Alfalfa Diseases and Insects
Development and Evaluation of Mississippi Sclerotinia-Resistant
Germplasm - R G. Pratt and D. E. Rowe................14
Performance of Sclerotinia Resistant Plants with
Crossing and Selfing - D. E. Rowe
and E. S. Halimi ..............................................................15
Benefits Assessment of Fungicide Usage in Alfalfa
in lowa - F. W. Nutter, Jr.
G. P. Munkvold, and S. W. Wegulo...............................................16
Evaluations of Resistance to New Biotype of
Blue Alfalfa Aphid, Acyrthosiphon
kondoi Shinji, "BAOK90" - A. A. Zarrabi, R. C. Berberet and J. L. Caddel ......17
Evaluating Alfalfa Host Plant Resistance to Potato
Leafhopper - M. McCaslin
D. Whalen and W. Helming ......................................................18
Potato Leafhopper Resistance in Perennial Glandular-Haired
T. C. Elden and M. McCaslin...................................................19
Myristic Acid Metabolism in the Pea Aphid:
Possible Implications for Host
Resistance Studies in Alfalfa - J. W. Dillwith, P. A. Neese, R. D. Madden,
B. J. Irvin and R. C. Berberet.................................................20
Long-Term Prospects for Effective Biological
Control of the Alfalfa Weevil in
Oklahoma - R. C. Berberet and A. D. Bisges.....................................2 1
Development and Evaluation of Mississippi Sclerotinia-Resistant (MSR) Alfalfa Germplasm
R. G. Pratt and D. E. Rowe
USDA, ARS, Forage Research Unit
Mississippi State, MS 39762
Mississippi Sclerotinia-resistant (MSR) alfalfa germplasm was released
jointly by USDA-ARS and the Mississippi Agricultural and Forestry Experiment
Station in 1995 (2). MSR is the first germplasm of alfalfa developed specifically
for resistance to S. trifoliorum (Eriks), the causal organism of Sclerotinia
crown and stem rot. It also expresses similar resistance to _. sclerotiorum
(Lib.) de Bary. MSR was produced as a by-product of efforts to develop
and evaluate techniques for screening for resistance in excised leaf tissues.
Initial screening was performed by stem-tip inoculations, and a moderately
resistant population, termed "STR", was developed (1). The STR
population then was used as a resistant control to help develop leaf-inoculation
techniques. Those techniques that appeared promising were each applied
as trial screening procedures to several hundred plants of cultivar Delta,
to the STR population, or to populations produced by other leaf-inoculation
techniques. The most resistant-appearing plants identified by trial screening
procedures were polycrossed and progeny were evaluated. From four populations
of progeny that appeared to have increased resistance to S. trifoliorum,
2,772 plants were evaluated by one or more leaf-inoculation techniques.
Nine of the most resistant-appearing individuals were polycrossed to produce
Syn-1 seed of MSR. These nine plants each resulted from one or two previous
cycles of selection for resistance.
The principal techniques used to select resistant plants during development of the MSR germplasm were stem-tip inoculations, performed as described (1), and inoculations of excised unifoliate leaves, first trifoliate leaflets, and discs of tissue from later leaves. Excised leaves and tissues were incubated on agar in petri plates and inoculated individually with discs of agar from colonies of _. trifoliorum. Plants were scored and selected according to the rate and extent of necrosis that developed during incubation for 7 days. Similar inoculations with petioles were considered generally unreliable, but one parent plant of MSR was selected on the basis of petiole response. No plants were selected by whole-plant inoculations in either the growth room or the field during development of MSR.
