0761-B3

The Loss of Abies Sibirica Seed Stock from Megastigmus sp.

E.V. Bazhina and N.V. Belova ¹

Abstract

Stability of forest ecosystems is a common goal of forest management and nature protection. Stability is given when the populations of the involved forest tree species are persistent. Persistence of population results from its current state of adaptedness and from its adaptive and reproductive potential which due to seed stock. A study of fir (Abies sibirica Ledeb.) in the middle mountains of East Sayans demonstrates that seed-consumer plays a decisive role in the loss of fir seed stock. It was established that extent of seed elimination by the seed-consumer highly depends on initial amount of full seeds in stand, location of stands and features of the biology of seed-consumer. For the first time it was established that fir seeds are damaged by Megastigmus specularis Walley only.

Introduction

At present time the problem of Abies sibirica Ledeb. reforestation is becoming especially topical in the Southern Siberia mountains where intensive drying of fir-trees is observed. Fir drying has an influence on sex reproduction processes and results in biological diversity decrease and causes a disturbance in functioning of forest ecosystems in this region (Bazhina, Tretyakova, 2001).

Seeds are the most important and in many cases the only way of reproduction and preservation of biodiversity of conifers. The success of reforestation depends to a great extent on seed viability (their fullness, germinating power, energy of sprouting). External factors (extreme climatic conditions during pollination and fertilization, absence and low viability of pollen, air pollution, seed-consumers) have a great influence on the seed viability.

The significance of Abies sibirica seed-cosumers for forestry and their variety of species are studied insufficiently nowadays. In literature there is only some information about prevalence and harmfullness of insects, which larvae develop inside seeds. It is established, that in Irkutskaya Oblast territory two species of seed-consumers gen. Megastigmus, which cause a considerable damage develop in seeds of Abies sibirica (Golutvina, Stadnitsky, Grebenshikova, 1972). Specific variety and role of seed-consumers in loss of Abies sibirica seeds stock in the East Sayan middle mountains is practically unknown.

The purpose of this study is to determine specific variety of seed-consumers and loss of Abies sibirica seed stock caused by them in the middle mountains of East Sayans.

Materials and Methods

The study was carried on rezervation "Stolby" territory. Abies sibirica cones were collected in 2001 from trees, which grow in the river Laletina valley (elevation 500-540 m a. s. l.), in high-grassed aspen forest with admixture of fir and in the apper reaches of the Suhoy Kaltat river (south-west and north-west slopes of the Kedrovy stream, 640-680 m a. s. l.), in mixtoherboso-hylocomioso-calamagrostiosum fir-stands and in motley grassed fir-stands (830m.a. s. l., Kaydynsky range). The fir cones were husked, scales and seeds were counted by methods of D.A. Sabinin (Minina, Tretyakova, 1983). Seed productivity was measured as a quantity of seed scales producting seeds. Since, under each seed scale 2 ovules develop then the seed productivitywas calculated according to the formula: A=n/2N100 (%), where n- seed number is, N- total number of seed scales is. Seed quality was tested by X-rays method (Scherbakova, 1965). Quantity of full, empty and damaged seeds by insects was calculated in %. Five months later after collecting the seeds were placed on filtering paper in Petri dishes and hatcheries, 100-130 seeds in each for sprouting and getting imagos of seed-consumers. In the first collection 2600 seeds without stratification were analysed in all. In spring 2002 (May 5th) control seeds were collected in "Stolby". In the second collection 200 seeds wintered on soil were analysed. During sprouting the seeds from the first and the second collections were constantly moistened in Petri dishes. Average daily air temperature in the lab 24,0±0,2⁰С (min=21,5⁰, max=26,6⁰С).

Periodically 15-30 seeds from the hatcheries were dissected in order to check larva's condition. Every found alive larva was placed for observation in a separate hatchery on slightly moistened filtering paper. Imago were fed with concentrated glucose solution untill their natural death. Imago, dead and damaged larva were put into 70% alcohol. The larvae were boiled in KOH. Period of observation has lasted for 60 days for each collection of seeds. One female imago of the seed-consumer hatched, 12 pupae and 73 larvae were preparated from the first collection. 45 imagos hatched, 7 pupae and 43 larvae were preparated from the second collection.

