Introduction
The current need to search for environmentally friendly alternatives to the use of broad-spectrum synthetic pesticides for pest management has prompted a burst of research on insecticidal properties of plant-derived compounds (Salvatore et al., Reference Salvatore, Borkosky, Willink and Bardon2004; Dayan et al., Reference Dayan, Cantrell and Duke2009). Among these, essential oils (EOs) from plants in several families have been found to be toxic to a variety of insect pest species (Koul et al., Reference Koul, Walia and Dhaliwal2008). EO toxicity has been recorded to act by contact, ingestion, and as fumigants (Isman, Reference Isman2000; Regnault-Roger et al., Reference Regnault-Roger, Vincent and Arnason2012; Upadhyay et al., Reference Upadhyay, Dwivedy, Kumar, Prakash and Dubey2018), and can be applied to control at the egg, larval, pupal, or adult stages of a wide array of agricultural pests (Koul et al., Reference Koul, Walia and Dhaliwal2008; Tripathi et al., Reference Tripathi, Upadhyay, Bhuiyan and Bhattacharya2009; Isman et al., Reference Isman, Miresmailli and Machial2011). Besides their insecticidal properties, EOs and their components can have strong effects on insect behavior, frequently producing attraction or repellence, promoting or suppressing egg production in females, and in the context of the sterile insect technique, enhancing exposed male mating success (Isman, Reference Isman2000; Tripathi et al., Reference Tripathi, Upadhyay, Bhuiyan and Bhattacharya2009; Nerio et al., Reference Nerio, Olivero-Verbel and Stashenko2010; Regnault-Roger et al., Reference Regnault-Roger, Vincent and Arnason2012; Martinez et al., Reference Martínez, Plata-Rueda, Colares, Campos, Dos Santos, Fernandes, Serrão and Zanuncio2018; Segura et al., Reference Segura, Belliard, Vera, Bachmann, Ruiz, Jofreinse-Barud and Shelly2018).
EOs are volatile complex compounds produced as secondary metabolites of aromatic plants which in nature play an important role in plant defense as antibacterial, antiviral, antifungal, attractants, repellents, or insecticides (Koul et al., Reference Koul, Walia and Dhaliwal2008; Piesik et al., Reference Piesik, Łyszczarz, Tabaka, Lamparski, Bocianowski and Delaney2010; Regnault-Roger et al., Reference Regnault-Roger, Vincent and Arnason2012). They are characterized by two or three major class of compounds, terpenes or terpenoids, at high concentrations (20–70%), and numerous other class of compounds occurring at trace amounts (Bakkali et al., Reference Bakkali, Averbeck, Averbeck and Idaomar2008). In these complex mixtures, individual constituents rarely account for the major share of toxicity which appears to occur as a synergy among all constituents (Isman et al., Reference Isman, Miresmailli and Machial2011). Because EOs interfere with the octopaminergic nervous system in insects (Kostyukovsky et al., Reference Kostyukovsky, Rafaeli, Gileadi, Demchenko and Shaay2002; Jankowska et al., Reference Jankowska, Rogalska, Wyszkowska and Stankiewicz2017), and this target site is not shared with mammals, most EO chemicals are relatively non-toxic to mammals and fish (Koul et al., Reference Koul, Walia and Dhaliwal2008). Importantly, owing to their complex nature, resistance is slower to evolve than in the case of synthetic pesticides (Regnault-Roger et al., Reference Regnault-Roger, Vincent and Arnason2012)
