Forster, B.; Knizek, M.; Grodzki, W. (eds.) 1999: Methodology ofForest Insect and Disease Survey in Central Europe.
Proceedings ofthe Second Workshop ofthe IUFRO WP 7.03.10, April20-23, 1999, Sion-Chiiteauneuf, Switzerland.
Birmensdorf, Swiss Federal Institute for Forest, Snow and Landscape Research (WSL) 144-148.
THE DYNAMICS OF THE GYPSY MOTH POPULATION AT A LOW LEVEL OF ITS ABUNDANCE
Golubeva T.V.
Forest Institute ofthe National Academy of Science ofBelarus, Department ofF orest Protection,
71 Proletarskaya Street, 246654 Gomel, Belarus Marchenko J.l.
Republican Enterprise ofF orest Protection and Monitoring <<Bellesozashchita»
of the Ministry for Fotrestry,
70 Bagration Street, 220037 Minsk, Belarus
The gypsy moth (Lymantria dispar L.) is one of the most abundant and hazardous forest pests. This pest is chiefly destructive to foliage of oaks. At times, infestations of the gypsy moth are discovered in different areas in Belarus (Marchenko, 1998). Numerous studies (Vasechko, 1990; Logoida, 1992; Liamtsev, 1997) were conducted on population characteristics of this pest. Most of investigators describe the population when mass outbreaks occur (Benkevich, 1984; Nealis and Erb, 1993). However, of great interest is the study of the gypsy moth population when its abundance is low since it is important to define factors that govern its development. We did research on the gypsy moth population at a stationary plot of 15.5 hectares that can be regarded as a reservation of a chronic infestation of this pest in the Gomel Province. This is a 50-year-old insular stand of Yield Class V surrounded by ameliorated agricultural land. The percentage of oaks is 86.4%. The stand density is 0.6-0.7.
The environmental conditions (loamy sand and deep groundwater) do not favor the development of oak trees there. Perhaps, these unfavorable conditions are responsible for physiological deterioration of the stand which is thus has become a reservation of the gypsy moth.
The Department of Forest Protection of the Forest Institute launched the study of the gypsy moth population in 1991 when this was in depression (in the years 1991-1996 Dr.
Marchenko J.I. was a chief scientist). At this period the lowest abundance was 0.1 egg mass per 100 trees. A slow increase in the abundance of this phytophage has been observed recently. Since that time the staff of the department has been performing extensive investigations using common methods such as making records of the pest at different developmental stages, revealing and estimating the factors responsible for mortality at each developmental stage, estimating the factors of replenishment of the population (fecundity and male to female ratios), making records with the use of traps, isolators, model branches and trees as well as a visual method.
All insects, including the gypsy moth, are known to pass through several instars. At each instar the size of the population reduces due to different biotic and abiotic factors. The extent of the involvement of these factors or their components is distinct at each developmental stage. In our analysis of the data obtained we should like to dwell on characteristics of the gypsy moth population of only two recent generations, 1996-1997 and 1997-1998.
It should be noted that the distribution of egg masses over the area is not uniform. The extent of aggregation reduces as the abundance increases. Over seven generations the number of egg masses increased by a factor of 38, the number of eggs per female averaging 300 to 500 (Fig.). Over the two generations mortality at the egg stage due to both the biotic and abiotic factors averaged 28.7% (Tables 1 and 2). Interestingly, over the 1996-1997 generation
larvae were found with no evidence of a disease or a damage. The eggs were not infested by parasites, either. The percentage of egg masses affected by predators, namely, Dermestes, mammals and birds varied from 10.3 to 11.8%. The percentage ofnon-fecundated eggs was insignificant, 0.4 - 0.8%.
2500
Ill CD
2000
0 0
....
1500
Ill Cl Cl CD 1000
...
0..
CD J:lE 500
z :s
0
1992 1993 1994 1995 1996 1997 1998
generation, years
Fig. The dynamics of the gypsy moth population density at the egg stage over seven generations
A drastic decrease in the population size is observed at larval instars. For instance, over the two generations mortality of larvae was 83.6% as against the larvae hatched. The action of the abiotic factors is the most noticeable only in first instar larvae since these are still susceptible to the effect of temperature differences, many larvae migrate and some of them have to do very little feeding since hatching is non-synchronous with foliage expansion of oaks. This is the so-called «infant mortality». Over the two generations the percentage of dead individuals was 21.6% as against the percentage of the individuals died at the larval instars.
In successive developmental stages and as a larva ages the abiotic factors decrease significantly in importance, while the biotic factors come into absolute importance in the dynamics of the population. Just at this period parasitic insects contribute to the biological control of the gypsy moth. For instance, mortality of second and third instar larvae was often caused by Braconidae of the genus Apanteles. Occasionally mortality of the gypsy moth individuals was caused by parasitic tachina flies. Mortality of fourth, fifth and sixth instar larvae was also mainly caused by tachina fly, Parasitigena silvestris R-D. Mortality of some larvae was due to Apanteles liparidis and Apanteles melanoscelus.
