Foliage-browsing Lepidoptera (Insecta) in deciduous forests of Ukraine for the last 70 years

As insect development depends on temperature, so the change in biology, behavior habits, frequency, and severity of outbreaks of foliage-browsing insects considered pests can follow the climate change. The reactions of species to the same climate changes can be specifi c, therefore, an unpredictable change in their ratio in the community will affect the consequences of climate change. The details of such changes must be studied to quantitatively assess future trends and the threats to deciduous forests. The aim of this research was to evaluate the representation in deciduous forests the foliage-browsing lepidopterous insects if different groups of size, lifestyle, voltinism, trophic relations, and ability to mass propagation in different periods of assessment for recent 70 years. In the analysis, we used a list of 118 lepidopterous species of foliage-browsing insects of deciduous forests, compiled on the basis of archival data from 1940–1975, and in the course of our own fi eld research from 1975 to the present in the forests of Ukraine. Following trends were expected to be confi rmed for these time intervals: to increase the number of species of small size, the number of species with hidden lifestyle, multivoltine species, polyphagous species, and so-called indifferent species. For each species, all these parameters were identifi ed and proportions of species of each category for certain time intervals (1940–1950, 1960–1970, 1980–1990, and 2010–2020). Their distribution for size, voltinism, lifestyle, trophic features, and outbreak potential at these time intervals was compared using χ2-test. Among lepidopterous foliage-browsing insects of deciduous forests of Ukraine, the increase for recent 70 years was proved for the proportion of indifferent species (do not able to mass propagation), small species (with wingspan below 20 mm), as well as species with hidden (leaf-miners) and semi-hidden lifestyle (leaf-rollers). All trends are expressed the most obviously in 1940–1950 and further periods. The hypothesis about decrease the proportion of the univoltine and monophagous species for this period is not supported statistically.

Introduction. Climate change brings to forest weakening and increases its susceptibility to insect damage (Frank, & Just, 2020;Halsch et al., 2021). At the same time, insects under new climate conditions accelerate development, change their behavior, and hibernation habits (Branco et al., 2019;Jactel, Koricheva, & Castagneyrol, 2019). Such changes must be studied in detail to determine threats to forests.
Some phyllophagous insects are able to cause considerable damage to tree foliage in the forest, protective, and urban stands during particular periods, so-called outbreaks when the number of these pests increases drastically (Исаев, Пальникова, Суховольский, Тарасова, 2015). The incidence, severity, duration, and intervals between such outbreaks were analyzed for different periods to fi nd the algorithm for predicting the following increase of threat to a forest (Мєшкова, 2002). It was found that the species which are able to mass propagation begin feeding early in spring, and conditions for their population growth are formed the most often. Duration of outbreaks for different foliage-browsing insects is proved to be connected also with the features of their seasonal development and increases from western to eastern regions (Мєшкова, 2009).
As insect development depends on temperature (Verberk, Atkinson, Hoefnagel, Hirst, Horne, & Siepel, 2021), one can see the change of its dates and rates, including dates and intensity of diapause, as well as their synchrony with the host plant, particularly in spring. It results in changes in survival, fecundity, area boundaries, outbreak parameters (incidence, severity, duration, and intervals between such outbreaks), injuriousness, and the species composition of foliage-browsing insects (Knell, & Thackeray, 2016;Kirichenko, Augustin, & Kenis, 2019).
The study of statistical reporting on the dynamics of foliage-browsing insects also indicates a decrease in the number of species capable of forming outbreaks. This may be due to a change in the structure of both the deciduous stands themselves and the insects living in them. Thus, taking into account information on changes in the structure of complexes of foliagebrowsing insects in recent decades, it is necessary to quantitatively assess future trends and the threats to deciduous forests.
Objects and methods. Object of research -the trends of the habits of foliage-browsing insects in deciduous forests. Subject of research -representation of the foliage-browsing insects of deciduous forests depending on size, lifestyle, voltinism, trophic relations, and ability to mass propagation.
The aim of the research was to evaluate the representation of the foliage-browsing lepidopterous insects of different groups by size, lifestyle, voltinism, trophic relations, and ability to mass propagation in different periods of assessment for recent 70 years in deciduous forests of Ukraine.
In the analysis, we used a list of 118 species of foliage-browsing insects of deciduous forests, compiled on the basis of archival data from , and in the course of our own fi eld research from 1975 to the present.
Five hypotheses considered concerning trends in structure change of foliage-browsing insects of deciduous forests.
According to the fi rst hypothesis, there is a trend to increase the number of species of small size. For analysis, all species were divided into 3 groups by wingspan: large (over 40 mm), middle (21-40 mm), and small (below 20 mm).
According to the second hypothesis, there is a trend to increase the number of species with hidden lifestyles. All detected insect species were divided into 3 groups by lifestyle: open-living caterpillars feed on foliage without shelters, cobwebs, leaves held together, etc. Hidden-living insects feed under leaf cuticles (leaf-miners, for example). Semi-hidden-living insects most of the life cycle feed in the shelters, for example, cobwebbed, glued, or folded leaves (Соколова, Швиденко, Кардаш, 2020).
