Evaluation of food attractants based on fig fruit for field capture of the black fig fly, Silba adipata (Diptera: Lonchaeidae)

The black fig fly, Silba adipata McAlpine (Diptera: Lonchaeidae), was first detected on commercial fig crops in the municipality of Ciudad Ayala, Morelos, Mexico in 2019 (NAPPO 2020) and subsequently in the states of Aguascalientes, Coahuila, Estado de México, Hidalgo, Michoacán and Puebla in 2021 (Bautista et al. 2021; (SENASICA 2021). The state of Morelos is the primary producer of fig fruit at the national level, with a production value in excess of 10 million Mexican pesos (SIAP 2023), followed by the states of Baja California Sur, Veracruz, Puebla and Hidalgo (Gobierno de México 2019).

The management of S. adipata with chemical pesticides has proved ineffective due to the cryptic nature of the larva. Consequently, the adult stage of the pest is the primary target for management strategies. In Greece, Katsoyannos (1983), observed that S. adipata adults feed on the sweet exudates emanating from overripe figs as well as on fresh or dried sap oozing from freshly removed green fruit or leaves. The author then evaluated McPhail traps baited with 2 % ammonium sulfate or 1 ml of fresh fig sap obtained from the leaves or green fruit of the fig plant and determined that these food baits were highly attractive to S. adipata. Subsequently Katsoyannos and Guerin (1984) reported that hexanol, isolated from fig leaf volatiles, was as effective as 2 % ammonium sulphate in attracting S. adipata when tested in McPhail traps. In Morelos, Mexico, Perales-Rosas et al. (2021) confirmed that ammonium sulphate at 2 and 4 % was highly attractive to S. adipata. However, Illescas-Riquelme (2022) concluded, upon evaluating various food attractants (molasses, hydrolysed protein, vinegar + yeast, wine and fig juice), including hexanol plus 3 % ammonium sulphate in McPhail traps in the states of Jalisco and Morelos, Mexico, that these food attractants were not effective for use in the field. Therefore, considering the controversy surrounding the results obtained by different authors and the importance that the fig currently has in Morelos, Mexico, the aim of this study was to evaluate various food attractants produced by different parts of the fig plant to ascertain whether food attractants can be useful for field monitoring of S. adipata.

The present study was conducted in a 1.5 ha commercial fig farm located in San Juan Ahuehueyo (18.7150000°N, 98.9327778°W) in the municipality of Ciudad Ayala, Morelos, Mexico. The plants were 3 years old, and the fig variety cultivated was ‘Black Mission’. The traps were fabricated from 600 ml recycled polyethylene terephthalate bottles. Traps were painted yellow on the lower half and possessed four circular (0.5 cm diameter) holes at 2/3 height of the bottle (Figure 1) (DGSV 2017).

Figure 1:

Handmade 600 ml polyethylene terephthalate trap painted yellow from the middle downwards. Each trap had four circular holes arranged in a half-moon shape. The holes are indicated by arrows.

In the initial bioassay, five food-borne attractants were evaluated: 1) dried fig leaves in ethanol extract, 2) dried fig leaves in water infusion, 3) unripe fig fruit in water infusion, 4) fermented unripe fig fruit and 5) fermented ripe fig fruit. In addition, three commercially available attractants used by the National Campaign against Fruit Flies (SENASICA 2017) were tested: 1) ammonium sulphate, 2) hydrolysed protein, and 3) torula yeast. All treatments contained 250 ml of water, with nine replicates for each treatment (n = 81 traps total). The control consisted of 250 ml of water. The baited traps were placed in a completely randomised block design from September 2021 to January 2022. The traps were placed at a height of three quarters parts of the tree canopy (DGSV 1997). The traps were checked, rebaited and rotated every 15 days. Temperature and humidity data were recorded with a hygrothermometer (data logger DT-172 CEM, Calcuta, Bengala Occidental, India). Captured flies were placed in 100 ml capacity glass vials containing 70 % alcohol and labeled for transport to the laboratory where they were separated by families. Subsequently, the specimens were sent to Dr. Néstor Bautista (Colegio de Posgraduados en Ciencias Agrícolas, Unidad Montecillo) and Dr. Carlos Illescas (Departamento de Biociencias y Agrotecnología, Centro de Investigación en Química Aplicada, Saltillo, Coahuila, Mexico) for identification to species. Data were expressed as flies/trap /day and after checking the normality of the data were analyzed using ANOVA with Tukey’s post hoc test for multiple comparisons.

