The occurrence and harmfulness of box tree moth Cydalima perspectalis (Walker, 1859) (Lepidoptera: Crambidae) in the Podkarpackie Province (SE Poland)

Boxwood Buxus sempervirens L. is an ornamental bush very often found in utilitarian greenery. Boxwood is a host for several important mite and insect pests of which the box tree moth Cydalima perspectalis (Walker, 1859) (Lepidoptera: Crambidae) is currently the greatest threat to B. sempervirens in Poland. Uncontrolled feeding of caterpillars of this moth leads to complete defoliation of the affected plants, which results in rapid and often massive dying of boxwood bushes. The aim of the study was to assess its biology, harmfulness and analyze the expansion and extent of the box tree moth in southeastern Poland in the last five years. The deepening problem of the rapid and uncontrolled expansion of the box tree moth in the Podkarpackie Province was caused by the lack of a methodology for monitoring and controlling the population of this species and the lack of registered plant protection products dedicated to its control.


Introduction
Invasive species that usually do not have natural enemies in new areas and no environmental resistance (Thomas and Reid 2007;Wright 2014) can become extremely burdensome and cause significant damage to ecosystems.They can also cause serious losses in cultural landscapes (Paulavičiūtė and Mikalauskas 2018;Czerniakowski et al. 2018) (Figure 1).
The box tree moth, Cydalima perspectalis (Walker, 1859) (Lepidoptera: Crambidae), is an invasive species that causes very severe damage to boxwood, Buxus L., often leading to dieback in affected bushes.This moth originates from southeastern Asia (Mally and Nuss 2010) and has been spreading rapidly in Europe over the last decade (Paulavičiūtė and Mikalauskas 2018).The appearance and expansion of this species in Europe (Table 1) is the result of the accidental introduction of caterpillars on boxwood trees imported to Germany from Asia (Billen 2007;Krüger 2008;Blaik et al. 2016).
In Poland, the species was found for the first time in the town of Michałkowa in the Sowie Mountains (2012), and then in the vicinity of Opole and Kraków (2015) (Blaik et al. 2016) and in Greater Poland (Żurawlew and Markiewicz 2019).In the Podkarpackie voivodeship, the first adult specimens were observed in Grabiny near Dębica, Umieszcz near Jasło, and in the Zalesie district of Rzeszów (Bury et al. 2017).Currently, more than 50 localities in the voivodeship are identified as being affected by this harmful species.

Materials and methods
The observations were conducted in 2016-2020 using several research methods.

Field observations
In situ observations were conducted in 2016-2020 throughout the Podkarpackie voivodeship in urban, suburban, and rural areas consisting of detailed inspections of boxwood bushes growing in arranged greenery, parks, home gardens, cemeteries, and allotments.Field observations were conducted throughout the growing season from March to October, once a week, in each study year.

Light traps
Since adult box tree moths are attracted to artificial light, an attempt was made to monitor the flight period of specimens of different generations.Monitoring was conducted throughout the growing season in 2018 in Rzeszów.A barrier light trap fitted with a Philips TL 15W Actinic BL fluorescent lamp was used.
The light trap was hung outside the University of Rzeszów building at a height of about 12 m.Monitoring was conducted from April 15 to September 30, 2018.The lantern was lit throughout the night.The trap was checked daily throughout the monitoring period, and the individuals caught were counted, and the date on which they were caught was recorded.

Breeding method
Artificial breeding was conducted in the spring of 2018 when the caterpillars awakened from winter diapause.Thirty caterpillars at the same developmental stage were collected on April 19, 2018, for breeding.The caterpillars were kept in cylindrical plastic containers (40 cm high and 25 cm in diameter) with openings permitting air circulation.These were placed in a room lit by sunlight with an average day length of 14.5 hours, at a temperature of 23 °C, and with an air humidity of approximately 42%.Thirty caterpillars that were collected from infested bushes were observed and fed systematically every two days with fresh boxwood twigs until they reached the pupa stage.Observations were performed twice daily to monitor the quantity and quality of the food supplied to the caterpillars and to monitor the progress of each developmental stage.The immature stages were weighed on a RADWAG PS 1200/C/1 scale.