According to the third hypothesis, proportion of multivoltine species tends to increase. We considered as multivoltine ones those species which are known to be able to have more than one generation per year.
According to the fourth hypothesis, the proportion of polyphagous species tends to increase. We considered as polyphagous those species which can feed on more than one plant genus.
According to the fi fth hypothesis, the proportion of eruptive species tends to decrease. We considered the prodromal and eruptive species those ones which are capable of a multiple increase in abundance. However, in prodromal species, the abundance remains at the lower stationary state, while in eruptive species it can remain at lower or upper stationary level (Исаев, Пальникова, Суховольский, Тарасова, 2015).
For each species, all these parameters were identifi ed and proportions of species of each category for certain time periods (1940-1950, 1960-1970, 1980-1990, and 2010-2020) were evaluated.
Descriptive statistics of the data obtained were performed by employing the mean ± standard error of the mean. The proportions of species inside each analyzed characteristic were compared using the chi 2 test (Атраментова, Утевская, 2008). Microsoft Excel software and statistical software package PAST: Paleontological Statistics Software Package for Education and Data Analysis (Hammer, Harper, & Ryan, 2001) were used.
On some objects, the functional relationship between body size, development duration, and temperature of animals has been proven (Chown, & Nicolson, 2004). It was shown that against the background of the current climate warming, the size of the insect body decreases. At the same time, in regions with lower average annual temperatures, the increase in beetle size is statistically signifi cant.
The proportion of open-living lepidopterous foliagebrowsing insect species tends to decrease, semi-hidden species tends to increase, and hidden species tends to increase and then to decrease again (Fig. 2). The difference is the most pronounced between 1940-1950 and further periods (χ 2 -24.4-38.5; р < 10 -5 ) than between these periods (χ 2 -0.2-1.8; р > 0.1). The increase of the proportion of hidden species, particularly miners, is registered also in urban stands (Kirichenko, Augustin, & Kenis, 2019;Branco, Nunes, Roques, Fernandes, Orazio, & Jactel, 2019;, but they seem not to be dangerous for a forest. The trend of increase the proportion of bivoltine species and decrease the proportion of the univoltine species is rather slight (Fig. 3), and the difference is not statistically proved (χ 2 < 0.4; p >0.1). So this hypothesis is rejected, however, it may be due to an insuffi cient dataset. Most of the papers are devoted to an increase in the number of generations, this is logical since an earlier start and a later end of the growing season and an increase in the sum of temperatures during this period are favorable for the development of a larger number of generations (Knell, & Thackeray, 2016;Мєшкова, 2019;Teder, 2020). An increase in the proportion of multivoltine species has recently been reported in Finland (Pöyry, Leinonen, Söderman, Nieminen, Heikkinen, & Carter, 2011). However, in some cases the last generation does not enter the diapause before winter, contrary to expectations, but dies (Invasive stink bugs, 2017).
The shift under the warming the phases of the food plant and the caterpillars can bring to the earlier hatch than the available food appears, and entomophages will appear earlier or later than the phytophages hatch. In this case, those phytophages and entomophages that can adapt to feeding on a large number of hosts will gain advantages, that is, polyphages will receive advantages over monophages (Robinet, & Roques, 2010).
At the same time, in our set of data, the hypothesis of increase the proportion of polyphagous species and decrease the proportion of the monophagous species is not supported (χ 2 < 0.4; p > 0.1) (Fig. 4). It is possible that when considering a particular outbreak, conclusions regarding the ratio of polyphages to monophages will change. So, according to our research in the 80s in the focus of the Archips crataegana (Hübner, 1799), the caterpillars of all foliage-browsing species that lived there preferred to feed on oak leaves, but after severe defoliation of trees, the insects moved to other tree species and successfully completed their development (Мєшкова, 2002). In the years when the outbreak was collapsed, the largest populations of all species were again recorded in oak trees. That is, in the years of high insect population density and lack of oak foliage, these species became polyphages.
Thus, the data obtained confi rm that the reactions of species even to the same climate changes are species-specifi c. Therefore, an unpredictable change in their ratio in the community (Wagner, Fox, Salcido, & Dyer, 2021) will affect the consequences of climate change (Gilman, Urban, Tewksbury, Gilchrist, & Holt, 2010). Conclusions. Among lepidopterous foliagebrowsing insects of deciduous forests of Ukraine, the increase for recent 70 years was proved for the proportion of indifferent species (do not able to mass propagation), small species (with wingspan below 20 mm), as well as species with hidden and semihidden lifestyle. An increase in the proportion of smallsized species as well as for species with a hidden and semi-hidden lifestyle is known also for other groups of insects. Small size and hidden and semi-hidden lifestyle allow survival under high anthropogenic pressure.
The hypothesis about decrease the proportion of the univoltine and monophagous species for this period is not supported statistically.
However, both hypotheses should be tested in the future again, taking into account that polyphagia helps species that have lost the advantages of feeding on one host plant, in particular, as a result of a shift of the synchronicity of development, and multivoltine development allows a rapid increase in population number.