Influencia del mantillo sobre la persistencia de Steinernema brazilense (Nematoda: Steinernematidae) en cultivo de cana de azucar.
Sugarcane crop in Brazil occupies > 9 million hectares of land, just Sao Paulo state accounting for > 5 million hectares. Farmers expect that planted areas provide at least 5-6 ratton crops that mean the number of regrowth following a harvest. Meanwhile, several factors may affect the cane production in a way that the number of profiting ratton crops become much lower. Among these aspects, is notorious the importance of insect pests with emphasis on soil dwelling species that are the most diverse group, many of which are difficult to manage with chemical insecticides (Pinto et al. 2009). One promising alternative to this problem is to use entomopathogenic nematodes (EPNs) of the genera Steinernema and Heterorhabditis (Nematoda: Steinernematidae, Heterorhabditidae) which are soil dwelling organisms that kill these pests and recycle on the cadavers (Leite et al. 2012).
EPNs are mutualistically associated with bacteria of the genera Xenorhabdus and Photorhabdus, which are voided from the nematode intestine into the hemolymph, propagate and kill the host insect by septicemia, usually within 48 h when infect susceptible hosts such as Galleria mellonella (Griffin et al. 2005). These nematodes possess a number of attractive qualities as biocontrol agents including a relative durable infective juvenile (IJ) stage, a broad host range, high virulence, great ability for host seeking, suitability to mass production and safety (Shapiro-Ilan and Gaugler 2002).
Several studies have tested EPNs against sugarcane pests, especially against the sugarcane billbug, Sphenophorus levis (Vaurie, 1978) (Coleoptera: Dryophthoridae), the larvae of which feed on sugarcane underground stems + rhizomes, damaging up to 80% of the stems and reducing cane productivity by 30%. Adults usually remain underground becoming also potential target for EPNs. Previous field trials applying Steinernema brazilense (strain IBCB n6) in the sugarcane rows (1 x [10.sup.8] infective juveniles [ha.sup.-1]), demonstrated that this nematode controls the billbug and enhance cane productivity (Leite et al. 2012).
The successful application of EPNs to protect sugarcane crops in Brazil requires consideration of several aspects, including the widespread use of mechanical harvesting, which discards the cane leaves in the field, resulting in a thick lay of mulch covering the ground. In this study, the influence of mulch on the persistence of S. brazilense (strain IBCB n6) in a sugarcane field was examined.
The nematodes were obtained by in vitro production using the sponge method (Bedding 1984) and were used soon after being collected, with the infective juveniles (IJs) showing more than 90% viability.
The study was done in a second ratton of a mechanically harvested sugarcane crop (variety SP 803280, planted in rows 1.5 m apart) located in the municipality of Santo Antonio de Posse in Sao Paulo state. Two identical experiments located 400 m apart were carried out, both starting on 10 December 2007, 91 days after the first harvest on 10 September 2007, when the plants were on average 40 cm height. For each experiment, four treatments were considered: (1) soil ground treated with nematodes and covered with mulch, (2) soil ground not treated with nematodes, but covered with mulch (control), (3) bare soil treated with nematodes, and (4) bare soil not treated with nematodes (control). Each treatment was replicated five times and each replicate consisted of a 10 x 7.5 m plot containing five rows 10 m long, randomly distributed in blocks. The plots were 5 m apart within blocks, and 4.5 m apart (three rows) between blocks. Both experiments were set up in an area covered with mulch, and plots without mulch (bare soil groups) were raked to remove any covering. The nematodes were applied just once, at a dose of 1 x [10.sup.8] IJs [ha.sup.-1], on 24 December 2007, using a manually operated backpack sprayer fitted with a jet tip that delivered 3 L [plot.sup.-1] (400 L [ha.sup.-1]) in a 15 cm wide strip along both sides of each row. For the mulch group, a 15 cm wide strip along both sides of each row was uncovered to allow the nematodes to be applied directly to the soil. After application, the sprayed area was covered with mulch again.
