Role of meteorological factors in reducing the production of Ips acuminatus Gyll. in pine stands of Eastern Polissya

Abstract

Introduction

Intense group dieback of pine trees took place in the pine stands of East Polissya during 2015–2018. Ips acuminatus spread in the dieback foci and accounted for a low vitality of the trees inhabited by it. The aim of the research was to study the role of meteorological factors in reducing the production of Ips acuminatus in the pine stands of Eastern Polissya.

Materials and Methods

The research was conducted in pure middle-aged and mature pine stands, which had different spread degree of group dieback foci. Population indices of Ips acuminatus were determined in accordance with generally accepted methods of forest-pathological and entomological research.

Results

After winter 2018–2019 mortality of the beetles hibernating under the bark was 80% of the total number of imagoes detected. This indicator was 3–5 times higher compared to the previous years. In the bark beetles’ wintering zone, at a relative height of the trunks of 0.2–0.4, the tissues of the trees’ trunks remained viable; there also was a high resinous activity during that year. Therefore, the imago died as a result of resin exudation.

Acting dieback foci in the pines were recorded at the end of summer 2019, that is after the completion of the first generation of apical bark beetles. The dieback foci were mostly of group type and included up to 10 trees.

In the dieback foci, the beetles started colonizing pine stems in the zone of thin bark at a relative height of more than 0.5. The insects failed to inhabit the lower parts of the trunks due to resin exudation. A partial resin exudation covered a young generation of bark beetles at a relative height of 0.75. It prevented the active development of thebark beetle population and resulted in a considerable decrease in their quantity. In middle-aged and mature stands, the production rate of the first generation of Ips acuminatus in the thin bark zone of the attacked trees varied from 29 to 33 pcs. per dm². The development of beetles focused at a relative height of trees of 0.8, where resin exudation was not sufficient to prevent it.

The beginning of the 2019 growing season proved to be very humid. In April, it fell by 58% more compared to the long-term norm during this period, and in May, the figure was close to the long-term norm. The value of the Hydro-thermal Coefficient significantly exceeded the average long-term norm due to the combination of excess rainfall and reduced temperature. The humidity level during that period ensured sufficient water supply for pine trees to have an intensified ability to exudate resin as well as high resistance to insect settling.

Conclusions

In the pine stands of Eastern Polissya, Ukraine, group dieback foci began to intensively spread after a very dry second half of the 2016 growing season and lasted for 2017–2018. Those years were extremely arid at the beginning of the growing season when Selyaninov’s Hydro-thermal Coefficient was 0.1–0.5 and precipitation deficit made up to 10–30 % in various months. After the beginning of the 2019 vegetation period, which was favourable for tree growth with the hydrothermal coefficient of 1.7, the spread of pathological pine tree dieback in Eastern Polissya decreased significantly. It has stabilized with the predominance of single and group types as a result of excess atmospheric precipitation and reduced temperature. The renewed water supply to the trees intensified their resin exudation, which, in turn, considerably hindered insects’ attempts to populate stems at a relative height of 0.51–0.75 and below. The average production along the stems of inhabited trees ranged from 29 to 33 beetles per dm².

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