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ŠUMARSKI LIST 1-2/2013 str. 54     <-- 54 -->        PDF

1988) generally decreases in the course of the growing season (Hrdlička, Kula 2001, Šrámek et al. 2009, Rodin, Bazilevich 1967).
A hypothesis: The increased content of nitrogen in leaves can affect positively the development of caterpillars of species feeding during the growing season and negatively of species occurring as late as the second half of the growing season. The aim of the paper is to verify responses of caterpillars of gypsy moth reared on leaves of birch (Betula pendula Roth) with the differentiated content of nitrogen.
Gypsy moth Lymantria dispar L. (Lepidoptera: Lymantriidae) – an Eurasian species spread from Europe to North America where it became the most important pest (McManus, Csóka 2007). It is a widely polyphagous species attacking more than 300 species of trees and shrubs from at least 14 plant families (Doane, McManus 1981, Lechowicz, Mauffette 1986). In Europe, it prefers oak, hornbeam, beech, chestnut, but also birch, linden, willow, poplar, maple, alder and larch. At the same time, it does harms to fruit trees (Schwenke 1978). In the USA, even defoliation of conifers (particularly of spruce) occurs at gradations (McManus, Csóka 2007).
The growth and mortality of caterpillars of L. dispar, duration of their development, the weight of pupae and fertility of females are affected by a host plant (Barbosa, Greenblatt 1979, Roden, Mattson 2008, Kinney et al. 1997).
In Central Europe, gypsy moth hatches usually in August and September. Females create clutches containing 500–800 eggs on the bark of broadleaved species. In an egg, the embryogenesis of which was finished during the growing season, a diapausing caterpillar overwinters. Caterpillars hatch in April or at the beginning of May, climb to crowns of trees or spread on web fibres to the surroundings (Schwenke 1978). Male caterpillars come through 5 (exceptionally 6) instars, female 6 (exceptionally 7) instars (Leonard 1966). Under Central-European conditions, caterpillars develop 6–12 weeks depending on weather and food quality. At the turn of June and July, they pupate on trees. Pupae are fixed to the stem bark by means of thin web. After 2–3 weeks, moths hatch (Schwenke 1978).
Birch (B. pendula) grows as an admixture in cultivated forests, on derelict land often disturbed by anthropogenic effects (mine dumps, spoil banks). In air-polluted areas of the ČR, it is a dominant substitute tree species (Slodičák et al. 2008) creating also spontaneous monocultures. The species occurs on poor dry soils as well as on extremely acid sites (pH 3.5–5.0); it is nearly missing on mesotrophic sites (particularly limestone) (Hejný, Slavík 1990). It responds negatively by decreased increment to reclamation liming (Kula 2009). Increased inputs of nitrogen at simultaneous drought stress cause intensive summer defoliation (Kaňová, Kula 2004) and the high level of nitrogen limits height and diameter increment (Kula, Pešlová, Martinek 2012).
Methods of research
Metode istraživanja
Annual future nutrient plants of birch B. pendula were planted out into containers (volume 10 l) with soil substrate taken in the Cambic mineral horizon of a forest soil and placed into plastic greenhouses in a forest nursery (Brno – Řeč­kovice, altitude 220 m) (Kula, Pešlová, Martinek 2012). Changes in the content of nitrogen in the substrate were caused by the application of ammonium nitrate (NH4NO3) to a plant in three treatments: 0.5 g (T1), 1 g (T2) and 1.5 g (T3). The fertilization was carried out in the year of outplanting (2006) in regular five-week intervals (four-times), in further years (2007–2009) five times in the growing season. The amount of applied ammonium nitrate was derived from nitrogen depositions in the Ore Mountains according to the ČHMÚ (Czech Hydrometeorological Institute) data.
According to the ICP Forests methodology, mature leaves were sampled from the upper half of a birch crown (except four terminal) at the turn of August and September. In the leaves, the content of nitrogen was determined after desiccation at 70 °C according to the method of Kjeldahl using a tecator Kjeltec analyzer UNIT 2300 (Kula, Pešlová, Martinek 2012).
On 22 March 2009, eggs of gypsy moth were transported from oak stands of the Lednice-Valtice area (48°44’45.085"N, 16°49’7.276"E). The rearing of caterpillars was carried out in Petri dishes. In total, 600 caterpillars of the 1st instar were divided into 60 Petri dishes at 10 pieces. Each of the treatments (T1, T2, T3) including control (T0) showed 15 repetitions. The rearing of caterpillars was carried out under controll. Day light 10 hours, temperature 17.8 °C (14 June increased to 20.3 °C), RH (relative humidity) 60 %.
Without exposure 6 hours, temperature 13 °C (14 June increased to 15.5 °C), RH 85 %.
Transitional light conditions with 20 % intensity twice at four hours with the continuous change of temperature and humidity.
Twigs with leaves from nutrient plants (B. pendula) according to treatments (T0–T3) were sampled at an interval of 48-hours in plastic greenhouses and transported to a laboratory in a tempered box with a laboratory temperature (circa 20 °C) in the course of 1 hour. Leaves with the determined area were used for the rearing.
The instar of caterpillars was noted continuously [according to the cranium width 0.6–1.2–2.2–3.2–4.4–6 mm (Schwenke 1978)] and mortality of caterpillars, pupation and pupa weight. Due to the increased consumption of food caterpillars were reared individually from the 4th instar. The area of feeding was determined as a difference between the input area of a leave and the leaf remainder after the caterpillar feeding using an ADC BioScientific Leaf Area Meter AM300.