MONITORING OF RESISTANCE TO THREE INSECTICIDES ON PEPPER WEEVIL (Anthonomus eugenii) IN POPULATIONS FROM BAJA CALIFORNIA SUR, MEXICO

Rosalía Servín, Ricardo Aguilar, José L. Martínez, Enrique Troyo and Alfredo Ortega

Rosalía Servín Villegas. Biologist, Instituto Politécnico (IPN), Ciudad de México. M.Sc. in Animal Biology, Universidad Nacional Autónoma de México (UNAM). Investigador Titular, Centro de Investigaciones Biológicas del Noroeste, S.C. (CIBNOR). Address: Mar Bermejo #195, Playas El Conchalito Apdo. Postal 128, La Paz, B.C.S., México, 23090 e-mail: rservin@cibnor.mx

Ricardo Aguilar Aguilar. Biologist, Universidad Autónoma de Nuevo León, México. M.Sc. in Animal Biology, UNAM. Técnico Académico Titular, CIBNOR. e-mail: aguilar6@prodigy.net.mx

José L. Martínez Carrillo. Agronomical Engineer, Universidad Autónoma de Sinaloa, México. M.S. and Ph.D. in Entomology, University of California at Riverside. e-mail: jlmc@cirno.inifap.conacyt.mx

Enrique Troyo Dieguez. M.Sc and D.Sc, UNAM. Investigador Titular, CIBNOR. e-mail: etroyo@cibnor.mx

Alfredo Ortega. M.Sc. and D. Sc., IPN. Investigador Titular, CIBNOR. e-mail:aortega@cibnor.mx.

Toxicity levels of three insecticides, carbaryl, endosulfan and methomyl, were determined in populations of the pepper weevil Anthonomus eugenii. Insect populations were obtained from three locations in Baja California Sur, Mexico: Benito Juárez (27º53’N, 113º46’W), San José Viejo (23º07’N, 109º43’W), and San Juan de los Planes (23º58’N, 109º56’W). The highest LC50 values were found for carbaryl (178.4, 635.4 and 5462.1 µg/ml), endosulfan was intermediate (36.5, 34.1 and 47.1 µg/ml, respectively); and the lowest LC50 values corresponded to methomyl (1.3, 1.7 and 2.7 µg/ml), to Benito Juárez, San José Viejo and San Juan de los Planes, respectively. It is concluded can be inferred that pepper weevil populations in the farming zone of San Juan de los Planes are resistant to the three insecticides used, and are significantly more resistant to carbaryl, than are pepper weevil populations in San José Viejo and Benito Juárez. Levels of insecticide susceptibility correspond to the amounts of insecticides used against the pepper weevil in the three locations, and are a response to the selective pressure exerted by those chemicals.

KEYWORDS / Anthonomus eugenii / Carbaryl / Endosulfan / Insecticides / Methomyl / Pepper Weevil /

Se determinaron los niveles de toxicidad en poblaciones de picudo del chile Anthonomus eugenii a tres insecticidas, carbaril, endosulfan y metomil. Las poblaciones de insectos fueron obtenidas de tres localidades en Baja California Sur, México: Benito Juárez (27º53’N 113º46’W), San José Viejo (23º07’N, 109º43’W) and San Juan de los Planes (23º58’N, 109º56’W). Las CL50 más elevadas corresponden al carbaril (178,4;635,4 y 5462,1 µgr/ml); endosulfan tuvo valores intermedios (36,5; 34,1 y 47,1 µg/ml); mientras que los valores más bajos correspondieron a metomil (1,3; 1,7 y 2,7 µg/ml), para Benito Juárez, San José Viejo y San Juan de los Planes respectivamente. Se puede concluir que las poblaciones de picudo del chile de San Juan de los Planes son resistentes a los tres insecticidas utilizados, teniendo mayor resistencia a carbaril, en comparación con las poblaciones provenientes de San José el Viejo y Benito Juárez. Los niveles de susceptibilidad corresponden con las cantidades de insecticidas usados en contra del picudo del chile en las tres localidades de estudio y son una respuesta a la presión selectiva ejercida por esos compuestos.