On the whole, over the two generations the percentages of gypsy moth larvae died due to parasites, diseases and predators were 27.3%, 6.8%, and 19.7%, respectively. The actual loss of the larvae (calculated with the data on the abundance at the beginning and at the end of the age interval) appeared to be greater than that deduced from the experiment. In other words, we failed to reveal other factors of mortality. These might apparently be predators that feed on the gypsy moth larvae such as representatives of different species of Carabidae and Silphidae as well as birds. The effect of mice must not be ruled out.
Age interval
1 Eggs
Larvae, L1
Larvae, L2 and L3
Larvae, L4 - L6
Pupae
Moths, incl.
females
Mortality over
Table 1.
Mortality and survivorship of the gypsy moth population over the 1996 - 1997 generation
Population density Mortality over the given Factors of mortality by the beginning of age interval
age interval, pcs/100
pcs/1 00 trees % trees
2 3 4 5
Non-fecundated 1600 0.8 13
Frosts 23.6 378
Diseases 0 0
Parasites 0 0
Predators 10,3 164
Total: 34.7 555
<<Infant mortality» 1045 23.2 243
Diseases 0 0
Parasites 2.9 30
Predators 4.6 48
Total: 30.7 321
Diseases 724 9.0 65
Parasites 25.3 183
Predators 10.2 74
Undetermined factors 5.2 38
Total: 49.7 360
Diseases 364 7.7 28
Parasites 44.8 163
Predators 8.5 31
Undetermined factors 7.1 26
Total: 68.1 248
Diseases 116 24.1 28
Parasites 32.8 38
Predators 12.1 14
Undetermined factors 6.0 7
Total: 75.0 87
29 14
the generation: 98.2% 1571
Age interval
1 Eggs
Larvae, Lt
Larvae, L3
Larvae, L4 - L6
Pupae
Moths, incl.
females
Table 2.
Mortality and survivorship of the gypsy moth population over the 1997 - 1998 generation
Population density Mortality over the given Factors of mortality by the beginning of age interval
the age interval, pcs/100
pcs/1 00 trees % trees
2 3 4 5
Non-fecundated 2301 0.4 8
Frosts 11.6 267
Diseases 0.9 21
Parasites 0 0
Predators 11.8 271
Total: 24.7 567
<<Infant mortality>> 1734 20.6 357
Diseases 1.5 27
Parasites 0.5 9
Predators 5.7 98
Undetermined factors 11.6 201
Total: 39.9 692
Diseases 1042 2.7 28
Parasites 12.0 125
Predators 14.8 154
Undetermined factors 13.2 138
Total: 42.7 445
Diseases 597 1.3 8
Parasites 20.7 124
Predators 8.8 53
Undetermined factors 12.1 72
Total: 42.9 257
Diseases 340 19.0 65
Parasites 33.0 112
Predators 12.6 43
Undetermined factors 14.2 48
Total: 78.8 268
72 37
the generation: 96.9% 2229
Counts of the pupae collected in traps showed that over the period of pupation the size of the gypsy moth population had also reduced drastically. For instance, the percentage of pupae died over the two generations averaged 77 .6%. It was determined that mortality of most of the pupae (32.9% of the total number of dead pupae) was caused by parasites, among which were Tachinidae and Sarcafagidae, Parasorcophaga uliginosa Kram. in particular. Insect- eating birds played a definite· part in the reduction of the population. The gypsy moth pupae were also annihilated by Carabidae whose imagines were often found in traps.
At the imago stage the abundance was calculated using the number of healthy pupae and deposited eggs recorded as the base. At this stage the reductions appeared insignificant and were apparently caused by insect-eating birds who were observed feeding on gypsy moth females when they deposited eggs. Traces of dermested beetles on the deposited eggs were evident. At the imago stage the abiotic factors can influence the population indirectly. For instance, precipitation can impede flights of the moths when they copulate and deposit eggs.
As in the former year, the trend has been toward equalizing the sex ratios, suggesting that viability of the gypsy moth population is increasing.
According to the results obtained over a generation the gypsy moth population has reduced by 96.9 to 98.2%. The pests survived not only reproduction, but also a rise in abundance of the new generation. Mortality was chiefly caused by the biotic factors (predominating at the vegetation, postembryonic period) that are of great importance in the light of developing the elements of biorational control and self-control of abundance of forest pest populations.
References
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2. Vasechko G.l. The development ofll'insky's ideas in understanding ofthe dynamics of the gypsy moth abundance. Journal ofForest Science.l990. No 6. 41-47. (in Russian).
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(Lepidoptera, Orgyidae) abundance in the oak stands of the Transcarpathians. Journal of Ecology. 1992. No 1. 72-77. (in Russian).
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5. Marchenko J .1. The dynamics of the gypsy moth abundance in the conditions of Belarus. Proceedings of the International Symposium on Biological and Integrated Forest Protection. (pp. 71-72) Pushkino, Moscow Province, 7-11 September 1998 (in Russian).
6. Nealis V.G., Erb S.A. Sourcebook management of the gypsy moth. Forest Canada.
Ontario Region. Great Lakes Forestry Center. 1993. 48.
