Insecticidal activity of native isolates of Spodoptera frugiperda multiple nucleopolyhedrovirus from soil samples in Mexico.
Insect pests (Control)
Del Rincon-Castro, Ma. Cristina
Sanchez-Pena, Sergio R.
Siller, Melchor Cepeda
|Publication:||Name: Florida Entomologist Publisher: Florida Entomological Society Audience: Academic Format: Magazine/Journal Subject: Biological sciences Copyright: COPYRIGHT 2011 Florida Entomological Society ISSN: 0015-4040|
|Issue:||Date: Sept, 2011 Source Volume: 94 Source Issue: 3|
|Geographic:||Geographic Scope: Mexico Geographic Code: 1MEX Mexico|
The fall armyworm (FAW), Spodoptera frugiperda (J. E. Smith)
(Lepidoptera: Noctuidae), is the main insect pest of corn in Latin
America. Control of FAW larvae requires 2-4 applications of chemical
insecticides (Hruska & Gould 1997). FAW larvae are susceptible to
infection by a baculovirus, specifically the Spodoptera frugiperda
nucleopolyhedrovirus (SfMNPV) (Shapiro et al. 1991). Use of this virus
may serve as an alternative to chemical control; it would reduce the
risk of resistance development and environmental pollution; due to its
specificity, biosecurity, persistence and virulence level (Fuxa 2004).
Essentially all isolates used against FAW have been isolated from
infected insects. There are no prior reports on obtaining and using
SfMNPV isolates from soil. In this study the biopesticidal activities of
different isolates of native NPV on FAW larvae were measured.
FAW larvae were obtained from a colony established under controlled conditions (25 [+ or -] 2[degrees]C, 12:12 L:D h and 50-60% RH). Larvae were reared on an artificial diet (Southland Products Incorporated). Corn plots were sampled in Coahuila, Nuevo Leon and Nayarit, Mexico. Soil samples were taken by scraping away the upper 5 cm of soil, and then collecting soil at depths of 5-10 cm. Each sample consisted of 600 to 800 g soil. Samples were processed according to the methodology described by Richards & Christian (1999). Thus each 25 g sample of sieved soil was incorporated twice in 100 ml of artificial diet, and first and second FAW instars were placed on the amended diet. Larvae that died of polyhedrosis disease were considered to be infected by a NPV isolate. These NPVs isolates were amplified in vivo in FAW third instars and purified by filtration and centrifugation as described by Munoz et al. (2001). The concentrations of viral occlusion bodies (OBs) were quantified with a hemacytometer and stored in aliquots of 500 [micro]l of distilled water at 0[degrees]C until required.
The median lethal concentration ([LC.sub.50]) and median lethal time ([LT.sub.50]) of each viral isolate recovered was determined by the diet surface contamination technique. The diet surface in each container was inoculated with 1 of 7 NPV concentrations ranging from 2.0 x [10.sup.1] to 4.0 x [10.sup.6] OBs/[mm.sup.2]; and 20 FAW larvae per concentration were infected in each of 3 replicates. Larvae used as the control in the bioassay were placed in cups with artificial diet treated only with sterile distilled water, i.e., no virus. Mortality was measured daily for 25 d. Based on the highest mortality and the shortest time to death among toe 10 isolates, the most infective isolate, i.e., SfMNPV-[AN.sub.2], was selected (Table 1). Bioassays were conducted on all 5 FAW instars (Table 2). The [LC.sub.50] and [LT.sub.50] were determined using 7 concentrations ranging from 1.0 x [10.sup.1] to 1.0 x [10.sup.7] OBs/[mm.sup.2]. Larval mortality was recorded every 12 h for 25 d. These bioassays were replicated 3 times.
Mortality was corrected by Abbott's formula (Abbott 1925), and the means of treatments were separated using the Tukey's test (P < 0.05). The [LC.sub.50] values were calculated by the probit method using the statistical program SAS (SAS 2002). [LT.sub.50] values were estimated with the Generalized Linear Modeling Program (GLM).