Tephritid fruit flies are major pests of fruit production all over the world and therefore have been the target of many and diverse pest management strategies (Suckling et al., Reference Suckling, Kean, Stringer, Cáceres-Barrios, Hendrichs, Reyes-Flores and Dominiak2016). Several tephritid pests species all over the globe, such as the Medfly (Ceratitis capitata Wiedemann), the Mexican fruit fly (Anastrepha ludens Loew), the South American fruit fly (Anastrepha fraterculus Wiedemann), the olive fly (Bactrocera oleae Rossi), the oriental fruit fly (Bactrocera dorsalis Hendel), and the melon fly (Zeugodacus cucurbitae Coquillett), have been found to be affected by EOs of several plant species (Pavlidou et al., Reference Pavlidou, Karpouhtsis, Franzios, Zambetaki, Scouras and Mavragani-Tsipidou2004; Salvatore et al., Reference Salvatore, Borkosky, Willink and Bardon2004; Chang et al., Reference Chang, Il Kyu Cho and Li2009; Papachristos et al., Reference Papachristos, Kimbaris, Papadopoulos and Polissiou2009; Robacker, Reference Robacker2009; Ruiz et al., Reference Ruiz, Juárez, Alzogaray, Arrighi, Arroyo, Gastaminza and Vera2014). The effects of EOs and some of their individual components on tephritids vary from toxicity to attraction/repellence, increases or reductions of lifespan, oviposition enhancement and deterrence, and enhancement of male copulatory success (Shelly, Reference Shelly2001; Ioannou et al., Reference Ioannou, Papadopoulos, Kouloussis, Tananaki and Katsoyannos2012; Chang et al., Reference Chang, Il Kyu Cho and Li2009; Barud et al., Reference Barud, López, Tapia, Feresin and López2014; Oviedo et al., Reference Oviedo, Van Nieuwenhove, Van Nieuwenhove and Rull2017; Segura et al., Reference Segura, Belliard, Vera, Bachmann, Ruiz, Jofreinse-Barud and Shelly2018). EOs and their components can be exploited in tephritid pest management for monitoring, mass trapping, attract and kill, male annihilation, and the sterile insect technique (Benelli et al., Reference Benelli, Flamini, Canale, Cioni and Conti2012; Canale et al., Reference Canale, Benelli, Conti, Lenzi, Flamini, Francini and Cioni2013; Royer et al., Reference Royer, Khan and Mayer2018).
In the case of medfly, the toxic effects of EOs have been examined for 16 plant species in nine plant families (Ghabbari et al., Reference Ghabbari, Guarino, Caleca, Saiano, Sinacori, Baser and Verde2018). A large part of this research has focused on attraction, enhancement of male mating success, and toxicity (Jang and Light, Reference Jang, Light, McPheron and Steck1996; Bazzoni et al., Reference Bazzoni, Sanna Passino, Moretti and Prota1997; Niogret and Epsky, Reference Niogret and Epsky2018; Benelli et al., Reference Benelli, Flamini, Canale, Cioni and Conti2012; Segura et al., Reference Segura, Belliard, Vera, Bachmann, Ruiz, Jofreinse-Barud and Shelly2018). Less attention has been paid to the effect of EOs on medfly female oviposition behavior (Ioannou et al., Reference Ioannou, Papadopoulos, Kouloussis, Tananaki and Katsoyannos2012; Ghabbari et al., Reference Ghabbari, Guarino, Caleca, Saiano, Sinacori, Baser and Verde2018). In the case of the South American fruit fly, research on EOs is limited to two studies on the effect of volatiles of fruits on male sexual behavior (Vera et al., Reference Vera, Ruiz, Oviedo, Abraham, Mendoza, Segura and Willink2013; Bachmann et al., Reference Bachmann, Segura, Devescovi, Juárez, Ruiz, Vera and Fernández2015) and two studies on toxicity to immature stages and adults (Ruiz et al., Reference Ruiz, Juárez, Alzogaray, Arrighi, Arroyo, Gastaminza and Vera2014; Oviedo et al., Reference Oviedo, Van Nieuwenhove, Van Nieuwenhove and Rull2017).
