The weevil Scyphophorus acupunctatus Gyllenhal (Coleoptera: Curculionidae) is considered the most important pest of agaves worldwide (Solís-Aguilar et al. 2001; González et al. 2007). Both larvae and adults of S. acupunctatus cause direct damage while feeding, which is estimated to be about 10 to 25% of the total volume of the agave bole (Solís-Aguilar et al. 2001; González et al. 2007; Aquino Bolaños et al. 2007; Figueroa-Castro et al. 2013). Additionally, the weevil may be a vector of agave pathogens, and wounds caused during weevil feeding may serve as an entry point for these pathogens (Waring & Smith 1986; Aquino Bolaños et al. 2011; González et al. 2007). For instance, there is a positive relationship between blue agave plants with symptoms of agave bole rot disease and level of weevil infestation. At high levels of infestation, approximately 70% of the total volume of the agave bole can be damaged (Figueroa-Castro et al. 2013). The agave weevil has several generations per year and its life cycle occurs inside the agave plant, making detection, monitoring, and management difficult.

As part of the integrated pest management strategies for this weevil, there is an increasing interest in developing a semiochemical-based system for monitoring or mass trapping of this pest. A number of studies have investigated the chemical ecology of this weevil, resulting in the identification of an aggregation pheromone (Ruiz-Montiel et al. 2003; Rojas et al. 2006; Ruiz-Montiel et al. 2008; Rodríguez-Rebollar et al. 2012). These studies also reported that the response of agave weevil to the pheromone is synergized by the presence of the host plant volatiles. Currently, a trapping system using traps baited with aggregation pheromone plus fresh agave tissue is used for monitoring S. acupunctatus at blue agave (Agave tequilana Weber var. Azul; Asparagaceae) plantations (SENASICA 2014). However, little is known about the factors that affect the efficiency of the food bait used for monitoring the agave weevil. In this study, we investigated the effect of amount of fresh agave tissue and bagging of the plant tissue on capturing S. acupunctatus in blue agave.

Materials and Methods

Results

EFFECT OF FOOD BAIT AMOUNT ON WEEVIL CAPTURE

In the first experiment, the number of agave weevils captured was affected by the treatment used (F = 7.40; df = 4,15; P = 0.0017). Traps baited with pheromone plus 400 g of agave captured more weevils in comparison to traps baited with pheromone alone or 200 g of agave alone. The number of weevils caught by traps baited with 200 g of agave plus pheromone and 600 g of agave plus pheromone were intermediate and not significantly different from capture by traps baited with pheromone alone, 200 g of agave alone, or 400 g of agave plus pheromone (Fig. 1A). The traps captured more females than males (X² = 93.61; P = < 0.0001), although apparently not in the case of agave tissue used alone as bait (Table 1).

In the second experiment, the capture of weevils was affected by treatment (F = 10.69; df = 6,21; P = < 0.0001). Traps baited with pheromone plus 400 g of agave captured more weevils compared to traps baited with pheromone alone, 200 g of agave alone, and 200 g of agave plus pheromone. Captures of traps baited with 100 g of agave plus pheromone, 300 g of agave plus pheromone, and 500 g of agave plus pheromone were intermediate and not significantly different from the captures in traps baited with 400 g of agave plus pheromone (Fig. 1B). In the same manner as the previous experiment, the traps captured more females than males (χ2 = 43.76; P = < 0.0001) (Table 1) except that traps baited with agave alone captured similar numbers of females and males.

Fig. 1.

Mean (+ SE) number of Scyphophorus acupunctatus captured per trap baited with different amounts of fresh agave tissue in 2 trials. The 1st trial was performed from 18 Jan to 8 Feb 2013 (A), while the 2nd trial was carried out from 11 Apr to 23 May 2013 (B). Bars with the same letter are not significantly different (Tukey's, α = 0.05).

Discussion

We confirmed that host volatiles synergize the attraction of the agave weevil to its synthetic aggregation pheromone. Traps baited with pheromone alone or agave tissue alone captured fewer weevils than traps baited with pheromone plus agave. Earlier, Rojas et al. (2006) pointed out the possibility of synergism between the synthetic pheromone and the agave tissue in trapping this weevil. García-Ramírez et al. (2014) performed field experiments to investigate the influence of the food bait (200 g of pineapple) on the capture of S. acupunctatus in Mexican tuberose (Polianthes tuberosa L.; Asparagaceae) crop. In 1 trial they found that traps baited with synthetic pheromone plus food bait captured a higher number of weevils than traps baited with pheromone alone, but in a 2nd trial no effect of the food bait was found. The fact that plant volatiles synergize the responses of weevils to its pheromone has been reported for several species of the Rhynchophorinae subfamily (Gries et al. 1994; Oehlschlager et al. 1993; Giblin-Davis et al. 1996; Hallett et al. 1999; Rochat et al. 2000; Wibe et al. 2014). For example, Gries et al. (1994) found that traps baited with palm tissue or pheromone captured few Rhynchophorus phoenicis F. (Coleoptera: Curculionidae), in comparison to traps baited with palm pieces plus pheromone. Hallet et al. (1999) reported that the addition of palm volatiles synergized the attraction of Rhynchophorus ferrugineus (Olivier) (Coleoptera: Curculionidae) to its aggregation pheromone. The addition of the major flower volatile from wild strawberry flowers, Fragaria vesca L. (Rosaceae), to the aggregation pheromone of Anthonomus rubi Herbst (Coleoptera: Curculionidae) increased the trap captures by over 2—fold, as compared to the pheromone alone (Wibe et al. 2014). In contrast, Tafoya et al. (2007) found that the addition of prickly pear did not synergize the attraction of Metamasius spinolae (Gyllenhal) (Coleoptera: Curculionidae) to its aggregation pheromone.