Of the 120 soil samples collected, 10 samples were positive for SfMNPV. These 10 isolates of NPV were shown to be pathogenic against FAW third instars with mortalities ranging from 82 to 100%. Also, the virulence of these isolates varied as reflected in the [LC.sub.50] and [LT.sub.50] values (Table 1). The isolate SfMNPV-[AN.sub.2] from Coahuila proved to be the most infective and caused 100% mortality. The [LT.sub.50] values ranged between 6 to 11 d (Table 1). Martinez et al. (2003), reported mortalities of 63-100% and a [LC.sub.50] of 3.4 x [10.sup.4] OBs/larva, and a [LT.sub.50] of 3.9 d in third instar FAW treated with SfMNPV. We found the first 2 instars to be the most susceptible (Table 2). The [LT.sub.50] values ranged from 4 d in the first instar to 8 d in the fifth instar. Data on pathogenic isolates of NPV in FAW larvae have been reviewed by Escribano et al. (1999). The first instar was the most susceptible to SfMNPV; this result substantiated by Cisneros et al. (2002). The time and the concentration required for the virus to cause larval death both increase with succeeding instars (Martinez et al. 2003). Based on these results we conclude that in soils in Mexico there are native isolates of nucleopolyhedrovirus with potential for use in biological control of the FAW.
The FAW is the main insect pest of corn in Latin America. The larvae are susceptible to Spodoptera frugiperda multiple nucleopolyhedrovirus (SfMNPV). Ten isolates collected from soil of corn plots infested with FAW larvae in Coahuila, Nuevo Leon and Nayarit, Mexico were evaluated to control this pest. Bioassays were performed to determine the biological response of the third instar to SfMNPV infection in order to select the most infective isolate. The diet surface contamination technique was used. The isolate SfMNPV-[AN.sub.2] from Coahuila was the most highly infectious. Additional bioassays of the same isolate were performed in the 5 FAW instars to determinate mortality. [LC.sub.50] increased as the size of the insect increased from first to fifth instar. A similar pattern occurred with [LT.sub.50]. This study achieved the isolation from the soil of highly virulent SfMNPV isolates.
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CLAUDIO RIOS-VELASCO (1), GABRIEL GALLEGOS-MORALES (1), MA. CRISTINA DEL RINCON-CASTRO (2), ERNESTO CERNA-CHAVEZ (1), SERGIO R. SANCHEZ-PENA (1) AND MELCHOR CEPEDA SILLER (1)
(1) Departament of Parasitology, Universidad Autonoma Agraria Antonio Narro, Calzada Antonio Narro 1923, Saltillo, Coahuila, Mexico, C.P. 25315
(2) Departament of Food, Division de Ciencias de la Vida, Universidad de Guanajuato, Irapuato, Guanajuato, Mexico, C.P. 36500. Irapuato-Silao Km 9
TABLE 1. MEDIAN LETHAL CONCENTRATION [LC.sub.50] OF NATIVE ISOLATES OF SFMNPV ON THIRD INSTAR OF SPODOPTEBA FRUGIPERDA. Isolates (a) N (b) % Mortality (c) Lower limit SfMNPV-[NAV.sub.1] 420 52.63 6.7 x [10.sup.4] SfMNPV-[NAV.sub.2] 420 70.00 3.7 x [10.sup.5] SfMNPV-[NAV.sub.4] 420 89.47 5.5 x [10.sup.5] SfMNPV-[NAV.sub.6] 420 85.00 1.0 x [10.sup.4] SfMNPV-[NAV.sub.8] 420 90.00 9.9 x [10.sup.3] SfMNPV-[NAV.sub.9] 420 78.95 6.8 x [10.sup.4] SfMNPV-CAD 420 80.00 9.7 x [10.sup.4] SfMNPV-NAY 420 94.74 5.6 x [10.sup.3] SfMNPV-[AN.sub.1] 420 72.22 5.4 x [10.sup.4] SfMNPV-[AN.sub.2] 420 100.00 4.3 x [10.sup.2] Isolates (a) [LC.sub.50] (95%) (d) Upper limit