Results and discussion
Cydalima perspectalis expanded strongly into new regions of the Podkarpackie voivodeship in subsequent research periods.According to the literature, this species can expand its range from 5 km to even 10 km annually (Salisbury et al. 2012;Wiech 2017).In 2016, the species was noted in only three locations in the southern and central parts of the voivodeship.In subsequent years, the number of new sites where the box tree moth was observed increased gradually from eight in 2017, to 16 in 2018, to 51 in 2019, and 53 in 2020 (Figure 2).The details for all of the stands follow: 2016: EA24 Grabiny, EA30 Umieszcz, Sep 2016, EA73 Rzeszów-Zalesie, Oct 2016 (Bury et al. 2017  Tryńcza, EA67 Wilcza Wola, FA05 Wólka Małkowa, FA21 Przemyśl, Aug 2019, FA05 Grodzisko Dolne, FA24 Jarosław, EA34 Nagawczyna, EA66 Widełka, Sep 2019; 2020: EA89 Nisko, May 2020, EB40 Tarnobrzeg, Jun 2020. The results of monitoring conducted with a light trap throughout the research season are presented in Figure 3.In 2018, there were three box tree moth generations as evidenced by clear peaks in the number of imagines caught on June 10, August 5, and September 19.It is noteworthy that the highest number of imagines was in the first generation, while subsequent generations were less numerous; this could have been linked to the gradual decline in hostplant abundance in the vicinity of these observations.Bereś et al. (2021) also noted three generations in 2019 during monitoring in Poland.
This species produces two to three generations annually in Central Europe (Korycinska and Eyre 2011;Matošević 2013;Blaik et al. 2016;Matošević et al. 2017;Bury et al. 2017;Dobrzański et al. 2018;Plant et al. 2019), which was confirmed by our own monitoring.Simultaneously, Czerniakowski and Olbrycht (2017) report that artificial light can be used to determine box tree moth occurrence in cities.
During caterpillar development, it took an average of 13 days to the pupa stage and then eight days from the pupa stage to the appearance of imagines.In preparation for the pupal stage, the caterpillar forms a cocoon made of white silk strands wrapped around several adjacent leaves.Initially, the pupae are similar in color to caterpillars (Figure 4A), but over time they darken gradually until the body of the imago begins to show through the chitinous shell (Figure 4B).Twenty-eight imagines were bred from 30 caterpillars (Figure 5).The mature caterpillars were 3.6 to 4.0 cm long and the pupae were 1.5 to 2.0 cm long with an average weight of 0.23 g.
During the observations, cannibalism was noted when the caterpillars ate two pupae; this was probably linked to the insufficient quantity and/or   quality of leaves, and, therefore, it resulted from food stress.Cannibalism occurred once when the boxwood leaves dried prematurely, which could have had an impact on the time of transition from the caterpillar to the pupal stages.Devoid of most of their leaves, the boxwood shoots began to wither and soon became unattractive food for the caterpillars.However, C. perspectalis developed quite unevenly through the different developmental stages, so it is difficult to say whether this had a real effect, especially since all the remaining caterpillars eventually passed into the pupal stage, and imagines emerged from each of them.
After mating, the females began laying eggs.Pale yellow, oval, flattened eggs were laid on the lower sides of boxwood leaves in groups of 15 to 20 (Figure 6).It took an average of seven days for the eggs to mature.Nearly 1,000 small yellow caterpillars with black heads hatched and immediately started feeding (Figure 7A).The caterpillars developed very rapidly, gradually taking on the characteristic green colour with black stripes and light streaks along the entire length of the body (Figure 7B).Under laboratory conditions, the full development of one generation of moths was significantly shorter than in the field.The larvae collected on April 19, 2018, and held in the laboratory became imagines on May 10, while in the field, where the average air temperature during the same period was 15.9 °C (Institute of Meteorology and Water Management https://dane.imgw.pl/data/dane_pomiarowo_obserwacyjne)(accessed 19 April 2020), imagines were not observed until 11 days later.The comparison of laboratory and field observations shows that favorable abiotic conditions (mainly air temperature) can significantly accelerate the development cycle of the box tree moth, which could mean increased numbers of generations annually.
The progressive expansion of the box tree moth over the last five years in the Podkarpackie voivodeship resulted in the progressive disappearance of long-standing evergreen boxwood plantings from the cultural landscape that were highly valuable historically, aesthetically, and emotionally.
The failure to prepare appropriate strategies to control C. perspectalis resulted in the very rapid spread of this pest in the study area.First, it should be noted that, initially there were no registered chemical plant protection products to control this species, and only a few biological pesticides were available on the market that were based on Bacillus thuringiensis (Berliner, 1915), and they were for professional use only.Currently, products based on B. thuringiensis bacteria are also available for non-professionals.Chemical pesticides from the group of neonicotinoid derivatives (acetamiprid) are available for professional and non-professional use, and products from the group of macrocyclic lactones (emamectin benzoate) are available for professional use to control C. perspectalis (Register of Plant Protection Products of the Ministry of Agriculture and Rural Development https://www.gov.pl/web/rolnictwo/wyszukiwarka-srodkow-ochrony-roslin-zastosowanie) (accessed 09 September 2021).
In order to control outbreaks and the expansion of the box tree moth, all control activities should begin with carefully inspecting bushes in the spring as this will permit deciding whether or not steps should be undertaken.However, it should be remembered that in the period when wintering caterpillars appear on plants and start feeding, which is usually in late March and early April in Poland, the average daily temperature fluctuates around just 10 °C.The temperature requirements of the active substances in insecticides dictate that box tree moth control should begin with the pyrethroid group, which, unfortunately, is still not registered in Poland for the control of C. perspectalis.Products containing B. thuringiensis should be used immediately after C. perspectalis imagines lay their eggs, bearing in mind, however, that the effectiveness of this biological pesticide also depends on temperature.Similarly, products from the neonicotinoid group are most effective at temperatures above 20 °C.
From spring to the end of summer, the flight of the box tree moth must be monitored with pheromone traps, which will permit identifying the occurrence of subsequent generations of this species and preparing for its control.

Figure 1 .
Figure 1.Effects of box tree moth feeding.Photo by T. Olbrycht.

Figure 3 .
Figure 3. Result of light trap monitoring in 2018.

Table 1 .
The occurrence of Cydalima perspectalis in European countries.