Seasonal dynamics of English oak acorns infestation by carpophagous insects in forest shelter belts in Kharkiv region

Abstract

Introduction

English oak (Quercus robur L.) is one of the most common forest-forming species in Ukraine. Its proportion in the forest fund of Ukraine makes up 27%. and it is also widely represented in forest shelter belts and urban green spaces. Recently, the health of oak stands has deteriorated in many regions due to climate change and significant anthropogenic impact. To restore the existing oak stands and establish new ones, it is necessary to harvest a sufficient quantity of high-quality acorns in the stands with the best growth and health, in permanent forest seed plots and seed plantations. However, oak fruiting is irregular, and acorns are infested by carpophagous insects, whose species composition, prevalence, and harmfulness vary among regions and stands. Until recently, few studies were devoted to oak carpophagous insects in Ukraine. Interest in this group increased following the detection of the invasive acorn moth, Blastobasis glandulella (Riley, 1871) (Blastobasidae), which inhabits not only Quercus acorns but also fruits of Juglans and Aesculus. In eastern Ukraine, in addition to the above-mentioned alien species, three indigenous species of acorn pests have been identified: Curculio glandium Marsh., 1802 (Coleoptera: Curculionidae); Cydia splendana (H?bner, 1799) and Cydia amplana (H?bner, 1799) (Lepidoptera: Tortricidae). Their prevalence varied among regions (Chernihiv, Poltava, and Kharkiv) and among specific biotopes. Differences in seasonal development and timing of possible acorn infestation among these species complicate the planning and implementation of effective protection measures.

This research aimed to study the timing of acorn infestation by carpophagous insects and their composition during the growing season.

Materials and Methods

The research was carried out in forest shelter belts dominated by early-flushing English oak within the territory of the Educational, Scientific and Production Center “Dokuchaevske Experimental Field” of the State Biotechnological University (SBTU) (49°89' N, 36°45' E). Beginning in the first decade of April, trees were regularly inspected to determine the dates of the development of vegetative and generative organs of oaks and to record the presence of carpophagous insects. After budburst, the size of the leaves and shoots was recorded, and subsequently, acorn length and diameter were measured weekly using a caliper. Following the first detection of acorn infestation by carpophagous insects (first decade of July) and until acorn fall at the end of October, 100 acorns were collected weekly, their diameter and length were measured, each acorn was dissected and examined for presence of entry holes, cracks, and carpophagous insect eggs or larvae. 

Results and Conclusions

The oak vegetation began 15 days earlier than the long-term dates. The size of the leaves reached its maximum in the second decade of May, and the length of the shoots in the third decade of May. Acorn growth lasted from the third decade of May to the first decade of September, with a total of 2,149.7 °C of active temperatures. Among 1,200 acorns analyzed from the second decade of July to the end of October, 38.5% were inhabited by insects, in particular 23.4% by acorn weevil, 12.2% by tortricids, and 2.9% by acorn moth. Among acorns with larvae (462 specimens), 60.8% were inhabited by weevils, 31.6% by tortricids, and 7.6% by acorn moths. Acorn weevils inhabited acorns from the first decade of July, when acorn length did not exceed 10 mm. Cydia sp. infestation began in the third decade of July, whereas acorn moth infestation was first recorded in the first decade of August. Despite the accelerated hardening of the acorn shell beginning in the second decade of July, carpophagous insects were able to penetrate acorns through natural cracks and holes gnawed by other insect species.

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