Rhyssomatus subtilis, the black soybean weevil, has emerged as a major pest of soybean [Glycine max (L.) Merr. (Fabales: Fabaceae)] in northwestern Argentina during the last 3 yr. This species was detected in 30 localities of Salta, Santiago del Estero and Tucumán provinces comprising a total area of about 541,000 ha. This species was observed for the first time developing on dry bean [Phaseolus vulgaris L. (Fabales: Fabaceae)] crops and feeding on 3 unrelated weeds [Conyza bonariensis (L.) Cronquist (Asterales: Asteraceae), Brassica campestris (L.) Metzg. (Brassicales: Brassicaceae) and Sphaeralcea bonariensis (Cav.) Griseb (Malvales: Malvaceae)]. The damage produced by R. subtilis in dry bean crops is similar to that produced in soybean crops. The results obtained suggest that as of 2012, R. subtilis has greatly expanded its distribution since its first detection in 2006 and produces major direct and indirect damage in soybean and dry bean crops in northwestern Argentina.
The genus Rhyssomatus Schoenherr, 1837 includes 176 Neotropical and Nearctic species, many of which have agricultural importance because they are phytophagous during the larval and adult stages (O'Brien & Wibmer 1982; Wibmer & O'Brien 1986; Costa et al. 1988; Socías et. al. 2009).
Rhyssomatus subtilis Fiedler (Coleoptera: Curculionidae) “the black soybean weevil”, has emerged as a major soybean pest in Northwest Argentina (NWA) during the last 3 yr. This species was recorded for the first time on soybean Glycine max (L.) Merr. (Fabales: Fabaceae) in 2006 in the vicinity of La Fragua (Santiago del Estero province). In the following season, it was detected at Rosario de la Frontera (Salta province), and in 2008 it was observed in Nueva Esperanza (Santiago del Estero province), and 7 de Abril (Tucumán province) (Table 1 and Fig. 1) (Socías et al. 2009). From its first detection in La Fragua until 2012, R. subtilis has been detected more frequently in soybean cultures in different localities of Santiago del Estero, Salta, and Tucumán provinces. In Mexico, another species of Rhyssomatus has been detected in soybean crops, which was identified as R. nigerrimus Fahraeus, 1837. The biology, damage, and behavior of R. nigerrimus in the field are similar to those described for R. subtilis (Lopez-Guillén et al. 2012). The occurrence of R. subtilis causes significant losses in the NWA region. Adults produce damage by their feeding and oviposition during vegetative and reproductive stages of the soybean (Socías et al. 2009). The control of this pest is based mainly on the use of chemical insecticides (Casmuz et al. 2010a). Nevertheless, when the number of the black soybean weevils is high, the entire soybean crop is lost and has to be reseeded.
Given that R. subtilis had not been found in these areas before, it could be argued that this species is expanding its range in the region. For this reason, the objective of this study was to determine the geographical distribution and host associations of this species of Curculionidae in NWA.
Samplings from 2009 to 2012 were conducted to detect the presence of R. subtilis (Fig. 1) in different provinces of NWA. All localities monitored (42) were georeferenced. Samplings were made in soybean and dry bean [Phaseolus vulgaris L. (Fabales: Fabaceae)] growing areas during vegetative and reproductive stages. One hectare was monitored in each locality. In each hectare, 10 points with 10 to 15 plants of soybean and/or dry bean were sampled. By observations on the plants using a vertical beat sheet (Drees & Rice 1985), the presence of weevils was detected. The immature stages (eggs and/or larvae) were estimated by observation on the plants of damaged pods.
On soybean and dry bean crop plantations, R. subtilis was collected in 30 localities in Salta, Santiago del Estero and Tucumán provinces, located in an area of about 541,000 ha (Table 1 and Fig. 1). In general, the presence of the black soybean weevil was concentrated in localities in southern Salta, in northwestern Santiago del Estero, and in northeastern Tucumán province (Fig. 1).This species was not found in Catamarca province, which is southwest of Santiago del Estero province and the south of Tucumán province.