Se determinaram os níveis de toxicidade em povoações de picudo do chile Anthonomus eugenii a tres inseticidas, carbaril, endosulfan e metomil. As povoações de insetos foram obtidas de três localidades em Baixa California Sul, México: Benito Juárez (27º53’N 113º46’W), San José Viejo (23º07’N, 109º43’W) e San Juan de los Planes (23º58’N, 109º56’W). As CL50 mais elevadas correspondem ao carbaril (178.4,635.4 e 5462.1 µgr/ml); endosulfan teve valores intermédios (36.5, 34.1 e 47.1 µg/ml); enquanto que os valores mais baixos corresponderam a metomil (1.3, 1.7 e 2.7 µg/ml), para Benito Juárez, San José Viejo e San Juan de los Planes respectivamente. Se pode concluir que as povoações de picudo do chile de San Juan de los Planes são resistentes aos três inseticidas utilizados, tendo uma marcada resistência a carbaril, em comparação com as povoações provenientes de San José el Viejo e Benito Juárez. Os níveis de susceptibilidade correspondem com as quantidades de inseticidas usados em contra do picudo del chile nas três localidades de estudo e são uma resposta à pressão seletiva exercida por estes compostos.

Received: 06/13/2002. Modified: 09/26/2002. Accepted: 10/22/2002

The pepper weevil, Anthonomus eugenii Cano (Coleoptera: Curculionidae) is the primary insect pest of the different varieties of chili pepper grown in Mexico (Bujanos, 1993). This insect causes premature maturation, evidenced by the fall of floral buds and fruit, and by a darkening or yellowing of the fruit. The damage is caused by the larvae, which feed on the pulp and seeds of developing fruits. Flowers are susceptible to damage from ovoposition and from feeding by adult weevils, thus causing a significant reduction in the fruit production per plant (Pacheco-Covarrubias, 1985; Bernot, 1992). In Mexico, economic loss caused by the pepper weevil may range from 50 to 100% of total pepper production (Laborde and Pozo, 1982; Bujanos, 1993; Riley and King, 1994; SAGAR, 1997). To solve this problem, several types of insecticide have been used as chemical control methods, especially carbaryl, endosulfan and methomyl. Avila (1987) notes that to obtain good crops, farmers have invested up to 45% of the total crop cost in insecticides recommended for the chemical control of the pepper weevil. In response to increasing pesticide application, the pepper weevil has developed efficient resistance mechanisms against these chemicals.

In the state of Baja California Sur, despite arid conditions, the culture of chili pepper is very successful and is a significant source of employment. In the period 1993-1997, an annual average of 1600ha were planted, with a production of 28700 metric tons, and an approximate value of MX$ 70 million. However, losses due to pepper weevil damage during 1996-1997 were estimated at MX$ 10 million (SAGAR, 1997).

The effectiveness of several insecticides, either alone or mixed, for the management of the pepper weevil has been assessed in field conditions (Hernández and González, 1992; Pacheco-Covarrubias, 1993; Enríquez and Valenzuela, 1996), and the resistance of the insect to different insecticides has also been estimated using bioassays (Quiñones and Flores, 1991; López, 1996). To contribute to the efficient management of pepper weevil, this study assessed the resistance of the insect to three commonly applied insecticides, in three locations of Baja California Sur characterized by their geographic isolation from each other and by the quality of their chili pepper crop, generally exported or traded in other regions of Mexico. There are increasing restrictions for trading products contaminated with insecticides in international markets and a risk posed to domestic consumers by the increasing use of these chemicals. These reasons, combined with the evolution of the pepper weevil’s resistance to the chemicals and their costs, make it imperative for alternative insect management programs to be implemented. These programs should include regular assessment of the resistance of pepper weevil populations to different insecticides. The objective of this work was to evaluate the resistance levels of the pepper weevil to the three pesticides most commonly used in the selected locations, by determining LC50 and LC95 values, confidence limits, and dose-mortality slope values.

This study was conducted from December 1997 to March 1998, in the main chili pepper cultivating areas in Baja California Sur. The locations were Benito Juárez (27°53’N, 113°46’W) in the municipality of Mulegé; San José Viejo (23°07’N, 109°43’W) in the municipality of Los Cabos; and San Juan de los Planes (23°58’N, 109°56’W), in the municipality of La Paz. These areas are geographically isolated from each other by natural mountain barriers and by xerophytic vegetation on non-agricultural land, conforming agricultural valleys or oases. Within each location, parcels of land on which chili peppers ("Ancho San Luis" variety) are cultivated were selected. In each parcel, 10-12kg of dropped fruit were collected and deposited in plastic containers of 60cm diameter and 25cm depth. The containers were covered with a screen and kept in the laboratory until the adult insects hatched. These adult insects were collected with a microaspirator and transferred to screened wooden cages of 25 x 25 x 40cm. The most active insects were selected, eliminating those having abnormal mobility. The insects were fed until 4 days old on a diet of fresh cut chili pepper, before being used for the toxicological bioassays with the glass vial technique (Plapp et al., 1987; Cahill and Hackett, 1992).

