*3.2. Ostrinia nubilalis, Diabrotica virgifera virgifera, Cameraria ohridella (Selected Result Issues for Comparative Purposes)*

*Ostrinia nubilalis*. Although the species was recorded for the first time in Poland on millet (*Panicum miliaceum*) in the 1930s, and then in the 1950s on *Zea mays*, it became a severe pest of maize cultivation in Poland beginning of the 21st century. Published data show that from the 1950s to the 1990s, this pest was recorded in a limited number only in south-eastern Poland (No.: 15 and 16) and was closely related to the cultivation of maize in areas with optimal conditions for its cultivation. However, in the last two decades, the total area of maize cultivation in Poland has been systematically growing. In 2008, its cultivation area was over 730 thousand hectares, but in 2012 this area increased very quickly to over 1 million hectares [38,39]. Such a rapid increase in the area of maize sown was caused by the freezing of winter cereals and winter rape at the turn of 2011/2012, which prompted farmers to cultivate maize [40]. Since 2012, maize has been sown every year in Poland on more than one million hectares [41]. Data from the Central Statistical Office (CSO) show that maize is sown in all 16 voivodeships. There is, however, regional differentiation of maize sowing in terms of the use of the crop. In the south, grain is dominated, and in the north, silage is dominated [42]. The great interest in maize cultivation resulted in the densification of crops and the reduction of the distance between the cultivation of this plant in all provinces. The above-presented trend in the change of maize cultivation in Poland was favorable for the spread of pests of this crop.

Monitoring carried out in 2004–2008 stated that in the first decade of the 21st century, *O. nubilalis* began to quickly infect maize cultivation towards the northwest (No.: 1, 4, and 5)—Figure 11. Up to the end of 2008, damaged plants caused by caterpillars were recorded in 185 counties located in 14 voivodeships [31]. From 2009, it could be found in almost the entire country, except for central Poland and the eastern part of Pomerania (No.: 2 and 7). Notably, the expansion of this pest to the north of Poland reflects the increase in maize acreage in the first decade of the 21st century in central and northern Poland. The monitoring carried out in 2010–2012 showed the permanent presence of this species in almost all of Poland [32]—Figure 11. Apart from the increase in the area and the spread of maize cultivation, it is difficult to indicate for *O. nubilalis* a dispersion factor other than a slow, local expansion into subsequent adjacent areas of cultivation with often limited crop protection against pests.

**Figure 11.** Stages of expansion of *Ostrinia nubilalis* in Poland (grayed out areas) identified as part of the national monitoring of this species carried out by the State Plant Health and Seed Inspection Service (SPHSIS) and the Institute of Plant Protection-National Research Institute, Regional Experimental Station in Rzeszów (IPP-NRI) in 2004–2012 [31,32]-modified; 1–16: numbers assigned to individual voivodships; see Table 1.

*Diabrotica virgifera virgifera* in Poland was first recorded in August 2005 in the southeastern part of the country in Subcarpathian voivodeship (No.: 16) in three outbreaks, near Dukla, Ł ˛aka, and Jasionka, where six adults were collected [33]—Figure 12. The first recording of this species in Poland is closely related to the transit of freight from Hungary via Slovakia to Poland, the site at the Polish-Slovak border crossing in Dukla, and maize cultivation in the areas adjacent to the airport in Rzeszów (No.: 16). By 2007, *D. virgifera* had infested entire south-eastern Poland and the border areas with the Czech Republic, including almost the entire Opole Voivodeship (No.: 13, 15, and 16, and partially 9, 11, 12, and 14) [34]. Unpublished data from the SPHSIS archive, in which data from the national monitoring of this species in 2008–2013 are stored, clearly indicate a relatively slow but successive infestation of subsequent maize crops in neighboring administrative units (Figure 12).