Nematode persistence in the soil was evaluated using Galleria mellonella (great waxmoth) larvae to bait the nematode. Evaluations were done 14 (10 December 2007) and 7 (17 December 2007) days before nematode application, and 3 (27 December 2007), 15 (8th January 2008), 32 (25 January 2008), 76 (9th March 2008), 105 (7th April 2008), 160 (1st June 2008), 225 (5th August 2008) and 278 (27 September 2008) days after nematode application. The last evaluation was done immediately after sugarcane harvesting. For each evaluation, three soil samples were randomly collected from each plot, from the 20 cm wide strip along each side of the three central rows. From each spot surveyed, 1 kg of soil was collected and placed in a plastic chamber (15 cm diameter x 10 cm high) and transported to the laboratory, were five artificially reared larvae of G. mellonella were buried in each chamber. Before burying, the larvae were held inside a 10 cm x 6 cm metal-screen cage (aperture: 1 mm), together with 35 g of soil, and then buried in the chambers at a depth of 5 cm. To improve conditions for the EPNs, ~100 mL of water was added to each chamber. The chambers were incubated for five days at 25 [grados]C on the dark. At the end of this period, the metal-screen cages were removed, opened and the insect mortality due to Steinernema brazilense was assessed. Larvae killed by ENPs had a flaccid body, with worms leaving the cadaver after being placed in a white trap. If the ENPs isolated belonged to the genus Steinernema they were cross-bred (Kaya and Stock 1997) to confirm whether they belonged to the same species as that used in the experiments (S. brazilense). However, if the EPNs belonged to the genus Heterorhabditis, they were promptly identified by the red color of the cadavers. Molecular analysis through sequencing of 16S ribosomal RNA gene was done to confirm species identity. These identifications were necessary since the experiments were conducted at field conditions, where some other EPN species may be inhabiting the soil and infect the G. mellonella larvae.
Both experiments were combined as single one to result 10 replications. The mortality of G. mellonella was analyzed by one-way analysis of variance (ANOVA) followed by the Tukey studentized range test for multiple comparisons. Mortality rates were arcsine VxJ100 transformed prior to analysis and all means were transformed back to the original units for presentation. All statistical comparisons were done using SPSS version 10.0 software, with a p value < 0.05 indicating significance.
Results and discussion
No Steinernema were found in the two surveys done before application, indicating that this genus did not occurred naturally in both experimental areas (Table 1). In contrast, indigenous Heterorhabditis sp. was detected at a low density and accounted for <16% of the mortality of G. mellonella larvae. Three days post application (PA), S. brazilense accounted for 49.3% of the mortality in G. mellonella larvae in both nematode treated groups (with and without mulch), indicating that S. brazilense IBCB n6 was introduced successfully in the sugarcane field. Several factors may have contributed to this introduction, including the high rainfall that helped to maintain soil moisture and the soil type, which was slightly sandy (clay = 23.8%, silt = 22.8%, total sand = 53.4%) favoring the action of the nematodes. In agreement, Susurluk (2009) reported that colonization and persistence of the nematodes S. feltiae and H. bacteriophora in different field crops and rotation regimes were correlated with the weekly precipitation following nematode release.
After introduction, there was a decreasing in G. mellonella infection rates that was not significantly affected by the presence or absence of mulch. In our study, the soil samples were rehydrated for baiting with G. mellonella, implying that the decrease in the mortality of the greater waxmoth was directly related to the decrease in the nematode population. Bednarek and Gaugler (1997) found that increased organic matter (organic manure) appeared to stimulate nematode establishment and recycling. The progressive decrease in the percentages of G. mellonella infected by S. brazilense, with no significant difference ([F.sub.39 360] = 29.145; P = 0.124) up to the 32th day PA in the nematode-mulch group, and up to 76th day in the nematode-soil, suggests that this nematode persisted for about one to two months before losing its effectiveness as an agent for controlling sugarcane pests. However, despite this decrease, S. brazilense remained the main nematode responsible for G. mellonella mortality, except prior to application, when the indigenous Heterorhabditis sp. was the only nematode present. According to Kung et al. (1991), S. carpocapsae and S. glaseri survived for 32 days at low soil moistures of 2 and 4%, when soil samples were kept at RH of 100%.
