The oriental fruit moth Grapholita molesta (Busck) and summer fruit tortrix moth Adoxophyes orana (Fischer von Röslerstamm) (Lepidoptera: Tortricidae) are two of the most important lepidopteran pests that continually cause significant damage to commercial pome and stone fruits, including peach, apple, pear, nectarines, cherries, quince, and persimmons (Rothschild and Vickers, 1991; van der Geest and Evenhuis, 1991; Kocourek and Stara, 2005; Myers et al., 2007; Piñero and Dorn, 2009; Kirk et al., 2013). Grapholita molesta is assumed to be native to Asia (Rothschild and Vickers, 1991; Kirk et al., 2013) and A. orana is originally reported from Europe (Byun et al., 2012); they are now distributed throughout temperate regions of Asia, Europe, Americas, Africa, and Australia (Rothschild and Vickers, 1991; Li et al., 2021).

Grapholita molesta overwinters as mature fifth instar larvae inside the cocoons (Rothschild and Vickers, 1991; Notter-Hausmann and Dorn, 2010). The overwintered larvae pupate in the spring, and adults emerge and oviposit on host plants or near the overwintering habitats (Sarai, 1970; Ellis and Hull, 2013). Upon hatching, G. molesta neonates excavate tunnels and feed inside the tender twigs in spring (Neven et al., 2018; Liu et al., 2022), and the developing larvae are internal feeders in fruits in the mid- and late-growing seasons (Rothschild and Vickers, 1991; Neven et al., 2018). Adoxophyes orana also has five larval instars but enter overwinter at the third instar (Oku, 1966), and the overwintered larvae develop again in spring and feed on host plant flower buds and clusters, and young shoots and fruits (Oku, 1966; Damos et al., 2022). Apart from attacking the fruit, A. orana is a leafroller as the young shoots and leaves fed by the larvae may stick together (Pehlevan and Kovanci, 2014; Damos et al., 2022). Grapholita molesta larval feeding may cause up to 50–60% fruit losses in Texas State, USA (Anon, 2014) or even 90% in Santa Catarina state, Brazil (Reis Fo et al., 1988). Direct damage on fruits by A. orana larvae may result in yield losses varying from 10% to 80% in apple, peach, and pear (de Jong et al., 1971; Whittle, 1985; Stamenkovic and Pesic, 1998; CAB International, 2008). Moreover, fruits damaged by G. molesta and A. orana larvae may attract secondary pests, such as the nitidulid beetles Carpophilus spp., which act as vectors of brown rot (Monilinia spp.) fungal infection (Hossain et al., 2006). The damaged or infected fruits could not access the markets.

So far, the management of field populations of G. molesta and A. orana is mainly relied on the applications of insecticides and sex pheromone traps (mating disruption) (Trimble et al., 2001; Pastori et al., 2012) basing on a year-by-year calendar schedule programme and/or the phenological stage of the moths (Damos and Savopoulou-Soultani, 2010; Damos and Karabatakis, 2012; Damos and Savopoulou-Soultani, 2012a). Unfortunately, the efficacy of insecticides is usually lower than expected due to the feeding habits protecting larvae from insecticides (Rothschild and Vickers, 1991; Pehlevan and Kovanci, 2014; Neven et al., 2018; Liu et al., 2022; Damos et al., 2022), and the rapid development of insecticide resistance (Pree et al., 1998; de Lame et al., 2001; Kanga et al., 2003; Navarro-Roldán et al., 2017). Furthermore, mating disruption using sex pheromone may have a low control efficacy when the field populations are large (Kong et al., 2020; Ferracini et al., 2021; Liu et al., 2022), as males can easily locate their mates using visual cues as well as by chance encounters (Thorpe et al., 2006). For G. molesta and A. orana, the eggs/neonates and male adults of moths are the targets of pest control using insecticide and sex pheromone measures, respectively (Philips, 1973; Wang et al., 2017); therefore, the effective management of these species largely depends on precise prediction of timings of egg laying and hatching and flight peaks of male adults, especially in the overwintered generation.

Insects are poikilothermic, and temperature is the major factor regulating their development, growth and thus population dynamics. Degree-days (DD) are the most convenient way to predict a phenological event in insects (Prues, 1983; Damos and Savopoulou-Soultani, 2012b). Previous studies have acknowledged that degree-days models could be a useful tool in forecasting the phenology of many lepidopterous pests (e.g., Hrdý et al., 1996; Godin and Boivin, 1998; Del Tío et al., 2001; Milonas et al., 2001; Tobin et al., 2003; Pehlevan and Kovanci, 2017; Ivezić et al., 2023), including G. molesta and A. orana (Damos and Savopoulou-Soultani, 2010; Jones et al., 2013; Damos et al., 2022).