Oviedo et al. (Reference Oviedo, Van Nieuwenhove, Van Nieuwenhove and Rull2017) tested the effect of several biopesticides on Mediterranean fruit fly and South American fruit fly pupal mortality after topical application and on adult mortality after ingestion. Two EOs, Baccharis dracunculifolia DC and Pinus elliottii Engelm, produced 100% mortality on medfly pupae and strongly suppressed the hatching of A. fraterculus compared to a control, while toxicity through ingestion was greatest on adults for the extract of Solanum granulosoleprosum Dunal Prodr. plus Ricinus communis L. and moderate for B. dracunculifolia oil. All the plants from which these botanical extracts were obtained are very abundant in northern Argentina. While B. dracunculifolia and S. granulosoleprosum are native and widespread, P. elliottii is widely used in plantations and R. communis is an invasive plant. All of such plants have insecticidal properties on several species of Diptera (Ibrahim et al., Reference Ibrahim, Kainulainen, Aflatuni, Tiilikkala and Holopainen2001; Dinesh et al., Reference Dinesh, Kumari, Kumar and Das2014; Porto et al., Reference Porto, Motti, Yano, Roel, Cardoso and Matias2017; Chaaban et al., Reference Chaaban, Martins, Bretanha, Micke, Carrer, Rosa and Molento2018), and there are some artisanal commercial products available in an incipient natural product market.
Here, as a follow-up study, and considering the volatile nature of several components of EOs, we tested the effects of vapors of these plant-derived biopesticides on survival, through mean adult longevity and survival curves, of the two most important tephritid pest species in South America. In addition, because EOs and their constituents can produce other effects than mortality, we tested the effect of vapors of these products on female fecundity and fertility. The goal of these assays was to establish if these plant-derived biopesticides could be used as fumigants for pestiferous fruit fly management.
Materials and methods
Insects
Anastrepha fraterculus and C. capitata pupae were obtained from colonies held at the LIEMEN- PROIMI (Laboratorio de Investigaciones Ecoetológicas de Moscas de la Fruta y sus Enemigos Naturales – Planta Piloto de Procesos Industriales Microbiológicos) laboratories in San Miguel de Tucumán, Tucumán, Argentina, following methods and under environmental conditions described in Oviedo et al. (Reference Oviedo, Van Nieuwenhove, Van Nieuwenhove and Rull2017). Thirty grams of pupae of each species were placed in cylindrical (500 cm3) plastic containers and covered with a perforated organza mesh blocked with a piece of cotton to prevent adult fly escape. At emergence, newly emerged adults were left undisturbed for 2 h to allow cuticle sclerotization. Adults were extracted with an aspirator, constructed with plastic tubes, and transferred to experimental containers according to species and treatment.
Obtaining of plant extracts
The plant extracts assayed during the study were B. dracunculifolia EO and hydrosol, P. elliottii EO, and an extract of S. granulosoleprosum plus R. communis. Baccharis dracunculifolia and P. elliottii EOs were obtained using a stainless-steel distiller. Distillation was achieved through vapor generation by means of a boiler. The system works by water-saturated steam transport. The distiller capacity was 600 kg biomass, and average steam pressure reached 2.5 Kg. Leaves and twigs of up to 5 mm diameter of both plants were used for extraction. After cutting, plant material was left to ventilate for 15 h. Eighty minutes after the beginning of steam passage, extracts begin dropping in a Florentine glass. Water or hydrosol dropped in abundance in similar proportion to EOs. Oils were recovered using a hypodermic needle and the supernatant liquid corresponds to the hydrosol. The hydrosol was kept in high-impact plastic containers (1000 cm3), while the oils were stored in amber glass bottles. Both containers were stored under shelter away from direct sunlight. Distillation time according to the atmospheric pressure reached by the distiller was 6 h. The concentration of the B. dracunculifolia hydrosol used in assays was 8% (w/w). An 8% floral water or hydrosol is obtained from 8 kg of dry plants to produce 40 kg of hydrosol (Oviedo et al., Reference Oviedo, Van Nieuwenhove, Van Nieuwenhove and Rull2017).
The extract of S. granulosoleprosum + R. communis was obtained through alcoholic extraction by placing a (1:1, w/w) mixture of freshly chopped S. granulosoleprosum and R. communis leaves in a 500 ml amber glass jar. Plant material was completely covered with a commercial brand of 46.2° ethanol (Everest®, MEGA, Pederneiras, Sao Paulo, Brazil) which contains 0.1% Eucalyptus essence, plus 0.4% (v/v) of quaternary ammonium, and the jar was kept in storage under dark for 4 months. This extraction technique is cheaper and simpler than distillation and allows the combination of plants whose compounds may have synergistic effects.