Table 1.

Percent of Scyphophorus acupunctatus females and males captured in baited traps.

Our results indicate that the amount of agave tissue influenced the capture of S. acupunctatus. In the 2 trials, we found that traps baited with 400 g of agave tissue captured more weevils than traps baited with other amounts of agave. Previously, Giblin-Davis et al. (1996) found that traps baited with 1.5 kg of chopped Sabal palmetto stem plus aggregation pheromone, cruentol, captured almost 3 times more Rhynchophorus cruentatus (F.) (Coleoptera: Curculionidae) than traps baited with 0.5 kg of palm tissue plus cruentol. Tinzaara et al. (2007) reported that traps baited with pheromone plus 100 or 500 g of tissue of banana stem (fermented) captured a similar number of banana weevils as traps baited with pheromone alone; however, traps baited with pheromone plus 1,000 g of stem tissue of banana had higher weevil numbers. Al-Saoud & Ajlan (2013) found that pheromone-traps baited with 550 g of date fruit captured fewer red palm weevils than traps baited with pheromone plus 350 or 450 g of date fruit. In our case, it is likely that lower amounts of agave tissue emit less of the volatile compounds, which would explain the lower captures of S. acupunctatus. Interestingly, a larger amount of agave likely would produce more volatiles, and this also seemed to affect the attraction of the weevils, though the reason for this is not apparent. Thus, our data suggest that there is an optimal amount of plant tissue for trapping S. acupunctatus.

Fig. 2.

Mean (+ SE) number of Scyphophorus acupunctatus captured per trap baited with pheromone plus bagged or unbagged fresh agave tissue. The experiment was performed from 24 May to 14 Jun 2013. Bars with the same letter are not significantly different (Tukey's, α = 0.05).

Bagging the food bait affected the capture of S. acupunctatus. One explanation for this result is that the plastic bag and plastic container help the agave tissue to maintain freshness longer, thus allowing the agave tissue to release volatiles longer in comparison to unbagged food bait. Unbagged agave tissues dry sooner in comparison to bagged plant material. However, we do not know if weevils captured by traps baited with unbagged agave tissue occurred during the 1st days, when the agave was not dry, which would support this explanation. Another possibility is that the bagged agave could produce fermentation volatiles that synergize the attraction of S. acupuncatus to its aggregation pheromone. However, Rodríguez-Rebollar (2011) did not find any differences in the captures of S. acupuncatus by pheromone-baited traps plus agave tissue bagged for 1, 5, 10, and 15 d. Although, agave weevils can be attracted to plant volatiles (Altuzar et al. 2007), they also respond to general fermentation volatiles (Valdés-Rodríguez et al. 2005). Future studies could clarify whether specific volatiles from agave plants or general fermentation volatiles are the pheromone synergist in agave weevil.

In all experiments, more females than males were captured, although the sex ratio for S. acupunctatus on agave plants is 1:1 (Figueroa-Castro et al. 2013). Previous studies have reported similar results when trapping this weevil species in agave and Mexican tuberose crops (Ruiz-Montiel et al. 2008, López-Martínez et al. 2011, Rodríguez-Rebollar et al. 2012, Figueroa-Castro et al. 2013, Figueroa-Castro et al. 2016a, b, c). Our results indicate that the synthetic aggregation pheromone could be responsible for attracting more females than males to the traps because traps without pheromone (i.e., agave tissue alone) captured equivalent proportions of females and males. Traps baited with pheromone, even traps in which pheromone alone was used, caught more females than males. Our hypothesis is that synthetic aggregation pheromone could work as sexual signal sometimes, but this needs to be studied in future studies. With respect to this, Figueroa-Castro et al. (2015) noted that colonization of new agave plants could be started by females and males. In boll weevil, the male pheromone attracts both sexes (aggregation pheromone), or only females (sex pheromone), depending on time of year (Hardee & Harris 2003). In agave weevil, however, pheromone-baited traps capture more females than males during the entire year (Figueroa-Castro et al. 2013).

In summary, this study showed that the amount of food bait, and bagging the food bait, in the pheromone-baited traps affected the number of captured agave weevils. We also confirmed the synergism between the food bait and the synthetic aggregation pheromone. We suggest that traps baited with the synthetic aggregation pheromone plus 400 g fresh agave tissue bagged in plastic bags with holes be used for monitoring or mass trapping the agave weevil.