In this study, we monitored the seasonal flight dynamics of G. molesta and A. orana males using pheromone traps in peach orchards of different cultivars over three successive years. We then calculate the cumulative degree-days basing on the low temperature thresholds of the two pests to predict the thermal requirement for the first male captured in early spring, and male flight peaks and egg hatching during the growing seasons. Our results may deliver critical knowledge for the development of pest management strategies aiming to supress the populations of G. molesta and A. orana in field.

The first step to integrated pest management (IPM) of the identified species is the timely detection of a pest infestation. Sex pheromone traps have long been used for detecting presence of insects in the early season and long-term monitoring of pest population size during the growing seasons. We detect three distinct flight peaks for A. orana and 4–5 flight peaks G. molesta during the growing seasons ( Figures 2 , 3 ). According to previous studies (Milonas et al., 2001; Kumral et al., 2005; Damos et al., 2022), the number of flight peaks in lepidopteran moths may roughly reflect the number of field generations. However, Damos et al. (2022) argue that as the season progresses, the higher variation in the last moth flights could be observed, probably due to the extreme high temperatures occurring during the summer which slow down the development and cause bimodal peaks. In this study we did not record a high temperature during the 3rd flight peak in July in 2019 and 2020 compared to that in 2018 ( Figure 1A ). Alternately, according to Zhao et al. (2017), the continuous low soil water content due to the low precipitation in August 2019/2020 ( Figure 1B ) may result in the bimodal adult emergence peaks. Therefore, the additional fifth flight peaks of G. molesta in late season in 2019 (for Jinghong only) and 2020 (for all cultivars) could be from the same generation, and there were three and four generations detected for A. orana and G. molesta, respectively. The various number of flight peaks or generations between these two species may be attributed to their natural phenology, i.e., G. molesta overwinter as the mature instar larvae (Rothschild and Vickers, 1991; Notter-Hausmann and Dorn, 2010) and overwintered larvae pupate and adults emerge in the spring (Sarai, 1970; Ellis and Hull, 2013), while A. orana overwinters at the 3rd instar (Oku, 1966) and overwintered larvae feed and develop again before adult emergence and thus require more time to complete the lifecycle and start a new generation.

Invasive herbivores usually utilize a broad range of host plants. Previous research reveals that both G. molesta and A. orana prefer peach over other host plants, such as apple and/or pear, for larval feeding, development or oviposition (Myers et al., 2007; Piñero and Dorn, 2009; Najar-Rodriguez et al., 2013a; Najar-Rodriguez et al., 2013b; Li et al., 2021). In the precent study, we further show that among different peach cultivars, the total number of G. molesta male adults captured was significantly higher in Jinghong than in Luhua9 with significantly lesser male adults trapped in Jiubao ( Figure 4A ), while the A. orana population size was significantly higher in Jinghong and Jiubao than in Luhua9 ( Figure 4B ). We suggest a variation in plant nutrients existing between peach cultivars, which may impact the interactions between host plants and insect herbivores, regulating the herbivore fecundity (Awmack and Leather, 2002) and affecting their population and community dynamics (Joern et al., 2012). In addition, we also show that the population size of G. molesta was usually significantly higher than that of A. orana ( Figure 4 ), suggesting that G. molesta performed better on peaches than did A. orana. To understand the mechanisms behind the varying population dynamic between cultivars and varying population size between herbivore species, future studies should assess development and reproduction of G. molesta and A. orana reared on growing shoots/leaves and peach fruit of different cultivars. Furthermore, it should be noted that the differences in environmental conditions across years and possible in the relative effectiveness of pheromone traps/lures for these two species may lead to the variations of moth populations between different years ( Figures 2 , 3 ) and between species ( Figure 4 ). However, these assumptions require further investigations, e.g., independently testing the specific pheromone traps/lures in successive years and separated orchards.