MSR was evaluated for resistance to S. trifoliorum, in comparison to 25 cultivars, by whole plant inoculations in a growth room, and with natural infection in the field. Whole-plant inoculations were performed with infested and comminuted wheat and oat grain (1). In repeated whole-plant inoculation experiments with seedlings in a growth room, MSR had significantly (P=O.O1) greater survival of plants than the parent cultivar (Delta), a standard check cultivar (Vernal), and all of 23 entries in the 1992-93 and 1993-94 Southern Regional Alfalfa Variety Tests. The resistance of MSR in comparison to Delta was also expressed following inoculations with six isolates of _. trifoliorum and _. sclerotiorum from diverse states in the eastern USA. Field evaluations were performed by growing MSR and several cultivars in naturally infested soil with no artificial inoculum applied. In comparisons of fungicide-sprayed and unsprayed plots, MSR attained higher percentages of its yield potential in the presence of disease than did Delta and two other cultivars in each of two seasons. Scores of disease severity in individual spaced plants were lower for MSR than for cultivars in most experiments. Preliminary observations of one test in Ohio suggest that relative resistance to natural infection also is expressed there.
(1) Pratt, R. G., and Rowe, D.E. 1994. Responses to selection for resistance
Sclerotinia trifoliorum in alfalfa by stem inoculations. Plant Dis. 78:826-829.
(2) Pratt, R.G., and Rowe, D.E. 1996. Registration of Mississippi Sclerotinia-Resistant
Alfalfa Germplasm. Crop Sci. 36 (In Press).
D.E. Rowe and E.S. Halimi
USDA-ARS, Forage Research Unit and
Mississippi State University, MS State, MS 39762
One successful selection technique used for Sclerotinia crown and stem
rot, the stem-tip inoculation method (1), was first used with divergent
selection for resistance to Sclerotinia trifoliorum (Eriks) (2). One of
our early observations on disease development following the stem-tip inoculation
was that development of necrosis on some plants proceeded down the stems
for a few days and then stopped. The original evaluation procedure measured
disease 14 days after inoculation and was not useful for measuring disease
development at intermediate times. The evaluation procedure was modified
so that the inoculated, excised stem was held inside a plastic sleeve and
progress of the necrosis was marked on the plastic daily from day 4 to
14 after inoculation. This procedure was used to measure disease on stems
of alfalfa clones selected for resistance and absence of resistance (susceptibility)
from an earlier divergent selection study. Clones selected for susceptibility
developed necrosis at an average of 1.2 cm d-'. Clones selected for high
levels of resistance had much less disease and, interestingly, had two
types of reaction to inoculation. Those which stopped development of necrosis
about eight days after inoculation (hereafter referred to as type 1 resistance)
and those which had a slow rate of disease development but did not halt
the development of necrosis (referred to as type 2 resistance). Of the
eight resistant clones evaluated, six had type 1 resistance and two had
type 2 resistance. Thus potentially, there was evidence that there were
different genes conferring resistance as either type 1 or 2. It might be
possible to increase resistance by combining types 1 and 2 or crossing
might result in heterozygotes with little resistance.
In the development of synthetic varieties, the performance of the selfs and the crosses between resistant clones of the same and different types, between resistant and susceptible clones, and are all important. Crosses and selfs were made among 10 alfalfa clones: four with type 1 resistance, two with type 2 resistance, and four selected for susceptibility. The crosses were made using vacuum emasculation. Parents, progenies of selfs, and progenies of crosses were evaluated together using the modified stem-inoculation procedure. The focus was on both the level of resistance and on whether it was type 1 or type 2 in the progeny.
Progenies from crosses of resistant and susceptible clones had intermediate levels of resistance. The six crosses among the four clones with type 1 resistance produced progenies with type 1 resistance. Crosses between the four clones with type 1 resistance and the two with type 2 resistance produced resistant progenies, four with type 1 resistance and four with type 2 resistance. The cross between the clones with type 2 resistance produced a progeny with same resistance and type. Selfing the resistant clones produced progenies with same type of resistance but with two exceptions. One of the selections with type 1 resistance produced a progeny with type 2 resistance and one of the selections with type 2 resistance produced a progeny with type 1 resistance. On average, selfing the susceptible selections slightly increased resistance and selfing the resistant selections slightly reduced the resistance.