SfMNPV-[NAV.sub.1] 1.9 x [10.sup.5] 9.5 x [10.sup.5] SfMNPV-[NAV.sub.2] 1.5 x [10.sup.6] 2.8 x [10.sup.7] SfMNPV-[NAV.sub.4] 7.4 x [10.sup.5] 9.9 x [10.sup.5] SfMNPV-[NAV.sub.6] 1.2 x [10.sup.4] 1.5 x [10.sup.5] SfMNPV-[NAV.sub.8] 2.3 x [10.sup.4] 4.8 x [10.sup.4] SfMNPV-[NAV.sub.9] 1.0 x [10.sup.5] 1.7 x [10.sup.5] SfMNPV-CAD 1.2 x [10.sup.5] 1.4 x [10.sup.5] SfMNPV-NAY 7.5 x [10.sup.3] 9.8 x [10.sup.3] SfMNPV-[AN.sub.1] 6.5 x [10.sup.4] 7.7 x [10.sup.4] SfMNPV-[AN.sub.2] 5.7 x [10.sup.2] 7.2 x [10.sup.2] Isolates (a) Slope ([+ or -] SE) Intercept ([+ or -] SE) SfMNPV-[NAV.sub.1] 0.52 [+ or -] (0.07) -2.73 [+ or -] (0.34) SfMNPV-[NAV.sub.2] 0.53 [+ or -] (0.10) -3.27 [+ or -] (0.59) SfMNPV-[NAV.sub.4] 2.93 [+ or -] (0.39) -17.20 [+ or -] (2.32) SfMNPV-[NAV.sub.6] 1.22 [+ or -] (0.10) -5.02 [+ or -] (0.41) SfMNPV-[NAV.sub.8] 0.72 [+ or -] (0.08) -3.12 [+ or -] (0.37) SfMNPV-[NAV.sub.9] 0.55 [+ or -] (0.06) -2.75 [+ or -] (0.28) SfMNPV-CAD 1.25 [+ or -] (0.13) -6.34 [+ or -] (0.67) SfMNPV-NAY 2.06 [+ or -] (0.23) -7.98 [+ or -] (0.92) SfMNPV-[AN.sub.1] 1.53 [+ or -] (0.18) -7.35 [+ or -] (0.90) SfMNPV-[AN.sub.2] 2.86 [+ or -] (0.34) -7.92 [+ or -] (0.97) Isolates (a) [chi square] SfMNPV-[NAV.sub.1] 0.98 SfMNPV-[NAV.sub.2] 0.96 SfMNPV-[NAV.sub.4] 0.77 SfMNPV-[NAV.sub.6] 0.99 SfMNPV-[NAV.sub.8] 0.96 SfMNPV-[NAV.sub.9] 0.96 SfMNPV-CAD 0.83 SfMNPV-NAY 0.99 SfMNPV-[AN.sub.1] 0.87 SfMNPV-[AN.sub.2] 0.99 NPV Isolates, SfMNPV-[NAV.sub.1, 2, 4, 6, 8 and 9]: isolated from corn plots at Navidad, Nuevo Leon; SfMNPV-CAD isolated from Cadereyta, Nuevo Leon; SfMNPV-NAY isolated from Nayarit; SfMNPV-[AN.sub.1, 2] isolated from Coahuila, Mexico. (b) Number of insects treated; (c) Percent mortality with the highest concentration (4.0 x [10.sup.6] OBs/[mm.sup.2]). (d) [LC.sub.50] values were expressed as OBs/[mm.sup.2] on the diet surface. Twenty 20 larvae per NPV concentration, 7 concentrations per isolate and 3 replicates; 20 untreated larvae (control) per replicate were used. SE = Standard error; [chi square] = Goodness of fit test. TABLE 2. MEDIAN LETHAL CONCENTRATION AND MEDIAN LETHAL TIME TO DEATH OF THE SFMNPV-[AN.sub.2] ISOLATE DETERMINED IN EACH OF THE FIVE SPODOPTERA FRUGIPERDA INSTARS. Instar N (a) Lower limit [LC.sub.50] (95%) (b) First 420 6.1 x [10.sup.1] 1.15 x [10.sup.2] Second 420 2.6 x [10.sup.2] 3.6 x [10.sup.2] Third 420 2.6 x [10.sup.2] 5.7 x [10.sup.2] Fourth 420 1.3 x [10.sup.5] 4.3 x [10.sup.5] Fifth 420 1.1 x [10.sup.7] 2.4 x [10.sup.7] Instar Upper limit Slope ([+ or -] SE) First 1.7 x [10.sup.2] 0.58 [+ or -] (0.04) Second 4.8 x [10.sup.2] 0.87 [+ or -] (0.07) Third 1.4 x [10.sup.3] 2.86 [+ or -] (0.34) Fourth 4.5 x [10.sup.6] 1.07 [+ or -] (0.31) Fifth 4.4 x [10.sup.7] 1.42 [+ or -] (0.21) [LT.sub.50] Instar Intercept ([+ or -] SE) [chi square] (95%) (days) First -1.61 [+ or -] (0.13) 0.72 3.96 Second -2.23 [+ or -] (0.22) 0.99 5.88 Third -7.92 [+ or -] (0.97) 0.83 6.55 Fourth -6.02 [+ or -] (1.72) 0.74 7.01 Fifth -10.54 [+ or -] (1.64) 0.81 7.82 (a) Number of insects treated (b) [LC.sub.50] values were expressed as OBs/[mm.sup.2] of diet surface; 20 larvae per NPV concentration, 7 concentrations per isolate and 3 replicates were used; also 20 untreated larvae (control) per replicate were used. SE = Standard [error;.sup.2] = Goodness of fit test.
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