Although the number of moths captured in orchards of each peach cultivar varied between flight peaks, the seasonal phenology of G. molesta or A. orana was quite stable with an identical timing of each flight peak between cultivars in a year ( Figures 2 , 3 ; Tables 1 , 2 ). The identical flight timing allows us to fit the overall data of different cultivars using a single phenology model. The precise prediction of flight timing coordinating with the degree-days technique (e.g., Rice et al., 1984; Tobin et al., 2003; Pehlevan and Kovanci, 2017; Rowley et al., 2017; Graf et al., 2018; Damos et al., 2022) may provide critical information to optimize the schedule of pheromone trap or pesticide applications. For example, the flight peak of overwintered G. molesta or A. orana was usually lower than the consequent peaks (i.e., the 2nd and 3rd peaks) ( Figures 2 , 3 ; Tables 1 , 2 ), which provides opportunities to prevent the outbreaks of field populations in the later seasons when the overwintered population size is low in early spring (Tshernyshei, 1995; Parry et al., 2019). Therefore, to trap the newly emerged male adults or disrupting female mating success in the 1st generation in peach orchards, deploying pheromone traps would be scheduled in April with a cumulative DD between 49.6 (for the earliest male captured in 2020) and 207.1 (for the 1st flight peak in 2018) for G. molesta and in mid-May–early June with a cumulative DD between 450.4 (for the earliest male captured in 2018) and 866.7 (for the 1st flight peak in 2019) for A. orana ( Table 3 ). The high variation of degree day calculations for the first emergence and first flight peak for G. molesta in different years ( Table 3 ) may be attributed to the various environmental temperatures before adult emergence ( Figure 1 ). The precise predictions of first adult flight and larval emergence basing on the calculated cumulative DD need future confirmation in orchards in early spring. Additionally, according to the thermal requirement for egg hatching [i.e., 79.4 DD for G. molesta (Croft et al., 1980) and 90.0 DD for A. orana (Charmillot and Megevand, 1983)], insecticide treatments would be applied in late-April–early May and late May–early June to reduce the field population density of neonates of G. molesta and A. orana, respectively.

Our results also demonstrate that compared to the overwintered population, the moth populations were higher during the growing seasons ( Figures 2 , 3 ). Therefore, to reduce the fruit damage, pheromone traps would be deployed in June to suppress G. molesta populations in the 2nd and 3rd generations and in mid- to early July to suppress A. orana populations in 2nd generation when the cumulative DD reached 573.8–1025.2 for G. molesta and 1539.7–1788.9 for A. orana ( Table 3 ). In addition, applications of short term-residue insecticides in early June to early July and mid-July to late July ( Table 3 ) might respectively suppress populations of G. molesta and A. orana neonate feeding on peach fruit. Furthermore, it is well known that there is positive relationship between the overwintering and next spring populations (Hu et al., 2015; Ju et al., 2017; Jorgensen et al., 2020). It is thus suggested that pheromone traps could be set up in late July–early August to trap G. molesta of the 4th generation and in mid-September to trap A. orana of the 3rd generation when the cumulative DD reached 1511.0–1617.4 for G. molesta and 2704.4–2662.8 for A. orana, which may reduce overwintering population size and thus decrease pest infestation in next year. However, application of chemical treatments to reduce the neonate population is not encouraged just before fruit harvest. Furthermore, mating disruption using sex pheromones may be less effective during the last generation, because moth survival is largely determined by some other factors such as the temperature, moisture and predators during overwintering.

In conclusion, results of this study show that there are three clear flight peaks reflecting three generations for A. orana, with four to five flight peaks observed but probably only four generations for G. molesta. We find that the population size of G. molesta in peach cultivars of Jinghong and Luhua9 is higher than that in Jiubao, while A. orana population is higher in peach cultivars of Jinghong and Jiubao than in Luhua9. We develop a logistic multiple-peaks model to predict the seasonal phenology of moths and integrate the results with the degree-days calculation, which may help optimize the precise timings of pheromone trap and insecticide applications aiming to suppress the male adult population size and reduce neonate population density of both G. molesta and A. orana. Future research will investigate the efficiency of precise timing and number of pheromone traps and insecticide applications in suppressing pest populations in field.

The raw data supporting the conclusions of this article will be made available by the authors, without undue reservation.

The manuscript presents research on animals that do not require ethical approval for their study.

HZ: Conceptualization, Data curation, Investigation, Methodology, Validation, Writing – original draft, Writing – review & editing. AM: Conceptualization, Investigation, Methodology, Supervision, Validation, Writing – original draft, Writing – review & editing. HR: Investigation, Validation, Writing – review & editing. XY: Investigation, Validation, Writing – review & editing. JH: Investigation, Validation, Writing – review & editing. JZ: Investigation, Validation, Writing – review & editing. HL: Investigation, Validation, Writing – review & editing. ZY: Investigation, Validation, Writing – review & editing. XW: Investigation, Validation, Writing – review & editing. XH: Data curation, Formal Analysis, Software, Validation, Writing – original draft, Writing – review & editing. JL: Conceptualization, Funding acquisition, Methodology, Project administration, Resources, Supervision, Validation, Writing – original draft, Writing – review & editing.