(1) Pratt, R.G., and D.E. Rowe. 1991. Differential responses of alfalfa genotypes to stem inoculations with Sclerotinia trifoliorum and S. sclerotiorum. Plant Dis. 75:188-191. (2) Halimi, E.S., D.E. Rowe, and M. Aung. 1994. Divergent selection in alfalfa for resistance to Sclerotinia crown and stem rot. Crop Sci. 34:1440-1442.
Benefits Assessment of Fungicide Usage in Alfalfa in Iowa
F. W. Nutter, Jr., G. P. Munkvold, and S. W. Wegulo
Department of Plant Pathology, Iowa State University, Ames, Iowa, 50011
Disease incidence, disease severity, percent defoliation, and disease
intensity (defoliation plus severity) assessments were obtained weekly
at field experiments located in Ames, IA (seeding year) and Nashua, IA
(third year stand) to determine the benefits of different fungicide scenarios
on disease development and yield. Nine treatments were arranged in a randomized
complete block design at both locations. Treatments l through 4 were the
fungicides chlorothalonil, cupric hydroxide, mancozeb, and propiconazole
at two applications per cutting cycle. Treatments 5 through 8 were the
same 4 fungicides applied once per cutting cycle, respectively. Treatment
9 was a nonsprayed control. Fungicides were applied with a hand-held CO2
powered boom sprayer operated at 40 psi. The first fungicide application
for each cutting cycle was applied when stem height had reached 6-8 inches
and a second application was made 11-14 days later.
The most prevalent diseases at both locations in 1995 were: Phoma spring black stem and leaf spot during the first cutting cycle and Cercospora summer black stem, Pseudopeziza leaf spot, and Leptospherulina leaf spot during the third growth cycle at Nashua (3 year old stand). Even in the seeding year in Ames, disease incidence (number of infected leaves/total number of leaves assessed) exceeded 60% for all treatments on the first assessment date (June 19, day of year 170) and incidence was nearly 100% for all treatments by day of year 192 (July 11). Disease incidence at the Nashua location (3 year old field) started at levels above 95% on the first sampling date (May 18, day of year 138). Fungicides had a significant impact on reducing disease incidence in subsequent samplings in the first growth cycle (P < 0.05) but less so in the second and third cycles. Incidence data, therefore, indicated that there was a higher risk of disease early in the season in established stands and a lower disease risk in the seeding year of new stands. There were significant differences among treatments for disease severity at both Ames and Nashua and disease severity was generally highest in the nonsprayed control plots. Percent defoliation assessment provided a measure of the impact of foliar diseases on leaf removal. Percent defoliation increased in all treatments over time in Ames and defoliation levels were significantly reduced by fungicide treatments at both locations for a number of assessment dates. In Nashua, percent defoliation levels exceeded 55% in nonsprayed controls by the end of the first growth cycle and approached 65% in nonsprayed control plots at the end of the second growth cycle. Disease intensity gave similar results as percent defoliation data. Overall, fungicides applied twice per growth cycle (especially mancozeb and chlorothalonil) provided better disease control than single applications per growth cycle.
The percentage difference between attainable yield (highest value using fungicides) and actual yield (nonsprayed control treatment) for each growth cycle was 27%, 34%, 16%, and 33% for Ames harvests 1 and 2 and Nashua harvests l and 2, respectively. In three out of four harvests, chlorothalonil applied twice per growth cycle resulted hl the highest yield. Propiconazole applied twice provided the highest yield for harvest 2 in Nashua, although this was not significantly different from two applications of chlorothalonil. In general, fungicides applied twice per growth cycle provided higher yields compared to those fungicides that were applied once per growth cycle.
Evaluations of Resistance to New Biotype
of Blue Alfalfa Aphid. Acyrthosiphon kondoi
A.A. Zarrabi, R.C. Berberet, and J.L. Caddel
Departments of Entomology and Agronomy, Oklahoma State University
Stillwater, OK 74078, U.S.A.
In 1989, the possible existence of a new biotype of BAA was reported in New Mexico (2). Two years later, a new biotype of BAA was suspected in Oklahoma (1). In 1995, the existence of a new BAA biotype designated as "BAOK90" was confirmed in Oklahoma (3). The purpose of research reported in this paper was to (1) develop adapted germplasm with resistance to "BAOK90" and (2) assess agronomic performance of BAOK90-resistant strains under field conditions.