Results

The study, conducted in the East Sayans middle mountains show that seed productivity of female cones is very high. This is a specific feature of Abies sibirica (Bazhina, 1998). In a good year in the East Sayans middle mountains (a territory of national park "Stolby") number of cones per tree can exceed 150 and seed productivity of each cone is 220-265 seeds (table 1).

One more feature of fir trees is stable seed production. Even in bad years absolute absence of yield in all trees is not observed. In 2001 Abies sibirica forms 30 (maximum 60) cones per tree in average. This is the average yeild. But a considerable amount of fir seeds is unviable (empty, without embryo). Besides, most of the seeds are damaged by seed-consumer.

X-ray analyses shows that part of damaged seeds exceeds 60% and can achieve 82.7-86.6% for individual trees (table 2). Part of fir seeds damaged by larvae varied greatly inside of one plot - variety coefficient was from 24,1% to 103,8%. There were differencies in damage by insects among different plots. Seeds of trees growing in high-grassed aspen forest with admixture of fir (500-540 m a.s.l.) get damaged most of all (from 25,0% to 86,6% for individual trees and 70,6% in average). Empty and decay seesds are 14,1%.

As elevation increases the amount of seeds damaged by insects decreases. At 640-680 m a.s.l, mixtoherboso-hylocomioso-calamagrostiosum fir-stands, the number of damaged seeds makes up 2.4-6.1% for some trees and is from 24,5% to 57,5% in average for different plots. It's necessary to note that part of empty seeds also increases here up to 17.8-35.3%.

Identification of imago and larvae shows that Abies sibirica seeds in national park "Stolby" are damaged by Megastigmus specularis Walley (Hymenoptera, Torymidae). In 2002 the flight of Megastigmus specularis Walley began in the middle of May. According to dissection of seeds, which were collected on May 5^th, 62,5% of alive larvae had already pupated and first imago hatched on May 14^th under laboratory conditions. During the first three days of the flight only males hatched. The alignment of sex was 1:1.4 (females- 57,9% and males - 42,1%). The period of flight was 8 days at average daily temperature of 24,0±0,2⁰С.

Condition of Megastigmus specularis considerably varied in different collections of seeds. Analyses of seeds of first collection 65,8% of larvae and 10,8% of pupa died in seeds, which were kept without stratification. Dissection of seeds after the end of pupation shows that from 23,4% to 75,0% of larvae was in diapause and 51,2±13,4% of alive larvae in average for all plots.

Analyses of seeds of the second collection in the beginning of the study and after the end of imago's flight shows that 19.4 - 23.8% of larvae died, 6,9% of imago couldn't go out of seeds, 24,0-25,0% of larvae stayed in diapause, and, finally 50,7% became imago during the wintering under natural conditions (% of total amount of insects). So, total death of Megastigmus specularis which wintered under natural conditions was comparatively low: 26,3-30,7%. There were no parasites of Megastigmus specularis found.

The study has shown that size of the population of the seed-consumer depends on amount of full seeds in stands (Figure 1). Ratio between seeds with larvae and full seeds is 42.5-73.3% on different plots. Amount of damaged seeds correlates with full seeds tightly (r=0,99**). Amount of seed-consumers correlates with amount of seeds with developed embryo for certain (r=0,71**).

Discussion

Megastigmus specularis Walley plays a major role in the loss of Abies sibirica seed stock in the middle mountains of East Sayans. Seed production of fir, weather conditions, pecularities of the seed-consumer's biology are the factors which regulate size of Megastigmus populations at average yield. In Russia this species is found in fir seeds in Ural also (Dolgin, 1988). It is established, that in Irkutskaya Oblast territory two species of seed-consumers gen. Megastigmus, which cause a considerable damage develop in seeds of Abies sibirica (Golutvina, Stadnitskiy, Grebenscikova, 1972). A. Rogues (1983) believes that Megastigmus specularis Walley has originated from North America and spreaded from Siberia to Finland and it also damages species gen. Abies (A.balsamea, A. lasiocarpa, A. nephrolepis, A. sacchalinensis, A. sibirica, A.veitchii).