Treatments
Experimental units consisted of 3 L (10 × 10 × 30 cm) plastic cages in which the front opening was covered with an organdy mesh tube to avoid adult fly escape. A plastic cup (100 ml) with a piece of cotton wick soaked with 50 ml of each solution for each different plant extract was placed inside each cage. Each plastic cup was then covered with a piece of organdy mesh tightly held with two rubber bands to prevent further direct contact of flies with products but allowing vapor/volatile release. Treatments were P. elliottii oil (PeO), B. dracunculifolia oil (BdO), B. dracunculifolia hydrosol (BdH), a 50% dilution of S. granulosoleprosum + R. communis 1 (S + R1), and pure S. granulosoleprosum + R. communis 2 extract (S + R2), ethanol 50% (Ol50), ethanol 100% (Ol100), and water as controls (table 1). Three individual cages were used per treatment. Then, 30 (15 days old) virgin, sexually mature, males and females (60 adults per cage in total) of each species were transferred into each treatment. Adult diet (a mix of hydrolyzed protein, sugar, gluten, and vitamins) and water was supplied in a plastic Petri dish (4.5 cm) during the assay. Three independent replicates (plastic cages) were set up for each treatment over three different test periods (overall nine experimental units per treatment).
Table 1. Product/ethanol ratio for plant-derived compounds, and ethanol and water controls
Finally, experimental units for each treatment were isolated into special shelves (80 × 80 × 30 cm) and covered with transparent crystal PVC plastic (150 μ) to avoid contamination among different plant extracts assayed, held under controlled temperature, humidity, and photoperiod [26 ± 1°C, 60 ± 10% RH, and 12:12 h (L:D), respectively] throughout the assay.
Adult mortality was checked daily through the observation of experimental cages over a 10 days period. Dead flies were removed with an aspirator, sexed, and recorded according to treatment.
Fecundity and fertility
To determine the effect of treatments on female reproductive potential (i.e. fecundity and fertility) for both fruit fly species, an agar sphere was introduced in each cage 24 h after assays set up. Spheres were wrapped in parafilm (Rolopac®) and hung with a thread, from the top of the cage. Spheres were removed and replaced daily for 10 days. The total number of eggs laid per day was counted according to treatment and fruit fly species. An egg subsample (100 eggs) of each treatment was used to determine both daily and total female fertility. Eggs were aligned over a dark cloth placed over a moist piece of cotton on a Petri dish bottom. The number of uneclosed eggs was counted after 6 days and percent eclosion calculated.
Statistical analysis
A three-way Generalized Linear Mixed Model (GLMM) with a Poisson distribution and log link function was used to determine mean adult fly longevity. Longevity was expressed as the number of lived days by adult flies exposed to different treatments. Explanatory variables were fruit fly species, treatment (exposure to volatiles), sex, and their interaction, while replicate (block effect) was included as a random factor. The analysis was followed by LSD (α = 0.05) tests to separate means.
Adult survivorship, expressed as the proportion of flies surviving under different treatments at the end of the 11 days observational period, was analyzed with a Kaplan–Meier test for those significant variables after GLMM. Survival functions were separated by a Gehan–Breslow (α = 0.05) tests.
Fecundity, expressed as the total number of eggs laid by females in cages under different treatments, was subjected to a two-way GLMM with a Poisson distribution and log link function while fertility (proportion of eggs hatched) data were analyzed with a two-way GLMM with a negative binomial distribution and log link function. Species and treatment were included as categorical variables while replicates and days (days 1–10) were included as random factors. Following analyses, means were separated with LSD (α = 0.05) tests. SPSS v.22.0 software was used for analyses.
Results
Average adult longevity
A GLMM analysis on adult longevity revealed a significant interaction among species, sex, and treatment (χ2 = 29.37; df 22, P < 0.01).