Eight adapted cultivars ('Aggressor', 'Apollo Supreme', 'Cimarron', 'Good As Gold', 'ICI 630', 'Magnum III', 'WL 320', and '5472') and two standard checks used for the original BAA biotype ('CUF 101' and OK 51) were screened for resistance to "BAOK90" under greenhouse conditions. Two strains OK 204 (including CUF 101) and OK 205 (excluding CUF 101) were derived from interpollination of the plants selected from these entries. These populations were then subjected to another cycle of selection for "BAOK90" to create strains OK 206 and OK 207, respectively. These strains were evaluated for resistance to "BAOK90" in the greenhouse tests with 16-d infestation period and rating 10 d after termination. In a field experiment at Stillwater, OK, five BAOK90-resistant strains (OK 204 synl, OK 204 syn2, OK 205 synl, OR 206 synl, and OK 207 synl), CUF 101, OK 51, 'OK08', and 'WL 457' were hand planted in plots of 1.2 by 3.0 m with 3 rows. Entries were arranged in a randomized complete-block design with six replications. The original intent was to evaluate performance of these entries under aphid infestation in spring, 1996. However, three cycles of unusually warm temperatures (ranging from 18-25 deg C) followed by hard freezes (-12 to -28 deg C) during February and March resulted in significant damage (WL 457) or loss of stands (CUF 101) of non-dormant entries. Though not originally planned, data were taken on freeze damage on a six point scale (l)= no damage; (2)= slightly damage; (3)= low damage; (4)= moderate damage; (5)= significant damage; and (6)= severe damage. For the surviving entries, each plot was infested On March 11 with ca. 500 BAA and ca. 100 BAA ten days later. Number of BAA per stem, plant height, freeze damage, and yield was recorded for each plot. Fifteen stems per plot were sampled for height and aphid numbers.
In greenhouse tests, the percent resistant plants of the selected strains
ranged from 20.2 to 25.0 and 16.8 to 20.3, for populations with and without
CUF 101 among germplasm sources, respectively. The percent gain for one
and two cycles of selection varied depending on the original population.
Nondormant entries were damaged more by freezing weather than the adapted
ones. WL 457 and CUF 101 had damage ratings of 4.7 and 6.0, respectively.
CUF 101 plots were replanted in May 1996 for future evaluations. Visual
observations indicated that aphid numbers peaked ca. 25 days after initial
infestation and then declined rapidly due to predation by lady beetles.
Stem elongation was halted by aphid feeding but recovered once aphids were
gone. Impact of "BAOK90" infestation on first-cutting yields
was somewhat confounded due to complex interaction of freeze damage and
short infestation period due to lady beetles. Short exposures to "BAOK90"
probably had a minor impact on forage production. Adapted strains, OK 205
and OK 207 with 3.7 T/ha had the highest yield compared to WL 457 (1.7
T/ha) and less adapted strains OK 204 (2.9 T/ha) and OK 206 (2.8 T/ha).
(1). Berberet, R.C., A.A. Zarrabi, and J.L. Caddel. 1982. Possible occurrence
of a new biotype of blue alfalfa aphid. Rep. 1992. 33rd North Amer. Alf.
Improv. Conf. P.20. (2). Kimmell, J.L., C.G. Currier, and B.A. Melton.
1989. Identification of a possible new blue alfalfa aphid biotype. Proc.
Of the Sixth Western Alf. Improv. Conf. pp.19-20. (3). Zarrabi, A.A., R.C.
Berberet, and J.L. Caddel.