**Figure 12.** Stages of expansion of *Diabrotica virgifera virgifera* in Poland identified as part of the national monitoring of this species by the State Plant Health and Seed Inspection Service (SPHSIS) in 2005–2013 [34]-modified and SPHSIS archives. The red points indicate the first described localities of *D. virgifera* in Poland in 2005 [33]; 1–16: numbers assigned to individual voivodships; see Table 1.

*Cameraria ohridella*. Horse chestnut-*Aesculus hippocastanum* is a tree initially found only in park and roadside plantings as an ornamental plant. Currently, it is also found in the wild almost all over Poland (Figure 13a) [43]. The horse chestnut pest, Horse-chestnut leaf miner-*C. ohridella*, was first recorded in Poland in 1998 in the Botanical Garden in Wojsławice, 50 km south of Wrocław (No.: 9) near the S8 clearway from the Czech border to Wrocław and on to Warsaw [35]. A year later, a mass appearance of Horse-chestnut leaf miner was found, and trees were heavily damaged in the country's southern regions, in the vicinity of Cieszyn, Pszczyna, and Racibórz (No.: 14). These are towns located in the border zone with the Czech Republic. Therefore, another unintentional introduction from the Czech Republic using the natural south-north migration route, the Moravian Gate and the communication routes running through it from the Czech Republic (A1 motorway and S52 clearway) cannot be ruled out. In 2001, insects of this species were noted in the provinces of southern Poland (No.: 9, 15, and 16) and, also, in central Poland (No.: 5, 7, 10, and 11), reaching Masuria in the north (No. 3). In 2002, the first damage to the chestnut tree was recorded on the German border in Słubice (No.: 4), on the Baltic coast (Kołobrzeg and Sopot—No.: 1 and 2) and in eastern and north-eastern Poland (Lublin and Białystok— No.: 8 and 12). These data indicate that from the site near Wrocław, where it was found for the first time (and perhaps also from the sites described in 1999 in Silesia—No.: 14), the expansion of insects was two-way-eastwards towards Ukraine and through Central Poland to the north-east towards Belarus, and the west towards the Polish-German border (Słubice) and Pomerania (Figure 13b). The explanation for such a rapid appearance of the Horse-chestnut leaf miner in Warsaw and Masuria is possible, only taking into account the use of road transport and the main south-north communication axes, such as the A1 motorway (Cieszyn-Katowice-Warsaw and further north) and the A4 motorway running

through Krakow to Rzeszów and the Polish-Ukrainian border. Fifteen years later, the same communication routes were used in their expansion by *Cydalima perspectalis* (Figures 7–9). In 2002 *C. ohridella* was already recorded nationwide in many scattered locations, confirmed by the anthropogenic dispersion pattern of this species in Poland, which is estimated on average at least 200 km per year. Currently, this species inhabits all of Poland, and its range coincides with the distribution of the common horse chestnut in Poland (Figure 13a).

**Figure 13.** (**a**) A map of the distribution of the horse chestnut (*Aesculus hippocastanum*) in Poland [43]; \*: each black point represents confirmed sites of *A. hippocastanum* in Poland; (**b**) the first recorded sites of the occurrence of *Cameraria ohridella* in Poland in the years 1998–2002, the arrows indicate the areas of the main directions of insect spread [35,36]-modified. 1–16: numbers assigned to individual voivodships; see Table 1.

#### **4. Discussion**

Such a sudden increase of knowledge in the *Cydalima perspectalis* range, which covered entire Poland for a few years, might be related to the fact that various media started talking about the appearance of the box tree moth due to the information campaign. For this reason, gardeners were more aware of the presence of specimens of a new pest species and were more likely to observe the boxwood plants, including detecting them more often.

