The weevil Aclees taiwanensis Kono 1933 (Coleoptera: Curculionidae), endemic to Taiwan, native to Asian subtropical and tropical regions (Hong et al., 2020;Meregalli et al., 2020a;Meregalli et al., 2020b), is continuously expanding its invasion. In France, the beetle was first detected on ornamental Ficus retusa (Rosales: Moraceae) in the greenhouse in the Provence- Alpes-Côte d’Azur (Perrin, 1997), but did not occur outdoors until 2019 (Mouttet et al., 2020). It was found in Italia in 2005 (Ciampolini et al., 2005), then quickly spread to many areas of the country (Farina et al., 2021). The species was formerly mis-identified as A. cribratus and subsequently as Aclees sp. cf. foveatus until the late revision by Meregalli et al. (2020a) certainly confirmed that A. taiwanensis had been introduced to Europe. The weevil is thought to have spread internationally mainly via the trade of contaminated fig plants and plant materials. The transportation of nursery stock, cuttings, and other plant components can unintentionally introduce the weevil to new regions (Hong et al., 2020;Mazur et al., 2024). Recently, the presence of A. taiwanensis has been documented in the southern region of the Korean Peninsula (Hong et al., 2020), which is closely genetically related to the Taiwanese colony (Kim et al., 2022).

A. taiwanensis infests plants within the genus Ficus, but the economic loss is primarily associated with F. carina, which is widely cultivated for fruit production (Farina et al., 2021;Hong et al., 2022). Adults feed on buds, leaves, and young fruits, but significant damage to fig plants is through their larval activities. The xylophagous larvae tunnel into the trunk near the soil surface, causing root damage and disrupting the plant's phloem flow, which ultimately leads to tree dieback (Farina et al., 2021). Additionally, detecting larval feeding activity at the onset of an attack is challenging since fig trees initially display no symptoms. By the time symptoms are noticeable, the infestation is usually well-advanced, rendering treatment ineffective (Bernardi et al., 2022;Ciampolini et al., 2007). As a result, eliminating larvae already residing within the trunk proves challenging and is often less effective than stopping adult weevils from laying additional eggs that cause further damage (Ciampolini, 2008).

Initial studies on environmentally sustainable approaches to managing this pest have been documented, though their effectiveness has fallen short of expectations. Iovinella et al. (2020) found that volatile organic compounds (VOCs) released by adult weevils of both sexes displayed characteristics of sexual attractants, indicating the potential use of pheromones for field monitoring of this species. Cutino et al. (2023) also reported the use of a synthetic, adhesive band wrapped around fig tree trunks to trap adult weevils in combination with the entomopathogenic fungus Beauveria bassiana. However, under field conditions, the mortality caused by the fungus alone remained relatively modest (Cutino et al., 2023;Gargani et al., 2017). Plant extracts such as olive and onion oil demonstrated highly promising efficacy, achieving up to 100% mortality within 3 to 24 hours of treatment (Gargani et al., 2021), but no large-scale field trials or commercial applications have been widely reported for fig weevil control. Given the swift rise in population and expanding geographic spread of A. taiwanensis, it is crucial to pursue effective preventive strategies, with particular emphasis on chemical control methods. To date, pyrethroids and spinosad are the only compounds that have been reported for controlling A. taiwanensis infestations (Ciampolini, 2008;Lodolini et al., 2020). Unfortunately, no chemical pesticides are currently registered for A. taiwanensis in either the EU or South Korea. Data on the efficacy of alternative chemical treatments against A. taiwanensis remains limited. Consequently, there is a pressing need to establish a robust control strategy and identify a range of highly effective pesticides to support farmers with reliable management options and facilitate chemical rotation to mitigate potential resistance in this pest species.

In this study, we assessed the effectiveness of various commercially available insecticides in South Korea for managing the population of this pest. The control efficacy and toxicity profiles of each insecticide group are also discussed.

Materials and Methods

Insect source

Adults of A. taiwanensis of unknown age were collected from fig farms in Samho-eup, Yeongam-gun, Jeollanam-do (34.749059N, 126.510320E) starting in early May 2024. The insects were then maintained in insect breeding cages (40 cm × 40 cm × 40 cm) in the laboratory (T = 26 - 28°C, RH = 40 - 60 %, with 16L:8D photoperiod) for the experiment. Fresh fig leaves and water-soaked cotton were provided as food for the weevil.

Pesticides used

Pesticides used in this study were purchased from the local market. Among them, twenty pesticides contain a single active ingredient (a.i.), representing various chemical classes including carbamates, organophosphates, pyrethroids, neonicotinoids, spinosyns, nereistoxin analogues, benzoylureas, diamides, and isoxazolines. The remaining three formulations were mixtures, each containing two a.i. All pesticides were applied based on concentrations recommended by the respective suppliers (Table 1).

Efficacy testing trials

All pesticides above were diluted in water at 3 concentration levels, including half (0.5x), equal (1x), and double (2x) the recommended concentration (x). Each concentration was applied to 10 weevils with 3 replicates (n=30). The control was water-treated.