M. McCaslin, D. Whalen and W. Helming
Forage Genetics - West Salem, Wisconsin
The potato leafhopper (PLH) is a serious pest of alfalfa over much of the eastern half of the United States. Significant losses in both yield and forage quality are common in areas where PLH are prevalent. Although timely application of insecticides has proven to be an effective mechanism of controlling alfalfa losses due to PLH, only a fraction of affected alfalfa fields are treated. Host plant resistance offers an opportunity to improve control of PLH damage, and lessen dependence on insecticides. Genetic resistance to potato leafhopper in alfalfa has been recently reported.
The goal of this study was to develop a method for quantifying resistance to PLH and relating different levels of resistance to yield losses under natural PLH infestation at West Salem, WI in 1995. PLH populations were very high during the 1995 season. Performance of twenty varieties was compared with and without insecticide application to control PLH. Varieties were characterized for PLH resistance by scoring spaced plants on a (1-5) scale, l=no damage and 5=severe stunting and yellowing. Plants scored l or 2 were classified as resistant. Average severity index (ASI) and % resistance were calculated. The twenty varieties in the test fell into four categories (mean for each category): highly resistant (%R=56, ASI=2.53), resistant (%R=43, ASI=2.77), moderately resistant (%R=21, ASI=3.50), and susceptible (%R=0, ASI=4.89). The seven commercial varieties in the test were all classified as susceptible.
In the unsprayed treatment, the July 28, 1995 harvest mean yield for highly resistant, resistant, moderately resistant, and susceptible varieties were 10.45, 9.54, 7.87 and 4.89 FW Lbs /plot, respectively. Comparing yield between sprayed and unsprayed treatments allows an assessment of the magnitude of yield loss due to insect damage. In the July harvest, this loss ranged from 2% with the highly resistant group to 33% for the susceptible group. This is the first data to support efficacy of alfalfa host plant resistance to PLH under field conditions. Yield of PLH resistant lines was comparable to commercial checks in the sprayed treatment .
Regression analysis was used to examine the relationship between ASI
and yield loss from insect damage (yield NoSpray/yield Spray) for the twenty
test entries. In these trials, ASI was very predictive (r=.92**) of loss
in yield due to PLH feeding .
l . McCaslin, Mark. 1994. Breeding for potato leafhopper resistance
in alfalfa. Proceedings of the 34th
North American Alfalfa Improvement Conference, Guelph, Ontario.
2. D. B. Hogg and M. McCaslin. 1994. Performance of potato leafhopper
nymphs and adults on selected
clones of glandular haired alfalfa. Proceedings of the 34th North American Alfalfa Improvement
Conference, Guelph, Ontario.
T. C. Elden1 and M. McCaslin2
USDA-ARS, Beltsville, Maryland 20705
Forage Genetics, West Salem, Wisconsin 54669
Perennial glandular-haired alfalfa (Medicago) species and hybrids have
been implicated with resistance to the potato leafhopper, Empoasca fabae
(Harris). However, the development of potato leafhopper resistant alfalfa
germplasm utilizing the glandular hair trait has met with limited success.
The objectives of this study were to quantitate and determine the association
of glandular hairs with potato leafhopper resistance in nineteen selected
clones from an alfalfa population developed for improved vigor and disease
and potato leafhopper resistance. The density of glandular hairs on the
leaf midveins, petioles, and stems were measured. Selected clones were
screened in a laboratory no-choice test for resistance to adult potato
leafhopper feeding damage, survival, and oviposition.
Differences in glandular hair densities among clones were highly significant for all plant parts measured. As the plants aged, glandular hair density among plants within clones increased and density variation decreased. Significant variation was observed among clones for all the potato leafhopper resistance variables measured. Percent adult mortality ranged from 13 to 96 and nymphal populations ranged from 0 to 33. Associations between glandular hair densities on petioles and stems with all the potato leafhopper resistance variables were significantly correlated but the correlations were not strong. Stem glandular hair density had the highest associations with the potato leafhopper resistant variables.
Potato leafhopper adult mortality occurred within the first 24 hours of a test. Physical entrapment of leafhopper adults and nymphs in a glandular hair exudate was not observed in our study. Results of this study suggest that the glandular hairs present on the selected clones used in this study are associated with a compound which is toxic to the potato leafhopper. This study demonstrates that the glandular hair trait is potentially useful in developing potato leafhopper resistant cultivars. 1995. New biotype of Acyrthosiphon kondoi (Homoptera: Aphididae) on alfalfa in Oklahoma. J. Econ. Entomol. 88:1461-1465.