In a second bioassay, only the two attractants with the highest trap catch were evaluated. The attractants were as follows: 1) fermented unripe fig fruit and 2) fermented ripe fig fruit. Each treatment was in 250 ml of water. The control consisted of 250 ml of water. Nine replicates were tested for each treatment (n = 27 traps total). The traps were set from January to September 2022, checked, re-baited and rotated as previously described. The flies captured were handled as described above. The obtained data were analyzed using a two-sample independent t-test.

In the initial bioassay, fermented unripe fig fruit and fermented ripe fig fruit emerged as the most effective food attractants, capturing the highest number of S. adipata among all food attractants evaluated (F = 4.814, df = 5, 48, P = 0.002) (Figure 2). Captures did not differ significantly between these two attractants. Over the trapping period, the highest S. adipata catches were obtained in November (Figure 3). In the second bioassay, equivalent numbers of S. adipata were captured by both attractants (0.04 flies per trap per day) (t = 0.438, df = 16, P = 0.670). The highest number of S. adipata captures was recorded in February, observing two population peaks throughout the year (Figure 3).

Figure 2:

Silba adipata captured per day in handmade yellow traps baited with: Torula (TOR), dried fig leaves in ethanol extract (DFLEE), dried fig leaves in water infusion (DFLWI), unripe fig fruit in water infusion (UFFWI), fermented unripe fig fruit (FUFF), and fermented ripe fig fruit (FRFF). Bar heights represent means (SEM). Bars labelled with the same letter are not statistically different (F = 4.814, df = 5, 48, P = 0.002). Results for ammonium sulphate (AS), hydrolysed protein (HP) and control (water) are not displayed as no captures were recorded.

Figure 3:

Captures of black fig fly, Silba adipata, during both trap periods using handmade 600 ml yellow traps made from recycled polyethylene terephthalate. It is evident that throughout the year (across the two bioassays) there were two population peaks for S. adipata, the first and largest during November and the second in February. FUFF = fermented unripe fig fruit. FRFF = fermented ripe fig fruit.

Overall, captures with the two best attractants (ripe fig fruit and unripe fig fruit) were low compared to the results published by Katsoyannos (1983), who was the first to document S. adipata captures with McPhail traps baited with ammonium sulphate (563–813 flies per trap per day) or with fig fresh sap (latex) (102–128 flies per day per trap), and also for Katsoyannos and Guerin (1984) later reported captures of 60 S. adipata flies per trap per day in McPhail traps baited with hexanol. In Mexico, hexanol has not been a good attractant for S. adipata. Perhaps environmental conditions cause rapid volatilization or degradation into other types of compounds, affecting its attractiveness to this species. The elevated number of captures reported by these authors also stand in stark contrast to those documented by Perales-Rosas et al. (2021), who observed that the maximum S. adipata captures occurred when utilizing plastic bottles with yellow bottoms baited with 2 % and 4 % ammonium sulphate (11 and 18 flies per trap per day, respectively) in Tepalcingo and Zacualpan de Amilpas, Morelos, Mexico. Peak captures occurred from the end of October to the beginning of December, which coincides with that reported here for the state of Morelos, México. Why captures were much greater at the study sites in Greece compared to Mexico is unknown. The differences may be attributed to the type of traps utilized, given that both our study and Perales-Rosas et al. (2021) employed handmade traps made of polyethylene terephthalate bottles. In the near future it would be interesting to evaluate the two attractants (fermented unripe fig fruit and fermented ripe fig fruit) with the highest captures from our study with other trap types of different shape, size, color and with larger entry holes such as the Multilure or the bucket trap being used by the Dirección General de Sanidad Vegetal (DGSV 2016). The Tephri trap, successfully used to monitor S. adipata populations in Portugal (Gonçalves 2021), could also be evaluated. Another factor that could explain the low captures of S. adipata in our study are the low population densities of this fly in the study region. Paniagua-Jasso et al. (2024) obtained only 1.6 larvae per fruit in Telixtac, Axochiapan, Morelos, Mexico, a locality near where we conducted our study.

To our knowledge, there is currently no established economic damage level for S. adipata; however, the flies per trap per day index obtained in this study are well above the flies per trap per day index established for fruit flies of the Anastrepha genus (SENASICA 2017). The present study demonstrates that throughout the year there are two population peaks of S. adipata, the first and largest in November (also reported by Perales-Rosas et al. (2021) and a second population peak in February (Figure 3). Therefore, although our catches were low using the food attractants evaluated, these data can help growers to plan the timing of management measures for this fig pest.

In conclusion, fig fruit-based food attractants may be an effective alternative for trapping or monitoring S. adipata in the field and thus for implementing management measures during the first outbreaks of this pest in fig crops.