Assuming that it usually takes two years from the first appearance of insects to boxwood until they are entirely defoliated, we can assume that the data sent by users of the gardening website are just such a consequence of a two- or three-year delay in detecting the pest [19,44]. Information from observers indicates that the natural expansion of this insect in Poland was a secondary factor, an example of which is the 8-year long settlement route in southern Poland in the latitudinal direction along the Carpathian arc. It should be assumed that the main factor was accidental, untargeted transfers of insects with infected plants through the use of road transport and resale of infected plants in subsequent parts of the country. An example is that in 2019 the presence of *C. perspectalis* was noted in Gda ´nsk, at a distance of over 300 km from the previous year, the closest place of the outbreak in Płock (Figures 8 and 9). It cannot be ruled out that *C. perspectalis* reached the Baltic coast independently by sea transport. It can also be stated that most of the new insect occurrence sites are associated with large cities. The rapid expansion in the last three years was also favored by warmer, above the long-term norms, average daily and monthly temperatures. The hot and long autumn of 2019 was the reason for the third generation of insects' mass appearance.

In Poland, the box tree moth easily survives during the winter period. In spring, as the temperature rises, the over wintering caterpillars start foraging, so the first adults appear at the beginning of April [45]. In Central Europe, the insect develops 2–3 generations a year, depending on weather conditions [17]. In 2018 in south-eastern Poland, due to the dry and hot summer, the 4th generation of this pest was likely to be developed because active moths were found in November [45]. Adults *C. perspectalis* can fly up to 10 km per year, then a long-range invasion of the pest (especially in Europe) is favored by bulk freight or boxwood cargo transport. Local dispersion is facilitated by, e.g., hedges, horticulture, nursery gardens, or internet sales [46].

In the last 30 years, over 30 species of insects, alien to the Polish entomofauna, has been found in Poland [28]. However, some of them were particularly spectacular and of real economic importance. These include the invasions of *Ostrinia nubilalis* (European corn borer), *Diabrotica virgifera virgifera* (Western corn rootworm), and *Cameraria ohridella* (Horse-chestnut leaf miner). There are many differences and similarities to that found for *C. perspectalis* in their pace and spread directions.

For example, in the monitoring studies carried out in the United States on *D. virgifera*, it was found that adult insects, under favorable conditions, move an average of several dozen kilometers per year (under the most favorable conditions, a natural dispersion of insects over a distance of over 100 km was recorded). The effect of such a pattern of spreading the Western corn rootworm was the appearance of insects only in the areas adjacent to the previously infected [47,48]. In Poland, for *D. virgifera*, spreading over much smaller distances was observed, up to several dozen kilometers per year. However, spreading was favored by lowland terrain and the fragmentation of field crops of maize, which, with the doubling of maize crops in 2005–2013, caused a progressive invasion of *D. virgifera* to the north and north-east. The only exception was the presence of *D. virgifera* in 2013 in Podlaskie voivodeship (No.: 8), which was undoubtedly brought to this area using transport or plant material. Studies on the spread of *D. virgifera* in Europe in countries such as Italy, France, Germany, Austria, and Switzerland have confirmed that the rapid spread of the species is related to the total acreage of maize cultivation in a given area. Areas that cover more than 50% of the acreage are classified as "high-risk areas" for this species' invasions [49]. These data fully corresponded to the *D. virgifera* dispersion pattern in Poland, which confirm the rapid invasion of the species in traditional maize cultivation areas (south-eastern Poland) and the increase in the range of occurrence with the increase in the acreage after 2008. A similar pattern of insects spread from pest-infested areas to nearby pest-free areas was observed for *O. nubilalis*. An additional factor contributing to the spread of this species was the abrupt increase in the acreage of maize cultivation in 2012 [41].

A completely different spreading strategy was observed for the third mentioned above invasive pest. The high rate of spread of *C. ohridella* and the rapid increase in its occurrence range is mainly attributed to road transport. In many cases, the favorable factors were tree stands located near the main communication routes, from which people, animals, or wind further transmitted insects to other trees. It is estimated that the distribution range of Horse-chestnut leaf miners increased in Europe at a rate of about 60 km to 114 km per year [50].