Myristic Acid Metabolism in the Pea Aphid: Possible Implications for Host Resistance Studies in Alfalfa
J. W. Dillwith, P. A. Neese, R. D. Madden, B. J. Irvin and R. C. Berberet
Department of Entomology, Oklahoma State University
Stillwater, Oklahoma, 74078, U.S.A.
The identification of biochemical markers in aphids that respond to
host quality in a predictable manner is a desirable goal to facilitate
screening for host plant resistance in alfalfa. It has been reported that
the medium chain fatty acid myristic acid (14:0) content of spotted alfalfa
aphids, Therioaphis maculata (Buckton), varied with the condition and susceptibility
of the host alfalfa (l). Myristic acid levels in the pea aphid (Acyrthosiphon
pisum (Harris) were also shown to vary with the quality of the diet (2).
We have further characterized the effect of host plant on the myristic
acid content of the pea aphid by comparing fatty acid metabolism in aphids
reared on faba bean and alfalfa. Faba bean has been reported to be a better
host for the pea aphid than alfalfa (3).
Pea aphids from a clonal laboratory colony were continuously cultured on faba bean (Vicia faba L.) cv. 'Windsor' or alfalfa (Medicago sativa L.) cv. 'OK08' at 25 deg. C with a photoperiod of 14 hours. In host switching experiments aphids were reared on the first host and nymphs were collected at birth and maintained separately on the same host until three days after molting to adults. Adults were then placed on the second host and samples were removed at 6 hour intervals for 24 hours and then at 24 hour intervals for up to 216 hours. Fatty acid content of aphids was determined by gas chromatography (1). Biosynthetic studies were conducted by removing the aphids from the plant and feeding them on an artificial diet containing 30% sucrose and 1-l4C acetate. Radiolabeled fatty acids were analyzed by radio-HPLC. To determine the effect of growth regulators on fatty acid metabolism aphids were reared on faba bean and were treated topically with either Precocene II, ethoxyprecocene, piperonylbutoxide, Juvenile hormone III or methoprene.
Pea aphids reared on alfalfa contained approximately three times more myristic acid than aphids reared on faba bean. Thc myristic acid was present exclusively in the triglyceride fraction. The myristic acid content of alfalfa and faba bean leaves were not significantly different suggesting that differences in fatty acid intake were not responsible for the differences. Biosynthetic studies indicate that the rates of synthesis of myristic acid is the same on both hosts and that accumulation in aphids reared on alfalfa must be due to lower rates of turnover. When aphids were switched from alfalfa to faba bean the myristic acid content dropped to levels expected for faba bean reared aphid within 24 hours. Aphids switched from faba bean to alfalfa took much longer to accumulate elevated levels of myristic acid. The host plant effect on myristic acid levels was mimicked by treating aphids with growth regulators suggesting regulation of aphid fatty acid metabolism by juvenile hormone. Treating aphids with methoprene resulted in the same effect as moving aphids from faba bean to alfalfa and occurred in the same time frame.
1. Bergman, D. K., J. W. Dillwith and R. C. Berberet. 1992. Spotted alfalfa aphid, Therioaphis mactulata, fatty acids relative to the condition and susceptibility of its host. Arch. Insect Biochem. Physiol. 18: 1-12.
2. Febvay, G., J.-F. Pageaux and G. Bonnet. 1992. Lipid composition of the pea aphid, Acyrthosiphon pisum (Harris)(Homoptera:Aphididae), reared on host plant and on artificial media. Arch. Insect Biochem. Physiol. 21:103-118.
3. Ellsbury, M.M. and M.W. Nielson. 1981. Comparative host range studies on the blue alfalfa aphid, Acyrthosiphon pisum (Harris) (Homoptera:Aphididae). U.S.D.A. Tech. Bull. 1639. 14pp.