By 76 days PA, the nematode S. brazilense was also found in the control plots (confirmed by cross-breeding identification), which were located 4.5 - 5 m from the neighboring plots where the nematodes were sprayed. This finding indicates that the nematode spread throughout the experimental area, migrating at least 5 m in 76 days, i.e., at least 1 m every 15 days. According to Weischer and Brown (2001), some species can move 3-6 m in approximately two months, or 1.5-2.0 m per month, which is similar to the rate observed in the current study. Passive dispersion by rain, wind, soil, humans or insects may occur over kilometers whereas active dispersal generally occurs over only a few centimeters (Smart and Nguyen 1994).
The high percentage of G. mellonella infected by S. brazilense in control-mulch group, not significantly different ([F.sub.39 360] = 29.145; P = 0.124) from the nematode-treated groups at 3th day PA (49.3%), but significantly greater ([F.sub.39 360] = 29.149; P < 0.001) compared to the control-soil group at 76th day PA (4,7%), probably reflected the greater suitability of mulch plots to enhance the nematode movement as well as to retain rainwater containing the nematodes from the treated plots, in addition to keeping the soil moist for a longer time. This result is in agreement to the study by Hsiao and All (1998), who showed that mulch can enhance the movement of some entomopathogenic nematodes in agricultural systems, with S. carpocapsae moving 3.5 cm/day on bare soil and 7.5 cm in rye mulch-covered soil.
Beyond 76 days PA, the percentages of G. mellonella infected by S. brazilense decreased in almost all treatments and remained low from 160 to 225 days PA, when the larva mortality was < 3.0%. From June to August (160-225 days PA), rainfall was low and may have accelerate a decreasing in the nematode population by drying the soil. This time has been a drought season for this region in Brazil. The level of moisture is one of the most important factors in the soil enviromnent to influence the survival, virulence and persistence of nematodes (Klein 1990).
Beyond 225 days PA, the nematode population started to increase again, resulting in 9.3% mortality of G. mellonella larvae in both control-mulch and nematode-mulch groups by 278 days PA (last evaluation). The increase in the nematode population coincided with the beginning of the rainy season. Our results show that S. brazilense survived the unfavorable conditions associated with winter dry season persisting in the soil by surviving as IJs or by recycling in the insect cadaver, for at least 278 days since its application.
S. brazilense can persist in the sugarcane field for at least 278 days after its application on the soil, and the mulch does not affect its persistence, favoring its spread compared to bare soil. The percentage of G. mellonella infected by the nematode decreased as the time approached to the draught season, dropping significantly after one-two month from the nematode application.
To Fundacao de Amparo a Pesquisa do Estado de Sao Paulo-FAPESP for financial support, and to Abengoa Bioenergia Corporation for the experimental support.
Received: 26-Jun-2014 * Accepted: 25-Nov-2015
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LUIS GARRIGOS LEITE (1,2), FABIO SILBER SCHMIDT (1,3), RICARDO HARAKAVA (1,4), ANTONIO BATISTA FILHO (1,5), FERNANDO HENRIQUE CARVALHO GIOMETTI (1,6), TATIANE DE CASTRO PIETROBON (1,7) and JULIE CHACON-OROZCO (8)
(1) Instituto Biologico, CEIB. CP 70, 13001-970, Campinas, SP, Brazil. (2) Ph. D. firstname.lastname@example.org. Corresponding autor. (3) M. Sc. email@example.com. (4) Ph. D. firstname.lastname@example.org. (5) Ph. D. email@example.com. (6) M. Sc. firstname.lastname@example.org. (7) M. Sc. email@example.com. (8) M. Sc. Universidade Federal Rural do Rio de Janeiro, Programa de pos-graduacao em Fitossanidade e Biotecnologia Aplicada, Rodovia BR 465, Km 7, 23851-970, Seropedica, RJ, Brazil. firstname.lastname@example.org.