In dry bean plantations, the weevil was observed in 2 localities each of Tucumán (Tapia and Trancas), and Salta provinces (Rosario de la Frontera and La Candelaria) (Table 1 and Fig. 1). Preliminary observations showed that adults feed on the crop both in its vegetative and reproductive stages. The crop in its vegetative stage is attacked by adults that feed on tender sprouts of both seedlings and well-developed plants. The damage caused during the vegetative stage of dry bean can cause the death of the plant or negatively affect its development. During the plant's reproductive stage (R8: pod fill) (Fernández et al. 1986), the female weevils deposit their eggs inside the pods (Fig. 2). After that, the larvae hatch and feed on the seed; and at the end of their last larval stadium, these larvae fall to the soil. In the soil, the individuals continue their development as hibernating larvae, pupae and adults during the winter season. Subsequently with the arrival of the first rains, the emergence of adult occurs.
GEOGRAPHICAL DISTRIBUTION AND HOST ASSOCIATION OF RHYSSOMATUS SUBTILIS (COLEOPTERA: CURCULIONIDAE) IN DIFFERENT PROVINCES OF ARGENTINA.
It is important to mention that the damage produced by weevils on pods (feeding and/or oviposition), facilitates the development of phytopathogens in the seeds, which may lower the quality and performance of bean seed. The behavior and the damage produced by of R. subtilis on dry bean crop are similar to that produced in the soybean crop (Socías et al. 2009). Therefore, the next important steps are to identify the duration of the life stages and to quantify the damage caused by this weevil to the dry bean crop.
In Rosario de la Frontera (Salta province), during the spring season, and before soybean sowing, R. subtilis adults were found on the following weeds: Conyza bonariensis (L.) Cronquist (Asterales: Asteraceae), Brassica campestris (L.) Metzg. (Brassicales: Brassicaceae) and Sphaeralcea bonariensis (Cav.) Griseb (Malvales: Malvaceae) (Table 1). This weevil was observed feeding on these weeds during their reproductive stages. Ovipositor perforations and eggs were not found in these plants. These are not true hosts, because — although the adults feed on them—the females do not oviposit in the plants and they are not utilized by the larvae. This observation was reported by Bentancourt & Scatoni (1996); Capinera (1999), and Casmuz et al. (2010b). They mentioned that some host plants such as grasses and various pasture weeds, play an important role as reservoirs for the insect pests, which move from one to another species of grasses or crops, but are not true hosts, i.e., the pest does not reproduce on them. In Rosario de la Frontera (Salta province) farms where R. subtilis adults were found on the above mentioned weeds, which had sustained intense attacks by the weevils in the previous year. It is valuable to know that these weevils can be found feeding on a range of plants not accepted for oviposition and larval development. This makes it clear that prior to the time of growth of the soybean or dry bean crops, it is possible for the weevils to seek alternate hosts for potential infestation. This could be beneficial for those who are studying the weevil and its range extension. This is the first report of this species developing on dry bean and feeding on the weeds mentioned. The results obtained in this study indicate that R. subtilis is expanding its distribution and produces direct and indirect damage on soybean and dry bean crops in northwestern Argentina. From its first detection in La Fragua in 2006 until 2012, this pest has dispersed about 87 km north, 136 km south and 99 km west in northwestern Argentina. Currently, this species of Curculionidae is located in an area of about 541,000 ha (Fig. 1).The expansion of the range of this pest could be facilitated by anthropic factors such as movement of agricultural machinery from weevil infested areas to weevil free areas. Similar observations were reported for Aubeonymus mariaefranciscae (Coleoptera: Curculionidae) (Giraldo & Alvarado 1990), an important pest of beet-root (Beta vulgaris L.), where its expansion is facilitated primarily by agricultural machinery. Therefore, it is crucial to adopt preventive measures to avoid or delay the expansion of R. subtilis to new regions.
We thank Javier Carreras Baldres (SrySig, EEAOC) for his assistance in the preparation of the map. This study was supported by Estación Experimental Agroindustrial Obispo Colombres.