The population of stem insects depends on many factors, such as the region and forest conditions. Also the type, intensity and prescription of a weakening of plantations, which determines the distribution of trees according to their sanitary status and their susceptibility to colonization by insects, are influencing, too.
The species composition of stem insects, including in Left-bank Forest-steppe of Ukraine, was studied and was clarified by many authors. However, the identification of the peculiarities of the population of these insects in pine stands weakened by individual factors receives little attention.
Material and Methods
The research in the plantations of the forest-steppe part of the Kharkov region was carried out in 2002–2013. The sample plots were laid in pure pine stands affected by a ground fire, root rot (Heterobasidion annosum (Fr.) Bref.) and by insects (Diprion pini L., Panolis flammea Schiff., Bupalus piniarius L.). The trees of I–III categories of sanitary condition with a diameter of 26–30 cm were selected in the first decade of April. They were cut into two-meter length parts and were inspected weekly for registration of inlet and outlet holes appearance.
We have investigated that six species among the invasive stem insect species in pine plantations in the region were detected more frequently. Tomicus piniperda L. (39 %) dominated in the focus of the root rot, Ips sexdentatus Boern. (47.3 %) and Tomicus minor Hart. (41.2 %) dominated in the ground fire area. Т. minor Hart. (42.3 %) dominated in the foci of foliage browsing insects. Colonization by Ips acuminatus Gyll. was observed in the area of thin bark. Сolonization by Т. minor was found in the area of thin and transition bark. Other aggressive species were found in the area of thick and transition bark. Depending on time when plants are weakened different fenocomplexes of insects begin to form under the bark of trees. Т. piniperda and T. minor were found in the spring. I. acuminatus and sexdentatus were found in spring and summer. Monochamus galloprovincialis Germ. and Phaenops cyaneus Fabr. were found in the summertime. Stem pests’ colonization of the different parts of a stem depends on the tree diameter. The number of the insects reduces as diameter decreases. The lower part of a tree was the most colonized by stem pests.
18 species of stem insects were detected during the examination of trapping trees. Among the aggressive representatives of stem pests, six species were the most frequent (Tomicus piniperda L., Ips sexdentatus Boern., Tomicus minor Hart., Ips acuminatus Gyll., Phaenops cyaneus Fabr. and Monochamus galloprovincialis Germ.). Tomicus piniperda L. dominated in plantations infected by root rot. Ips sexdentatus and T. minor dominated on the sites weakened by ground fire. The population of stem insects of the lower and upper parts of the tree section depends on the diameter of the segment. If the diameter is smaller the population is less.