The insecticides assessed were technical grade carbaryl, endosulfan and methomyl, which according to direct surveys of the farmers (published information was not available), are most commonly used against the pepper weevil pest. Five different concentrations with five replicates each were prepared for every insecticide, using acetone as a solvent. Each replicate included a control with acetone only. Five adult insects were introduced in 20ml scintillation vials whose caps were modified to allow the organisms to breathe, and the tubes were inverted to ensure continuous contact between the insects and the insecticides. After 24hr mortality was evaluated, considering as affected or dead any individual that was immobile or that had abnormal mobility. Bioassays were conducted at room temperature, between 22.5 and 28.5ºC, and relative humidity between 55% and 65%. The dose-mortality response lines for each chemical were obtained using the Probit program (Raymond, 1985).

The insecticide concentration lethal to 50% of the insects (LC50), the confidence range at 95% (CR 95%), the lethal concentration at 95% (LC95), the slope of each regression line, and the standard error (SE) for each population from the three studied locations are shown in Table I. For all three pepper weevil populations, methomyl had the highest toxicity, while endosulfan was intermediate and carbaryl was the least toxic.

Based on confidence range overlap (Figure 1) it was determined that sensitivity to carbaryl was significantly different among the three populations evaluated. The Benito Juárez weevils were more sensitive than those from San José Viejo, which in turn were more sensitive than those from San Juan de los Planes, where resistance to carbaryl was caused by the excessive use of this pesticide for more than 10 years (personal communication from producers), as this was the pesticide most recommended by governmental agencies.

The response to methomyl between pepper weevil populations from Benito Juárez and San Juan de los Planes showed a significant difference, but not that between populations from Benito Juárez and San José Viejo. With respect to endosulphan, there was no significant difference between the three populations evaluated, according to the overlap in the confidence ranges at the LC50 level.

Based on the results, it is evident that of the three pepper weevil populations evaluated in Baja California Sur, that from the agricultural zone of San Juan de los Planes has the greatest pesticide resistance problem, due to the selective pressure exerted by the compounds used to combat them. The populations of San José Viejo showed an intermediate response to the insecticides, while the lowest resistance was observed in the Benito Juárez populations. Carbaryl was the pesticide mostly used in the studied areas, and it was found that the doses required in San Juan de los Planes are 30 times higher than in Benito Juárez, and 8.5 times higher than in San José. Therefore, it is recommended that carbaryl no longer be used to control the pepper weevil in any of the mentioned areas. The susceptibility of the three populations to endosulfan and methomyl is quite similar. Therefore, it is suggested that if these pesticides are used, precaution should be taken to avoid repetition of the carbaryl problem. Other strategies should be implemented in order to reduce selection pressure on the pepper weevil and other insect pests in this región. These may include planting dates, destruction of plant residues, establishment of host free periods, alternative chemical or biological insecticides, and biological control.

There is little information published on pepper weevil resistance to the insecticides used in this study. Genung and Osaki (1972) refer methomyl, carbaryl and endosulfan resistance when conducting field assessments with these products. In test fields and commercial parcels Rolston (1977) found pepper weevils resistant to endosulfan and methomyl, reporting mortalities of 15% and 65%, respectively. In the agricultural valley of La Paz, Baja California Sur, Servín and Aguilar (2000) found insects resistant to carbaryl with an LC50 of 3565.7µg/ml, and to endosulfan, with an LC50 of 131.8µg/ml, consistent with the results presented in this study. According to the insecticide application record in the studied zone, the situation in San Juan de los Planes is critical.

It is important to consider that these pesticides are highly toxic to organisms other than the pepper weevil. The use of methomyl and carbaryl is restricted, as they are highly poisonous to human beings and other animals by inhalation or contact. In addition, some pesticides are unstable; 50% of the applied endosulfan residue is lost or degraded within three to seven days. The control alternatives for the pepper weevil are not encouraging; therefore, it is important to regularly assess the reincidence of this pest and to manage pesticide use in order to avoid increased resistance.

The authors thank the collaboration of Dinora Romero, Instituto Nacional de Investigaciones Forestales y Agropecuarias (INIFAP) and Amado Cota, CIBNOR, Sociedad Civil. This study was financed by the Consejo Nacional de Ciencia y Tecnología, Secretaría de Educación Pública, Centro de Investigaciones Biológicas del Noroeste, through Project AGEC-2, and by the Sistema de Investigación del Mar de Cortés through Project Code 990106010.

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