Overall, C. capitata adults (9.91 ± 0.05 days) were more susceptible to the exposure to volatiles than A. fraterculus (10.31 ± 0.50 days). The two most effective treatments (i.e. plant extracts) for both sex and species were P. elliottii oil (PeO) (7.47 ± 0.10 days) and B. dracunculifolia oil (BdO) (7.71 ± 0.11 days). In general, males (9.59 ± 0.06 days) had a lower longevity than females (10.29 ± 0.07 days).
Male and female adult longevity of C. capitata and A. fraterculus was strongly affected by PeO and BdO when compared to the other treatments. The PeO was the most effective treatment to decrease male and female adult longevity for C. capitata while BdO was most effective on A. fraterculus males and females (table 2).
Table 2. Adult longevity (±SD), expressed as the average number of days male and female Anastrepha fraterculus and Ceratitis capitata lived when exposed to one of five plant-derived compounds, diluted or pure ethanol, or a water (control)
Different letters denote significant differences within treatments and sex, and asterisk (*) denotes significant difference between species.
Adult survivorship
Females showed greater survivorship than males during the experiment. Regarding species, C. capitata was more affected by volatile exposure than A. fraterculus. PeO and BdO volatiles were the most effective treatments on adult survivorship. The lowest survivorship for both male and female C. capitata was recorded after exposure to PeO, whereas male and female A. fraterculus were more susceptible to BdO volatiles (fig. 1).
Figure 1. Adult survival (lx ± SE) for both female and male of Anastrepha fraterculus and Ceratitis capitata exposed to Pinus elliottii oil (PeO), Baccharis dracunculifoloa oil (BdO), Solanum granulosoleprosum + Ricinus communis (S + R1), Solanum granulosoleprosum + Ricinus communis (S + R2), water control (C), 50% ethanol (O150), 100% ethanol (O1100), and Baccharis dracunculifolia hydrosol (BdH).
Fecundity
A significant interaction between species and treatment (χ2 = 5.57; df 7, P < 0.01) was revealed after a GLMM analysis on fecundity.
A greater overall mean number of eggs was laid by A. fraterculus females (424.6 ± 64.9) when compared to C. capitata (340.3 ± 12.7). Flies exposed to the S + R2 (529.3 ± 21.2), BdH (549.3 ± 15.5), S + R1 (589.6 ± 67.6), Ol50 (635.5 ± 10.6), and Ol100 (696.9 ± 45.3) treatments laid a significantly greater number of eggs than those exposed to the water control (323.9 ± 17.6), while the PeO (128.1 ± 28.2) and BdO (138.4 ± 16.1) treatments resulted in the lower overall female fecundity than the water control.
Fertility
There were significant differences in fertility between species (χ2 = 9.30; df 1, P < 0.01) and treatments (χ2 = 4.72; df 7, P < 0.01); there was no significant interaction between species and treatment (χ2 = 0.87; df 7, P = 0.53). Overall, a greater percentage of egg hatch (i.e. fertility) was observed for C. capitata (89.7 ± 0.6%) than for A. fraterculus (86.8 ± 0.5%). Females exposed to PeO (82.0 ± 1.6%, A) and BdO (83.8 ± 1.4%, A) showed the lowest fertility while females exposed to the control (88.2 ± 1.3%, B), Ol100 (88.5 ± 1.3%, B), SR1 (88.6 ± 1.3%, B), BdH (89.0 ± 1.3%, B), and Ol50 (90.6 ± 1.3%, B) showed the highest fertility.
Both the BdO and PeO treatments produced the lowest fertility (egg hatch) for both fruit fly species when compared to the water control (table 3). By contrast, Ol50, Ol100, BdH, S + R1, and S + R2 produced the highest fertility values for both fruit fly species when compared to the water control. Volatiles from SR1, SR2, and BdH extracts increased female A. fraterculus fertility when compared to C. capitata (table 3).
