, and settle on the bottom of the test tubes (Heinrichs et al.1985). Counts of first instars were conducted by first shaking roots free of larvae,then pouring water from tubes into a Petri dish and counting larvae. Plants wereplaced back into tubes after counting and water was replenished. First instarswere counted daily for approximately 10 days, until no new larvae were found in


tubes for at least 2 days. The cumulative number of first instars found associatedwith a plant was, like egg density, a measure of antixenosis, provided that highlevels of mortality did not occur during the egg stage.

A third measure of plant resistance was obtained in most experiments bycounting pupae and late instars associated with the root systems of different ricelines. After removal of pots from infestation cages and removal of plants from potsfor egg and first-instar counts, plants were retained in the greenhouse in theabsence of weevils. Counts of immature weevils were conducted approximately 25days after initially infesting plants with weevils; at this time, immature weevilsconsisted mostly of larvae, with a few pupae. The contents of each pot (plants andsoil) were washed through a 40-mesh copper sieve. Material not passing throughthe sieve was placed in a basin containing saturated salt solution. Larvae andpupae floating to the surface of the salt water were counted. The number of larvaeand pupae associated with roots was a measure of antixenosis and, if survival tolate instars differed on different accessions, of antibiosis.

Analysis of data. Counts of eggs, first instars, and later instars were takenusing separate plants and provided independent measures of the susceptibility ofrice lines to infestation by the rice water weevil; thus, egg densities, first instardensities, and late instar densities were analyzed separately. There were at least10 infestation cages (replications) in all choice experiments, including prelimi-nary screens. There were 10 pots of each variety in the no-choice experiment. Allanalyses were conducted using SAS (SAS Institute 1990). Data from the choiceexperiments were analyzed using a mixed-model analysis (PROC MIXED) withinfestation cage as a random effect and rice line as a fixed effect. Data from theno-choice experiment were analyzed by one-way analysis of variance (completelyrandomized design) using PROC GLM in SAS. Means were compared using LSD.

Results

Preliminary screening of rice varieties. A total of 19 rice lines of diversegeographic origin and two modern U.S. varieties were chosen for study based ontheir performance in previous field and greenhouse experiments (Table 1). The 19lines were arbitrarily divided into three groups and preliminary choice screeningexperiments conducted. In these experiments, numbers of L. oryzophilus eggs perplant differed significantly among lines in all three groups (group 1: F � 3.62,df � 8, 63, P � 0.002; group 2: F � 3.16, df � 9, 81, P � 0.003; group 3: F � 7.88,df � 5, 44, P < 0.0001). In the first group (Fig. 1A), egg densities were highest inPI 420968 and PI 413958 and in the older cultivar ‘Saturn’. Egg densities werelowest on ‘Jefferson’, PI 319512, and PI 321278. In the second group (Fig. 1B), eggdensities were highest on PI 346840 and PI 415731 and lowest on PI 321264. Inthe third group (Fig. 1C), egg densities were highest on PI 160555 and lowest onWC 1541, ‘Jefferson’, PI 321310, and PI 321314. Based on the results of thesepreliminary screens, WC 1541 and PI 319512, PI 321264, PI 321278, PI 321310,and PI 321314 were chosen as potentially resistant lines for further study, and PI160555, PI 346840, PI 413958, PI 415731, and PI 420968 were chosen as poten-tially susceptible lines for additional study.

Choice and no-choice studies with resistant and susceptible lines. Inthe choice experiment with putatively resistant lines, plants of four of the lines—PI 319512, PI 321264, PI 321278, and PI 321310—had significantly lower egg


densities than both ‘Cocodrie’ and ‘Jefferson’ (analysis of variance [ANOVA] foregg densities: F � 16.74, df � 8,104, P < 0.0001; Table 2). PI 160555, a suscep-tible check, had egg densities approximately twice as high as both commercialvarieties. Numbers of first instars recovered from plants were lower than num-bers of eggs found on plants, but the patterns generally mirrored those observed

Table 1. Rice lines evaluated for resistance to the rice water weevil ingreenhouse experiments, Baton Rouge, Louisiana, 2002.