The weevils were dipped into the diluted pesticide solutions for 2 seconds and returned to the breeding dish (d = 100 cm × h = 40 cm; SPL Life Sciences, Gyeonggi, Korea) for mortality observation. One hour after treatment, a piece of fresh fig leaf was added to the dishes for the insects as food. The leaves were changed daily until the experiment was finished. The number of weevils that died in each dish was recorded 3 and 7 days after the treatment.

Acute toxicity test

Pesticides that resulted in >40% mortality after 3 days of treatment at the 1x concentration were selected for the acute toxicity test. Complementary dilutions of 0.25x and 1.5x the recommended concentration were added for the range test. The experimental protocol and sample size were consistent with the previous experiment. Mortality was recorded 3 days after treatment to estimate the 72-hour lethal concentration 50% (72hr-LC₅₀).

Data analysis

Mortality was corrected using the Abbott formula (Abbott, 1925). Data were subjected to a balanced ANOVA test followed by the post-hoc Tukey HSD to determine the differences among treatments. The LC₅₀ value and 95% confidence interval were estimated by probit analysis (Finney, 1971), where the corrected response mortality probit value was fitted against the log of the tested concentration (log10 ppm). The linear regression model parameters were estimated using Maximum Likelihood. Data analysis and graph design were performed using Minitab v.21 software (Minitab LLC, State College, PA, USA).

Results

Carbamate, Organophosphate, and Nereistoxin analogue insecticides were highly effective against A. taiwanensis, resulting in over 76.7% insect mortality 7 days post-treatment. Cartap hydrochloride demonstrated the highest control efficacy across all three dilutions, achieving a 100% mortality rate. The remaining insecticides exhibited low efficacy, with 8 out of 23 showing less than 10% effectiveness after 7 days of monitoring (Table 2).

The acute toxicity test results of four pesticides, classified under carbamate and organophosphate, are presented in Table 3. The goodness-of-fit test indicated that the model's probabilities closely matched the observed mortalities. The concentration of the tested pesticides had a statistically significant impact on the mortality response of A. taiwanensis (p < 0.001), with positive coefficients observed.

The 72-hour LC₅₀ estimates for carbaryl, carbosulfan, fenitrothion, and phenthoate were 327.60, 139.52, 578.51, and 500.35, respectively. The LC₅₀ confidence intervals for the two carbamate active ingredients (a.i.) fell below the recommended concentration, whereas the active ingredients from the organophosphate group exceeded their recommended concentrations (Fig. 1). This indicates that A. taiwanensis was more sensitive to carbamate than to organophosphate.

Discussion

To date, the fig weevil (FW) continues to expand swiftly and inflict substantial damage across fig cultivation regions (Farina et al., 2021;Hong et al., 2020;Kim et al., 2022;Mazur et al., 2024;Tani et al., 2023). This ongoing spread presents a major challenge for effective management, as previously tested control measures have yet to deliver satisfactory outcomes (Ciampolini, 2008;Cutino et al., 2023;Gargani et al., 2017).

Our findings indicate that 16 out of 23 tested insecticides exhibited limited effectiveness against this species, with mortality rates remaining below 70% after seven days (Table 2). Despite earlier recommendations by Ciampolini (2008), six pyrethroids evaluated in our study demonstrated poor control performance, even when applied at double the recommended concentration (Table 2). This aligns with a broader research showing widespread resistance of weevils to multiple pyrethroid compounds across the western United States (Rodbell et al., 2023). Spinosyns—the only tested Spinosad a.i. in our study— did not produce a significant increase in mortality. This contrasts with the results reported by Lodolini et al. (2020), potentially due to differences in application method, as Spinosad may be more toxic for A. taiwanensis when ingested rather than applied through the contact mode of action, as was the case in our trial.

In terms of contact mode of action, cartap hydrochloride emerged as the most effective insecticide, achieving 100% mortality in the tested insects even at a diluted concentration of 50% of the recommended level. Direct studies of the nereistoxin on the subfamily Molytinae or other weevils are limited, but they were highly effective against chewing and sucking pests, including lepidopterans and homopterans, with potential for broader pest control applications (Yan et al., 2024). In addition, carbamate and organophosphate insecticides are considered acceptable options, as they have demonstrated broad-spectrum activity against various weevil species (Vale and Bradberry, 2017). Both classes inhibit acetylcholinesterase (AChE), disrupting nerve function and causing paralysis in exposed insects. Organophosphates bind irreversibly to AChE, while carbamates bind reversibly (Fukuto, 1990), which may account for the differences in mortality observed in our study.

This study provides initial toxicity data for several commercially available insecticides against the fig weevil. Given the hardened cuticle and cryptic feeding behavior of A. taiwanensis, reliance on contact-mode applications alone is unlikely to achieve satisfactory control. Further research should investigate the systemic properties of potential control agents, considering the weevil’s chewing mouthparts, and should include field trials to corroborate laboratory results. Notwithstanding these constraints, the findings supply a preliminary baseline of susceptibility to common pesticides for informing management options, including the chemical rotation strategies for IPM.