R.C. Berberet and A. D. Bisges
Department of Entomology
Oklahoma State University, Stillwater, OK, 74078
From 1972-1986, six species of wasps that parasitize the alfalfa weevil, Hypera postica (Gyllenhal), were released in Oklahoma. All primary life stages of the weevil are subject to parasitism by one or more of these species which include Peridesmia discus (Walker) (host stage = egg), Bathyplectes anurus (Thomson) (larva), Microctonus colesi Drea (larva), Tetrastichus incertus (Ratzburg) (larva), Dibrachoides dynastes (Foerster) (pupa), and M. aethiopoides Loan (adult). The parasites have been released in as few as one county (P. discus) to as many as 29 counties (B. anurus) through the joint efforts of the personnel from the Oklahoma Agricultural Experiment Station, Oklahoma State Department of Agriculture, and USDA-APHIS. In addition to those released, there is one parasitic species, B. curculionis (Thomson), that has dispersed naturally across Oklahoma (1).
Since 1973, extensive sampling has been conducted in areas where releases have been made and through state-wide surveys to evaluate establishment of parasites and determine the prevalence of these species as control agents for the alfalfa weevil. Sites in northern (Payne Co.) and southern (Grady Co.) Oklahoma, where numerous releases were made, have been intensively sampled for 24 years on stands rotated every 3-5 years. Of particular concern in the sampling programs has been the population dynamics of species and the potential for long-term regulation of weevil populations. Since the first detection of Zoophthora phytonomi (Arthur) in 1983, sampling has included assessment of the extent to which this pathogen reduces population densities of the weevil and of the parasitic wasps. Prevalence of parasites and the pathogen has been determined both from dissections and rearing of weevils.
Effective control of H. postica is critical as peak larval densities typically
range from 200- 1000/0.1 m2. However, of the six parasites released, just
B. anurus (10 counties), M. colesi (6 co.), and M. aethiopoides (5 co.)
have been established. Only B. anurus has been recovered in sufficiently
high numbers to show potential for effective biocontrol. The other species
with biocontrol potential, B. curculionis, has dispersed into all counties
where alfalfa is grown. State-wide surveys have shown that parasitism by
this species from 1977-80 reached 40-50% at peak host densities and 90%
as host populations declined due to pupation. However, actual numbers of
larvae parasitized (% parasitism X larvae/0.1 m2) have rarely exceeded
100/0.1 m 2. Although the prevalence of B. anurus has been similar to that
of B. curculionis in close proximity to release sites, its effectiveness
is reduced due to limited dispersal from these sites. Competitive displacement
of n. curculionis by 13. anurus has been reported in some areas (2), however,
this is not evident in Oklahoma. Larval stages of both parasitic wasps
are often killed when host larvae are infected by Z. phytonomi. However,
competition for hosts is most detrimental to the second generation of B.
curculionis which coincides with peak prevalence of the pathogen (3). As
epizootics of Z. phytonomi occur only with frequent rainfall and warm temperatures,
the pathogen itself has not been a consistent regulator for alfalfa weevil.
In summary, natural enemies have not been effective in reducing weevil
populations below economic threshold levels and frequent use of chemical
insecticides is necessary to prevent serious losses in alfalfa production.
1. Berberet, R.C. and W.P. Gibson. 1976. Bathvplectes curculionis in Oklahoma: distribution and effective parasitism of the alfalfa weevil. Ann. Entomol. Soc. Am. 69:205-208.
2. Harcourt, D.G. 1990. Displacement of Bathvplectes curculionis (Thomson) by B. anurus (Thomson) in eastern Ontario populations of the alfalfa weevil, Hypera postica (Gyllenhal).. Can. Entomol. 122:641-645.
3. Goh, K.S., R.C. Berberet, L.J. Young, and K.E. Conway. 1989. Mortality of the parasite Bathyplectes curculionis during epizootics of Erynia phytonomi in the alfalfa weevil. Environ. Entomol. 18:1131-1135.