Table 3. Fecundity, expressed as the total number of eggs laid by females in treatment cages, (±SD) and fertility, percent egg hatch, (±SD) for Anastrepha fraterculus and Ceratitis capitata adults exposed to four different plant extracts and ethanol and water controls
Discussion
Exposure of C. capitata and A. fraterculus sexually mature adults to volatiles and vapors of both B. dracunculifolia and P. elliottii EOs resulted in lower longevity, survivorship, and female fecundity than the control. In general, the toxicity of C. capitata was greater for P. elliottii than for B. dracunculifolia while the reverse susceptibility was observed for A. fraterculus. Exposure to vapors of S + R had no effect on longevity but reduced adult's survivorship of both species. Interestingly, exposure to vapors of S + R, Ol50%, and Ol100% resulted in greater fecundity of both species than the control. By contrast, the fertility (% egg hatch) was high in all cases (>85%) and not different than the control.
Oviedo et al. (Reference Oviedo, Van Nieuwenhove, Van Nieuwenhove and Rull2017) previously reported 100% mortality for medfly pupae and a strong reduction in the hatching of A. fraterculus adults exposed by contact to both P. elliottii and B. dracunculifolia EOs. By contrast, adults of both species experienced only moderate mortality (<40%) when both EOs were delivered through ingestion, yet under such delivery system, S. granulosoleprosum and R. communis at a concentration of 8% produced 100% mortality. In the present study, exposure to vapors of P. elliottii and B. dracunculifolia EOs produced moderate adult mortality. Apparently, susceptibility to EO and other plant-derived compounds varies according to life stage and delivery mode. Similar observations have been made for several species of stored product pests (Rajendran and Sriranjini, Reference Rajendran and Sriranjini2008), yet no functional explanation for these differences has been formulated. In the case of frugivorous fruit flies, exposure to potentially toxic plant secondary compounds is greater for eggs and larvae that feed within the skin and pulp than for pupae in the soil or free-ranging adults. Perhaps it would be logical to expect that these two last stages should be more susceptible to the toxic effects of plant compounds.
Another noteworthy difference is that C. capitata appears to be overall more susceptible to P. elliottii while A. fraterculus is more sensitive to B. dracunculifolia. Both species are polyphagous and exploit plants across several families (Schliserman et al., Reference Schliserman, Aluja, Rull and Ovruski2014), nevertheless it has been argued that A. fraterculus original host is guava (Ovruski et al., Reference Ovruski, Schliserman and Aluja2003) and that the species or species complex has a strong evolutionary relationship with other host plants in the Myrtaceae (Uramoto et al., Reference Uramoto, Martins and Zucchi2008). Perhaps, host plant associations have selected for tolerance to specific plant secondary compounds, and such chemical specialization could account for the observed differences in our study and that of Oviedo et al. (Reference Oviedo, Van Nieuwenhove, Van Nieuwenhove and Rull2017).
The predominant terpenes in B. dracunculifolia EO are β-pinene (22.69%) and (+)-limonene (19.07%), while in the case of P. elliottii EO, α-pinene (39.25%) and β-pinene (34.79%) are by far the most abundant (Oviedo et al., Reference Oviedo, Van Nieuwenhove, Van Nieuwenhove and Rull2017). All of these terpenes have been found to be toxic to insects (Koul et al., Reference Koul, Walia and Dhaliwal2008) probably because they interfere with the octopaminergic nervous system (Enan, Reference Enan2001). Different species of insects display different susceptibility to different terpenes (Koul et al., Reference Koul, Walia and Dhaliwal2008); our results suggest that C. capitata may be more tolerant to limonene than A. fraterculus and perhaps more susceptible to α-pinene, a fact that could explain its predominance in orange where the two species co-exist (JR personal observation).
From an applied perspective, EO vapors have been tested as fumigants mostly for stored-product insect control. Achieved mortality for potentially successful candidate compounds reaches up to 100% (Upadhyay et al., Reference Upadhyay, Dwivedy, Kumar, Prakash and Dubey2018). However, although some compounds have shown toxicity comparable to methyl bromide or chloropicrin, the generally high molecular weight, high boiling point, and low vapor pressure of plant products are barriers for application in large-scale fumigations that must be overcome (Rajendran and Sriranjini, Reference Rajendran and Sriranjini2008). Herein, in the case of adult medflies and South American fruit flies, vapors of P. elliottii oil resulted at best in 27% survival of adults, which is in general the most susceptible life stage to fumigation with EOs (Rajendran and Sriranjini, Reference Rajendran and Sriranjini2008). Until EO fumigation effects on tephritid eggs and larvae, and most importantly on infested fruit can be examined, practical application of EO volatiles for pest management is more likely to yield results based on their behavior-modifying properties.