AccessionImprovement

statusaGeographic

originResistant (R) orsusceptible (S)b Referencesc

‘Cocodrie’ C USA — —‘Jefferson’ C USA — 1‘Saturn’ C USA S 1,3,4,5,6,7WC 1541

(PI 162076)B Japan R 1,2

PI 160555 L China ? 1,3,4,5PI 319512 B Mexico R 1,3,4,5PI 321264 B Philippines R 1,3,4,5,7,9PI 321278 B Philippines R 1,3,4,5,9PI 321310 B Philippines R 1,3,4,5PI 321314 B Philippines R 1,3,4,5,9PI 346840 B Argentina R to Spodoptera.

frugiperda11

PI 413955 B Indonesia S 1,8PI 413958 B Indonesia S 1,8PI 415633 B Philippines S 1,8PI 415660 C Turkey R? 1,8PI 415721 C Taiwan S 1,10PI 415727 C Taiwan S 1,10PI 415731 C Taiwan S 1,10PI 417833 B Sri Lanka S 1,10PI 417880 C China S 1,10PI 420968 C Turkey S 1,10

aB, breeding material; L, landrace; C, cultivated material.bPutative level of resistance relative to modern commercial varieties, based on performance in theprevious experiments.

cReferences: 1) Stout, unpublished data, 2001 field experiment using single-row plots; 2) Oliver et al.1970; 3) Robinson et al. 1979; 4) Robinson et al. 1980; 5) Robinson et al. 1981; 6) Smith & Robinson 1982;7) Cave et al. 1984; 8) Rice et al. 1994; 9) Stout & Riggio 1998; 10) W. C. Rice & Stout, unpublished data,2000 field experiment; 11) Pantoja et al. 1986.

>

Fig. 1. Densities of L. oryzophilus eggs (eggs plant−1 ± S.E) on rice lines in threepreliminary choice experiments (A, B, and C). Bars accompanied by thesame letter in each figure represent means that do not differ significantly(LSD; P > 0.05).


for egg densities (F � 11.95, df � 8,104, P < 0.0001). The highest number of firstinstars was found associated with roots of PI 160555, whereas numbers of firstinstars from PI 319512, PI 321264, PI 321278, and PI 321310 were lower thanfrom ‘Cocodrie’ and ‘Jefferson’. Fewer significant differences among lines in num-bers of larvae were found in the root wash (F � 12.11, df � 8,96, P < 0.0001).Counts of larvae from both WC 1541 and PI 160555 were significantly higher thanfrom ‘Cocodrie’ and ‘Jefferson’. None of the lines had significantly lower larvaldensities than the commercial varieties in the root wash, but densities on PI321310 and PI 321264 were numerically lower.

Differences among varieties in egg and larval numbers were less pronouncedin the no-choice experiment than in the choice experiment (Table 3). Overallpatterns of resistance were similar, although some qualitative differences wereobserved. PI 3195912, PI 321264, and PI 321278 had numerically lower egg den-sities than the commercial varieties, but the differences were not significant(ANOVA for egg densities: F � 1.73, df � 8,72, P � 0.11). PI 321310, whichscored “resistant” with respect to egg densities in the choice experiment, had thehighest egg densities in the no-choice experiment. Numbers of first instars asso-ciated with PI 321264, PI 321278, and PI 319512 were significantly lower thannumbers of first instars associated with ‘Jefferson’; however, the only line of thesethree for which numbers of first instars differed significantly from ‘Cocodrie’ wasPI 321264 (ANOVA for first instars: F � 2.81, df � 8,72, P < 0.01). The samethree lines also showed low numbers of late instars in the root wash; in addition,PI 321310, which had high egg densities, had the lowest larval densities of alllines evaluated in the root wash (ANOVA for root wash: F � 7.02, df � 8,72, P <0.0001).

In the choice test with “susceptible” lines (Table 4), egg densities in all PIswere numerically higher than egg densities in ‘Cocodrie’ and ‘Jefferson’. However,only egg densities from PI 415731 were significantly higher (ANOVA for eggdensities: F � 3.47, df � 5,55, P < 0.01). Similarly, first-instar counts were higher

Table 2. Results of choice test with putatively resistant lines, BatonRouge, Louisiana, 2002.