Table 1. Mortality (%) oiGaHeria mellonella larvae ([+ o -] standard error) caused by the nematodes Steinernema brazilense IBCB 116 (S) and Heterorhabditis sp. (indigenous) (H) in the S. brazilense-treated groups, with mulch (Nema-mulch) and without mulch (Nema-soil), and in the respective controls groups, with mulch (Control-mulch) and without mulch (Control-soil). Evaluation dates and days before and after nematode application Treatment 10/12/07 17/12/07 -14 -7 S Control-mulch 0.0 0.0 a a Nema-mulch 0.0 a 0.0 a Control-soil 0.0 0.0 a a Nema-soil 0.0 0.0 a a H Control-mulch 13.3 [+ o -] 3.0 8.0 [+ o -] 4.9 Nema-mulch 15.9 [+ o -] 6.9 5.3 [+ o -] 3.9 Control-soil 12.0 [+ o -] 3.9 2.7 [+ o -] 2.7 Nema-soil 13.3 [+ o -] 2.1 2.7 [+ o -] 1.6 Treatment 27/12/07 08/01/08 3 15 S Control-mulch 0.0 0.0 a a Nema-mulch 49.3 [+ o -] 1.6 45.3 [+ o -] 4.4 h gh Control-soil 0.0 0.0 a a Nema-soil 49.3 [+ o -] 4.5 40.0 [+ o -] 5.6 h gh H Control-mulch 18.7 [+ o -] 6.8 4.9 [+ o -] 4.9 Nema-mulch 9.3 [+ o -] 4.5 3.3 [+ o -] 3.3 Control-soil 13.3 [+ o -] 2.1 4.9 [+ o -] 4.9 Nema-soil 9.3 [+ o -] 3.4 2.1 [+ o -] 2.1 Treatment 25/01/08 09/03/08 32 76 S Control-mulch 0.0 34.0 [+ o -] 8.6 a fgli Nema-mulch 40.0 [+ o -] 3.0 21.3 [+ o -] 3.9 gh defg Control-soil 0.0 4.7 [+ o -] 2.5 a abc Nema-soil 36.0 [+ o -] 3.4 28.0 [+ o -] 6.5 fgli efgh H Control-mulch 13.3 [+ o -] 3.7 4.0 [+ o -] 2.7 Nema-mulch 8.0 [+ o -] 6.5 2.7 [+ o -] 2.7 Control-soil 14.7 [+ o -] 3.9 0.0 Nema-soil 6.7 [+ o -] 2.1 0.0 Treatment 07/04/08 01/06/08 105 160 S Control-mulch 6.7 [+ o -] 4.2 0.0 abc a Nema-mulch 14.7 [+ o -] 3.9 1.3 [+ o -] 1.3 cdef ab Control-soil 5.3 [+ o -] 2.5 1.3 [+ o -] 1.3 abc ab Nema-soil 8.0 [+ o -] 2.5 2.7 [+ o -] 2.7 abed ab H Control-mulch 0.0 0.0 Nema-mulch 6.7 [+ o -] 6.7 0.0 Control-soil 13.3 [+ o -] 6.3 0.0 Nema-soil 0.0 0.0 Treatment 05/08/08 27/09/08 225 278 S Control-mulch 0.0 9.3 [+ o -] 1.6 a bede Nema-mulch 2.7 [+ o -] 2.7 9.3 [+ o -] 4.4 ab abed Control-soil 0.0 4.0 [+ o -] 2.7 a abc Nema-soil 1.3 [+ o -] 1.3 ab 8.0 [+ o -] 3.3 abed H Control-mulch 0.0 10.7 [+ o -] 9.1 Nema-mulch 0.0 5.3 [+ o -] 2.5 Control-soil 0.0 5.3 [+ o -] 5.3 Nema-soil 0.0 9.3 [+ o -] 3.4 Means followed by the same letter in each row and column for S. brazilense (S) did not differ significantly by the Tukey test (P < 0.05). The data were arcsine x[raiz cuadrada de (100)] transformed prior to analysis.
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|Author:||Leite, Luis Garrigos; Schmidt, Fabio Silber; Harakava, Ricardo; Filho, Antonio Batista; Giometti, Fe|
|Publication:||Revista Colombiana de Entomologia|
|Date:||Jul 1, 2015|
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