The behavior-modifying properties of EOs and their constituents have been mostly exploited for tephritid pest management on account of their attractiveness, mostly for monitoring but also for male annihilation (Allwood et al., Reference Allwood, Vueti, Leblanc, Bull, Veitch and Clout2002; Tan et al., Reference Tan, Nishida, Jang and Shelly2014). In the context of the sterile insect technique, EOs have been used to boost sterile male mating performance (Segura et al., Reference Segura, Belliard, Vera, Bachmann, Ruiz, Jofreinse-Barud and Shelly2018), but their effect on female oogenesis or egg-laying behavior has been less exploited. In this study, exposure to vapors of both B. dracunculifolia and P. elliottii EOs resulted in lower egg recovery from both medfly and South American fruit fly females. Our experimental design does not allow to clearly establish if this reduction in the egg recovery is the product of a physiological effect inhibiting oogenesis or a behavioral effect on female egg laying. By contrast, females exposed to ethanol vapors and S. granulosoleprosum and R. communis extracts (obtained through alcoholic extraction) laid significantly more eggs than control. Ethanol is a ubiquitous ripening product of fruits in general (Buttery, Reference Buttery, Teranishi, Flath and Sugisawa1981; Straten and Maarse, Reference Straten and Maarse1983). Fruit ripening is associated with significant changes in color, puncture resistance, sugar, and ethanol content. Therefore, behavioral responses to ethanol may have been the target of natural selection for all frugivorous species (Dudley, Reference Dudley2004). In the case of Drosophila mojavensis, females exposed to ethanol vapors (4%) can exhibit a 30–75-fold increase in lifetime fecundity when compared to females exposed to water vapors (Etges and Klassen, Reference Etges and Klassen1989). Medfly and South American fruit fly females in our study also appear to be able to use atmospheric ethanol for egg production. Ethanol is an efficient substrate for lipid synthesis in wild-type Drosophila (Geer et al., Reference Geer, Langevin and McKechnie1985) and perhaps also for some species of Thephritidae.
From a practical perspective, the use of ethanol vapors could be examined to enhance the fecundity of mass-rearing colonies or when establishing the colonies of species with low fecundity for research purposes. By contrast, vapors of EOs negatively affecting egg production may be repellent to foraging females and could be exploited in push–pull mass trapping schemes. Push–pull strategies maximize the efficacy of behavior-manipulating stimuli through the additive and synergistic effects of integrating their use (Cook et al., Reference Cook, Khan and Pickett2007). EOs and other behavior-modifying volatiles could be exploited for the development of biorational pest control. Finally, the use of plant-derived chemicals in attract and kill strategies for pest management has been recently highlighted (Gregg et al., Reference Gregg, Del Socorro and Landolt2018), mainly because the use of sex pheromones for pest control is limited by male multiple mating and immigration of mated females into treated areas. Attractants for both sexes, and particularly females, can minimize these shortcomings. Along these lines, a four-component blend including ethanol has been found to be the key in attracting Drosophila suzukii to the mixtures of vinegar and wine (Cha et al., Reference Cha, Adams, Werle, Sampson, Adamczyk, Rogg and Landolt2014); our results suggest that ethanol in combination with other plant volatiles might be an important component in Tephritid fruit fly female trapping.
Acknowledgements
We are grateful to Mr Ricardo Verri for the supply of the essential oils and extract used in the present study. To Dr Sergio Ovruski (PROIMI-LIEMEN) for the supply of A. fraterculus and C. capitata pupae.
Financial support
This research was supported by grant of Secretaría de Arte, Innovación y Tecnología de la Universidad Nacional de Tucumán (PIUNT 26/G634).
Conflict of interest
The authors have no conflict of interest.