With large-scale spatial data of the occurrence of a given invasive species, several spatial models of the spread of insects to new, uninfected areas can be distinguished. These include, among other things, a diffusion model, a leptokurtic dispersal model and a stratified dispersal model [50,51]. Comparing the pace and directions of the insect's habitat mentioned above species in new areas of Poland, it should be stated that two basic ways of infesting new areas can be distinguished. The first is related to pests of arable crops, good examples of which are *O. nubilalis* and *D. virgifera*. These insects, closely related to crops, in this case, maize, take over successive areas gradually, usually over short distances (about 50 km in the direction of neighboring crops). An important factor contributing to the dispersion is the high density of the host plant crops and the short distance between them. On the other hand, the slowing down of the dispersion rate is associated with plant protection treatments and the applicable phytosanitary regulations. A diffusion model can best describe the spread of *O. nubilalis* and *D. virgifera* [50,51].

On the other hand, species of no economic importance, such as, for example, *C. ohridella*, take over new areas in a jump-like manner, sometimes over long distances, often

hundreds of kilometers, using land transport means (including water and air transport). Five years was enough for this species to be recorded throughout Poland, both on trees growing in areas not cared for by humans and those of a decorative and recreational nature. In the analyzes of the models of the spread of this species in Germany and France, it was found that this process is best described by a leptokurtic dispersal and a stratified dispersal model. It was also found that a stratified dispersal model incorporating the effect of human population density provides the best description of the spread of *C. ohridella* in many countries of Europe [50,51].

The use of social networks and dedicated websites dedicated to societies and interest groups' activities is not the first time this type of approach has been used to research the distribution of insects. *C. perspectalis* meets most of the insect criteria suitable for this type of social monitoring. It is a species that feeds close to humans, causes specific and massive boxwood dieback symptoms, visible to everyone, even to people who are not interested in entomology. It is only necessary to consider the time insects need from the first colonization of plants to their death-about two years [19]. The lack of natural enemies enables a more precise determination of the year of insects' appearance in a given area. Previously, this type of approach was used in the British Isles where, in addition to the official operating The British and Irish network of County Moth Recorders (CMRs), which was the primary source of fully reliable records of the species, the website of the European Boxwood and Topiary Society (EBTS) provides a facility for users to report occurrences of this species and we have accessed all such data for 2018 (www.ebts.org/bmctracker) (accessed on 22 February 2021) [11]. A similar approach that reflects Citizen Science's idea has been successfully used in recent years in Toronto (Canada), where the first appearance of *C. perspectalis* was recorded in August 2018 [52].

Observations made by Blaik et al. [25] and Bury et al. [26] were used by EPPO to map the distribution of *C. perspectalis* in Europe [53]. In turn, the map of *C. perspectalis* distribution in Europe conducted by CABI lacks detailed information on the occurrence of the species in Poland, including its first appearance [54].

Genetic studies based on mtDNA for cytochrome oxidase genes revealed two haplotypes— HTA and HTB—in Europe out of twelve identified in Chinese populations of *C. perspectalis*. These results support the hypothesis of multiple introductions of this pest from eastern China to Europe. Lack of precise trade regulations for ornamental plants and trade globalization facilitated the rapid spread of the pest [4,7]. The lack of genetic research data on the population in Poland does not allow for an unambiguous statement whether the spread of *C. perspectalis* is the result of a single introduction to the southwest of Poland from the Czech Republic or it is multiple introductions (from the Czech Republic, Slovakia, and Germany, or through the Baltic ports from Denmark or Sweden). Research of this type could provide an answer about the origin of this species in the cities of northern voivodeships (Szczecin, Gda ´nsk— Figure 8). It cannot be ruled out that these cities may have been infested with insects from Denmark or Sweden, where this species was first recorded in 2013 and 2016 [54].