Rice line Eggs First instars Larvae

‘Cocodrie’ 34.2 ± 6.6 d 12.1 ± 1.6 bc 3.5 ± 0.7 a‘Jefferson’ 28.1 ± 4.3 cd 18.4 ± 2.8 d 6.0 ± 1.3 abWC 1541 17.0 ± 2.2 bc 13.2 ± 2.0 cd 10.4 ± 1.5 cPI 160555 62.7 ± 9.0 e 25.9 ± 3.5 e 17.8 ± 1.9 dPI 319512 3.2 ± 0.8 a 3.1 ± 1.0 a 5.5 ± 0.9 abPI 321264 7.2 ± 1.8 ab 5.9 ± 1.1 a 3.2 ± 1.4 aPI 321278 8.6 ± 1.5 ab 5.5 ± 1.0 a 4.2 ± 1.5 abPI 321310 10.1 ± 1.2 ab 6.8 ± 0.8 ab 1.7 ± 0.8 aPI 321314 33.3 ± 6.7 d 18.8 ± 3.7 d 7.5 ± 2.2 bc

Densities of eggs and larvae are means ± SE (n � 14). Egg and larval densities are expressed as numberof eggs/larvae plant−1.Means followed by the same letter in a column are not significantly different (P > 0.05) according to LSD.


in all PIs than in the two commercial varieties, but the only significant differencewas between PI 420968 and ‘Cocodrie’ (ANOVA for first instars: F � 1.72, df �5,55, P � 0.15). In the root wash, PI 415731 showed significantly higher larvaldensities than all other lines (ANOVA for root wash: F � 4.99, df � 5,54, P <0.001).

Discussion

The 19 rice lines screened in these experiments exhibited significant variationin their susceptibilities to infestation by L. oryzophilus, with several lines show-ing significantly lower egg densities relative to ‘Cocodrie’ and ‘Jefferson’ (antixeno-sis). Egg densities on plants of the most susceptible rice line in each experimentwere 2.7–19.6 times greater than egg densities on plants of the most resistant linein each experiment (the extremely low numbers of eggs found on WC 1541 in thethird preliminary screen were considered anomalous). This variation in suscep-

Table 4. Results of choice test with putatively susceptible lines.

Rice line Eggs 1st instar Larvae

‘Cocodrie’ 6.6 ± 2.4 a 4.0 ± 1.1 a 4.9 ± 1.2 a‘Jefferson’ 6.3 ± 1.7 a 4.5 ± 1.5 ab 3.2 ± 0.7 aPI 346840 6.8 ± 1.8 a 5.1 ± 1.7 ab 4.9 ± 0.9 aPI 413958 7.4 ± 1.7 a 15.2 ± 9.8 ab 2.3 ± 0.7 aPI 415731 16.7 ± 3.1 b 14.4 ± 2.5 ab 9.3 ± 2.0 bPI 420968 11.6 ± 2.4 ab 17.1 ± 4.5 b 3.4 ± 1.0 a

Densities of eggs and larvae are means ± SE (n � 12). Egg and larval densities are expressed as numberof eggs/larvae plant−1.Means followed by the same letter in a column are not significantly different (P > 0.05) according to LSD.

Table 3. Results of no-choice test with putatively resistant lines, BatonRouge, Louisiana, 2002.

Rice line Eggs 1st instars Larvae

‘Cocodrie’ 25.1 ± 4.4 abc 23.9 ± 5.8 bcd 10.1 ± 2.0 cd‘Jefferson’ 33.7 ± 6.8 bc 27.4 ± 5.4 cd 8.8 ± 1.7 bcWC 1541 34.6 ± 5.9 c 29.6 ± 4.8 d 8.8 ± 1.5 bcPI 160555 28.8 ± 5.2 abc 23.4 ± 3.1 bcd 20.6 ± 4.3 ePI 319512 13.1 ± 2.8 a 13.4 ± 1.9 ab 6.8 ± 1.4 abcPI 321264 20.8 ± 6.4 abc 11.3 ± 1.4 a 3.5 ± 1.2 abPI 321278 14.7 ± 4.2 ab 16.3 ± 2.9 ab 5.1 ± 1.8 abPI 321310 39.8 ± 14.1 c 17.0 ± 4.2 abc 2.2 ± 0.6 aPI 321314 27.1 ± 7.6 abc 14.2 ± 3.5 ab 15.8 ± 3.1 de

Densities of eggs and larvae are means ± SE (n � 10). Egg and larval densities are expressed as numberof eggs/ larvae plant−1.Means followed by the same letter in a column are not significantly different (P > 0.05) according to LSD.


tibility is greater than that reported in several earlier publications (Smith &Robinson 1982, N’Guessan et al. 1994, Stout et al. 2001). In the choice experimentwith putatively resistant lines, four of the lines (PI 319512, PI 321264, PI 321278,and PI 321310) showed 60% or greater reductions in egg densities relative to themost resistant commercial cultivar. In the no-choice experiment, which used con-ditions probably more typical of the field (i.e. presentation of a single rice line toweevils), the differences among varieties were less marked, suggesting that thecontext in which a plant was presented to weevils influenced the level of resis-tance manifested. However, the qualitative patterns of resistance seen underchoice conditions were largely maintained. PI 319512, PI 321264, and PI 321264exhibited at least a 17% reduction in egg densities relative to the most resistantcommercial variety, and PI 319512, PI 321264, PI 321278, and PI 321310 showeda 20% or greater reduction in larval densities.