Only one imago going out of seed, death of pupa and many dead larvae of the first collection is explained by the fact that the seeds were kept in warmth. During wintering under such condition the larvae spent its energy reserves soon. Megastigmus sp. larvae finish developing in early August. By this time contents of seeds are completely eaten. During diapause the larvae are in deep physiological peace because they winter in seeds on soil. In case fir seeds are kept in warmth the larvae are not in full diapause and spend energy on breathing. This is the reason for high death of larvae and pupa. Total death without seeds being stratificated was very high. It was three times higher in laboratory than under natural conditions, where some larvae also die. State of larvae after the end of eating seeds (depends on seeds' quality) and to what extent they are protected by seeds' peel are, probably, the factors which cause larvae death during wintering. Size of the seed-consumer stays high because of long-term diapause of larvae and Abies sibirica stable seed production. Long-term diapause is a characteristic feature of all specialised seed-consumers (Dolgin, 1978).

It's necessary to note that there are complicated regularities in fir seed damage caused by seed-consumers. Lack of pollen and, therefore, development of empty seeds limits food stock of the seed-consumer. That is why size of seed-consumer is low in fir populations with high amount of empty seeds.

Conclusions

1. For the first time it was established that fir seeds in the middle mountains of East Sayan are damaged by Megastigmus specularis Walley only.

2. The seed-consumer plays a decisive role in the loss of fir seed stock. Its influence increases as elevation decreases.

3. Death of Megastigmus specularis Walley occurs during wintering while insects are in larvae stage.

4. Size of the seed-consumer stays high because of long-term diapause of some larvae (max=75,0%, min=28,6%, 51.2% in average) and Abies sibirica stable seed production.

5. Extent of seed elimination by the seed-consumer highly depends on initial amount of full seeds in stand.

Acknoledgements

This work was financially supported by the Krasnoyarsk Region Foundation of Sciences, grant № 10F052М. We expressed our gratitude to Mr. A.V. Knorre, director of national park "Stolby" who allowed our researches on the "Stolby" territory.

Literature cited

Bazhina, E.V., 1998. Seed productivity of macrostrobiles and quantity of Abies sibirica seeds in zone of influence Baikalsk pulpaner and paper factory. Lesovedenie. 2: 10-15.

Bazhina, E.V. and I.N. Tretyakova, 2001: To problem of fir forests drying. The successes of modern biology. 121: 626-631.

Dolgin, M.M., 1988. The pests of conifer cones and seeds. In: Biological and technological basis of creation of constant forest seed base in Komi Republic. Syktyivkar, p. 73-81.

Golutvina, L.S. and Stadnitsky, G.V. and V.P. Grebenschikova, 1972. To question of seed-consumers of conifers. P. 18-29. In: Forest protection. Sciences paper, N 144 Leningrad: LTA.

Minina, E.G. and I.N. Tretyakova, 1983: Morphogenesis and Sex in Conifers, Nauka, Siberian Branch, Novosibirsk, 215 pp.

Scherbakova, M.A., 1965. Determination of quality of conifers seeds by X-ray analysis. Krasnoyarsk, 35 p.

Roques, A., 1983. Les insectes ravageurs des cones et graines de coniferes en France. INRA, Paris.

Table 1. Abies Sibirica Seed Productivity of Female Cones in the Eastern Sayans Middle Mountains.

Table 2. Abies sibirica seed quality in the middle mountains of East Sayans (%)

Figure 1. Regression between potencial number of full seeds (%) and seeds with seed-consumer (%).

¹ 660036, V.N. Sukachev Institute of Forest, SiberianBranch of Russian Academy of Sciense, Krasnoyarsk, Russia,
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