Taproot Damage by Clover Root Curculio, Sitona hispidulus (Coleoptera: Curculionidae) and Other Factors Affecting Alfalfa Stand Longevity in Montana 

 

INTRODUCTIONAlfalfa is the major forage crop grown in Montana with over 1.8 million acres in production supporting the $2.5 billion livestock industry. Montana producers and extension agents have identified stand productivity and longevity as critical issues. A well-known insect pest of alfalfa, notorious for reducing stand longevity, is the clover root curculio Sitona hispidulus (F.). The larvae cause damage by feeding on alfalfa nodules and lateral roots, eventually creating open feeding wounds on the taproot that invite the entry of various pathogens. In this first year of a three-year study, we looked at state-wide incidence and severity of clover root curculio (CRC) taproot damage and alfalfa diseases as factors affecting stand longevity. Clover Root Curculio. Adults collected from sweep nets were 61%

S. hispidulus, 26% S. flavescens, and 12% S. lineelus. The clover root curculio is thought to be a complex of these three species, all of them known to cause economic damage (Bright 1994).

Most field sites showed no taproot damage in the early spring (Fig. 3). Fall CRC ratings showed a strong correlation with soil type; sandy soils have significantly less damage than silty and clay soils (p=0.003; Fig. 3). These findings are consistent with Pacchiolo and Hower (2004) who demonstrated that silt-clay loam provided soil cracks larger than 1.0mm for CRC larvae increasing access of first instars to alfalfa root nodules. Sandy loams did not contain large enough pores and the texture was abrasive to the larvae cuticle.

Plant Disease. Spring black stem (SBS) was present only in spring samples (Table 1). Incidence of alfalfa mosaic virus (AMV) and crown/root rot increased in fall (Table 1). Prevalence of alfalfa diseases did not seem to be correlated with CRC ratings.

Rebecca Baril, Cecil Tharp, Sue Blodgett, Ken Kephart1, and Dave Wichman2 Dept. of Animal and Range Sciences, Montana State University-Bozeman

1Southern Ag. Research Center, Huntley, MT 2Central Ag. Research Center, Moccasin, MT

FUTURE This statewide study will continue for two additional years. Other factors to be looked at include: field management practices, other insects, weeds, hay quality and yield. Concurrent studies are researching the use of nurse crops on alfalfa stand density and longevity, and at the effectiveness of carbofuran applications on clover root curculio taproot damage.

METHODSThirteen 1st year alfalfa fields in Montana (Fig. 1) were visited in spring and fall of 2005. •Ten plants per fields were dug and assessed for foliar and root diseases. •Taproot damage by CRC was determined on a six-increment rating scale, based on rough visual estimates of percent damage: 0=0%; 1=1-10%; 2=11-35%; 3=36-65%; 4=66-90%; 5=90-100.•Plants were tested for alfalfa mosaic virus using DAS-ELISA (Fig. 2).•Bi-monthly sweep samples taken by extension agents•Differences in taproot damage were analyzed using PROC GLM (Tukey Test; P=0.05).

LITERATURE CITEDBright, D.E. 1994. Revision of the Genus Sitona (Coleoptera: Curculionidae) of North America. Ann. Entomol. Soc. Am. 87(3): 277-306. Pacchiolo, M.A. and A.A.Hower. 2004. Soil and moisture effects on the dynamics of early instar clover root curculio (Coleoptera: Curculionidae) and biomass of alfalfa root nodules. Environ. Entomol. 33(2): 119-127.

AKNOWLEDGEMENTSWe would like to thank all of the producers and county extension agents who are involved in this study for their time and interest: Steve Seleg, Mark DeCock, David Christoffersen, Mike Knudsen, Glenn Droge, Ken Flikkema, Roger Schwarz, John Viall, Bill Griffin, Tom Stanton, Kevin Wiebe, Dee Boyce, Randy Parker, Dale Sand, and Jerry Johns, Dave Phillips (Fergus County Extension), Dan Clark (Teton County Extension), Paul Dixon (Yellowstone County Extension), Janna Kincheloe (Rosebud-Treasure County Extension), Marko Manoukian (Phillips County Extension), Steve Hutton (Pondera County Extension), Kent Williams (Custer County Extension), Eric Miller (Garfield County Extension), Wade Crouch (Cascade County Extension). This research is funded by the WSARE program.

Fig. 1. Field sites by county

Fig. 2. (A) AMV ELISA test showing positive results (blue) (B) Alfalfa leaf showing characterisitic AMV patterns

(A) (B)

S. hispidulus

Fig. 3. CRC taproot damage in spring and fall 2005 showing correlation with soil type

Fig. 4. CRC damaged taproots

Fig. 4. Alfalfa diseases. (A) severe crown and root rot (B) Spring Black Stem

Table 1. Incidence of spring black stem (SBS), alfalfa mosaic virus (AMV), and crown/root rot by producer fields

(A) (B)

Producer Field SBS AMV Crown/Root Rot AMV Crown/Root RotRoosevelt1 30% 10% none 60% 30%Roosevelt2 10% none none 10% 20%

Custer1 100% none none 20% noneGarfie ld none 10% none 20% 30%Phillips 20% none none none 50%Custer2 90% none none 10% 10%

Rosebud1 none none none none noneGallatin1 40% 20% 10% 70% 20%Rosebud2 none 10% none 10% 30%Gallatin2 100% 10% none 33% none

Fergus 30% none none 10% noneTeton 10% 20% none 30% 50%

Yellowstone 50% 30% none 40% 80%

FallSpring

RESULTSSitona spp.