There was a fair degree of consistency in the level of resistance manifested bylines both within and among experiments. In those experiments in which resis-tance was assessed by counting eggs in leaf sheaths and by counting larvae (firstand late instars) associated with roots, these three independent measures were ingood agreement. Furthermore, the performance of lines was consistent through-out experiments, with the exception of WC 1541, which scored highly resistant ina preliminary screen but only slightly resistant in the subsequent screening ofresistant lines. Notably, PI 321264, PI 321278, PI 319512, and PI 321310 hadlower egg or larval densities than at least one of the commercial varieties in everyexperiment in which they were included. Perhaps most importantly, those linesshowing greater levels of resistance than commercial varieties in these experi-ments also had exhibited resistance in several past studies (Table 1). These paststudies have included both greenhouse (Stout & Riggio 1998) and field experi-ments (Robinson et al. 1979, 1980, 1981) and have used a variety of methodsunder a range of conditions and weevil pressures.

The procedures used in the experiments reported here were designed primarilyto detect resistance to oviposition by female weevils, an antixenotic trait. How-ever, high rates of mortality from the egg to larval stage in these experimentsmight be indicative of antibiosis. Among the lines used in these experiments, onlyPI 321310 showed signs of antibiosis in both choice and no-choice tests. In thechoice experiment with resistant lines, this line had numerically higher egg den-sities than PI 319512, PI 321264, and PI 321278, but the density of late-instarson this line was lower than densities on all other lines. Even more convincing, eggdensities on this line in the no-choice experiment were the highest of any line, butlarval densities on this line were again the lowest. Lower levels of antibiosis maybe present on other lines, but verification of this will require experiments spe-cifically designed to test for antibiosis.

Very few attempts to incorporate resistance to L. oryzophilus into agronomi-cally acceptable varieties have been made, presumably because the levels of re-sistance found in rice lines are not high enough to serve as a primary means ofmanagement for this insect. However, the confirmation here of resistance inseveral rice lines, and the discovery of several lines with greater susceptibility toL. orytzophilus than commercial varieties, may provide impetus to efforts to breedfor resistance to this insect. The four most resistant lines—PI 319512, PI 321310,PI 321264, and PI 321278—were consistently less susceptible to infestation thanJefferson, the most resistant commercial variety in several prior field tests (Stout


et al. 2001); moreover, these four lines have each exhibited resistance in at leastfour previous field experiments (Table 1). PI 319512, PI 321310, PI 321264, andPI 321278 are breeding lines, the latter three from the Philippines and the firstfrom Mexico. These resistant lines may be useful as germplasm in a breedingprogram, particularly if the mechanisms of antixenosis in the four lines are dif-ferent (non-allelic) or if any of these lines also possess useful levels of antibiosisor tolerance. Furthermore, the identification of lines more susceptible than com-mercial varieties in addition to lines more resistant than commercial varietiesshould allow the production of mapping populations for marker-assisted selection(Yencho et al. 2000). The identification of lines with greater susceptibility toweevil infestation will also facilitate efforts to identify biochemical or morpho-logical characteristics that determine susceptibility to infestation.

The importance of alternative means of managing the rice water weevil hasincreased with the introduction and spread of this insect into Asian rice-producing regions. In areas where L. oryzophilus is univoltine, low levels of re-sistance might serve to reduce populations of overwintering weevils; reductions inpopulation size in areas where L. oryzophilus is multivoltine are potentiallygreater. Moreover, reductions in L. oryzophilus resulting from the use of resis-tant varieties may complement reductions from insecticides and other strategies(Heinrichs 1994, Stout et al. 2001). The combined use of host plant resistance andother management strategies will be important, because insecticides and culturalpractices effective against the rice water weevil often do not reduce populationsbelow economic thresholds (Stout et al. 2001).

Acknowledgments

This study was supported in part by a grant from the Louisiana Rice Research Board. Anearlier draft of the manuscript was improved by comments from Drs. B. Castro, Q. R. Chu,and J. Oard. Approved for publication by the Director of the Louisiana Agricultural Experi-ment Station, Manuscript No. 03-26-0892.

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