Due to the lack of nationwide monitoring of the box tree moth's occurrence in Poland, data on the presence of the pest came only from random observations. Without the involvement of state services dealing with the monitoring of alien origin species, it was impossible to create an accurate map of the range of this species in Poland. Our observations in 2018–2020 clearly show the growing range of *C. perspectalis* in Poland. The obtained data indicate that the main directions of the species spread in Poland were the main communication axes of the country. A good example to justify such a thesis is the sudden appearance of several confirmed positions in the Subcarpathian Voivodeship (No.: 16) in 2016–2017. Data on the spread of this insect in Hungary and Slovakia confirm its presence in large Hungarian cities as early as 2011, and the following year it will appear in Slovakia. In 2013 and 2014, the insect was recorded in Prešov (a large communication junction in eastern Slovakia) and Košice. These localities lie on the main communication axis from Slovakia to Poland (Košice-Prešov-Rzeszów) [37,55]. On the other hand, the detected presence of *C. perspectalis* in Lithuania in 2018 in Vilnius is unrelated to the spread of insects

along the axis of transit transport from southern Europe to the Baltic countries running through Poland [56]. *C. perspectalis* was recorded in the north-eastern voivodships bordering Lithuania only in 2020 in towns lying on the extension of the Warsaw-Ełk/Suwałki-Vilnius clearway. This assumption strengthens the thesis that the box three moth reaches new areas of Europe by sea transport, which may explain the earlier appearance of this species in large port towns (Szczecin, Gda ´nsk, Poland) in Poland as early as 2019 (Figure 8).

For *C. perspectalis*, an invasive pest of boxwood, a plant that grows in Poland only due to artificial plantings, a similar spreading pattern can be observed as in *C. ohridella*. It has been proven that the spread of this species is related to the use of land transport and the density of the human population—a stratified dispersal model. This model best explains the unintentional long-distance movement of the pest within one growing season, which may have resulted from the long-distance trade of infected plants. At the same time, an essential factor influencing the rate of spread of a species is the density of the human population, favoring local spread over short distances, locally matching the diffusion model [50,51]. Such a dispersion model is well reflected in the published maps for Switzerland, Hungary, and Slovakia, where the first recorded sites of this species begin with large cities and then along the main communication routes between them [7,11,37,55,57]. The time it took for *C. perspectalis* to infect all of Poland was only twice as long as that stated for *C. ohridella*. The years 2018–2020, in which citizen monitoring of the occurrence of the box tree moth was carried out, were crucial to the invasion of the whole of Poland.

Proposed by Nacombo et al. [5], a bioclimatic (CLIMEX®) model for *C. perspectalis* distribution in Europe, based on climate, ecological, and developmental parameters described for this species (e.g., diapause termination, thermal requirements and phenology) well reflects the directions of expansion of this pest in Poland. The places of occurrence of the box tree moth confirmed in 2020, as well as the number of raised records, correctly reflect the predictions obtained using the bioclimatic model (compare with Figure 9), pointing to the regions of south-eastern (No: 12, 14, and 16), central (No.: 5 and 10), and western (No.: 4) Poland. Similar analyses performed for the Slovak population confirm the model's usefulness [5,55]. Perhaps the model used should have been enriched, as was conducted for *C. ohridella*, with the human population density parameter, as it was conducted in the model to predict this invasive species for horse chestnut in a stratified dispersal model [5,50,51,55].

However, it should be noted that most of the obtained data were provided by gardeners and plant breeders, who often did not know about the appearance in Poland in the area where a new, alien species of pest lived. Some gardeners, boxwood growers, and institutions dealing with urban greenery and parks lost their boxwood bushes, topiaries, and hedges, which caterpillars utterly destroyed. Such a rapid and spectacular invasion of *C. perspectalis* in Poland makes it necessary to research to understand the biology of this species under Polish conditions. In this, it is essential to develop comprehensive methods of its control using biological and chemical methods, which will take into account Poland's climatic and weather conditions and the methods of growing boxwood [40,58,59].
