9. The level of threat to forests posed by the more important folivorous insect species

The poor composition of tree species and other stand characteristics combined with weather conditions and pollutant deposition are the main factors favouring outbreaks of folivorous insects. The population abundances of insect pest species change over time. There are years when the changes in population abundances are insignificant, but at times, under favourable conditions, the abundance of the insect pests can rapidly increase causing severe damage to forest stands.

The analysis of a threat to forests posed by insect pests during the last five years was based on results of the nun moth catches, as well as evaluation of abundances of other folivorous insects and pine-shoot beetles made on the first-level POPs located in the coniferous stands.

The nun moth (Lymantria monacha) is one of the most dangerous insect pests. Its abundance in older stands during the last five years was high and variable. In the years 1998, 2000 and 2002, the average countrywide abundances were remarkably higher (145.5, 214.3 and160.4 individuals/trap, respectively) than those in the years 1997 and 2001 (119.5 and 106.7 individuals/trap, respectively). Similar variability was also noted on a regional scale - see Fig. 16. The year 1999 saw a decrease in abundances of this pest in some, but not all the natural-forest regions (Mazury-Podlasie, Silesian and Sudeten natural-forest regions). The highest abundances of this pest were recorded in 2000 and the lowest ones in 2001.

The great threat posed by the nun moth during the last five-year period, in spite of its high variability over time, was observed in the Mazury-Podlasie, Wielkopolska-Pomerania, Mazowsze-Podlasie and Małopolska natural-forest regions. In the even-numbered years, the abundances ranged from 146.0 to 284.7 individuals per trap and in the uneven-numbered years - from 83.9 to 157.6 individuals per trap. The lowest number of trapped insects was found in the Carpathian natural-forest region (from 79.1 to 62.9 individuals/trap) and low in the Sudeten natural-forest region (from 51.0 to 115.3 individuals/trap).

In 2002, the threat by the nun moth was greater than in 2002 in all natural-forest regions with the exception to the Sudeten natural-forest region. The greatest increase in the number of captured insects when compared with 2001 was recorded in the Mazowsze-Podlasie natural-forest region. In 2002, the population abundances of this species were low in the Sudeten and Carpathian natural-forest regions (51.0 and 65.1 individuals per trap) while in other natural-forest regions they were very high (from 157.0 to 194.2 individuals/trap) - see Fig. 17.

The occurrences of the nun moth in younger stands were evaluated in 1998, 2000 and 2002, when the abundances of this pest in older stands were high. When compared to older stands, the abundances of insects in some natural-forest regions and in some years were lower, and higher in other. No regularity was found here.

The year 2000 saw the highest abundances in older stands and in the majority of natural-forest regions. In 2002, low abundances of this pest were recorded in the Sudeten natural-forest region (35.3 individuals/trap). In other natural-forest regions the abundances were high (from 139.8 to 166.7 individuals/trap).

Pine sawflies (Diprionidae) are the pest of pine, which recently have posed a threat to forest. In the period 1998-1999, their population abundances in the older stands was steady over a large area of the country. However, in 2000 and 2001, their abundances were elevated; the average figures for the country were similar (0.89 and 0.84 individuals/trap, respectively) and slightly higher than the means in the period 1998-1999 (0.54 and 0.59 individuals/plot, respectively). The year 2002 saw an increase in abundances of this pest and the country mean increased to 1.53 individuals per plot - see Fig. 18. In 5 natural-forest regions the abundances of diprionids increased, while a decrease was recorded in 2 regions. The number of insects exceeding 2 individuals per plot occurred in the Małopolska and Mazowsze-Podlasie natural-forest regions. An increase in the threat posed by diprionids during the last five years was also noted in younger stands. The threat to forests during that period was great in the Wielkopolska-Pomerania and Mazowsze-Podlasie natural-forest regions, and low in the Baltic and Sudeten natural-forest regions.

The pine looper moth (Bupalus piniarius) is another dangerous folivorous pest. In 1998, 1999 and 2002, the mean abundances for the country in older forest stands were 1.14. 1.37 and 1.29 individuals per plot, respectively. In the period 2000 and 2001, the mean values were low 0.30 and 0.23 individuals per plot, respectively. In the years with the elevated abundances of this pest over all country, the means for the Baltic and Wielkopolska-Pomerania natural-forest regions were markedly higher (from 1.19 to 2.72 individuals/plot) than in other natural-forest regions - see Fig. 19. In the years 1998 and 2002, the number of this pest in younger stands was also higher than in 2000. The most threatened by the pine looper moth in 1998 were stands of the Wielkopolska-Pomerania and Baltic natural-forest regions (from 0.97 to 1.70 individuals/plot). In 2002, the elevated abundances occurred in the Wielkopolska-Pomerania, Baltic, Mazowsze-Podlasie and Małopolska natural-forest regions (from 1.08 to 1.88 individuals/plot lot).

The abundance of the pine hawk moth (Sphinx pinastri) in the period 1998-2001 was stable. The mean population abundance for the country in older stands ranged from 0.20 to 0.33 individuals per plot. The elevated abundance of this pest during the last five years was noted in the Wielkopolska-Pomerania natural-forest region - see Fig. 20. In 2002, the abundance was higher than in 2001 in 6 natural-forest regions. The country mean increased to reach 0.46. The greatest number of the trapped insects was recorded in the Wielkopolska-Pomerania natural-forest region (0.72 individuals/plot). In younger stands, the abundances of pine hawk moth were also low. The abundance of this pest in the successive study years over all country amounted to 0.23, 0.14 and 0.20 individuals per plot. In the Wielkopolska-Pomerania natural-forest region the abundances of the pest in each measurement year exceeded the country mean.

The pine beauty moth (Panolis flammea) is one of the most dangerous pests of pine stands. During the last five years, the greatest threat posed by this insect to older stands was observed in the years 1999 and 2000. In 2001, the threat to forests markedly decreased in all natural-forest regions. The year 2002 saw a further decline in abundance of this insect over the prevailing area of the country. The increase in abundance was recorded in some regions, however the figures were not alarming. The greatest number of these insects was found on the plots in the Carpathian natural-forest region. The 2000 saw an increase in abundance of the pine beauty moth both in younger stands and older stands. The country mean increased from 0.19 in 1998 to 2.37 individuals per plot in 2000. The greatest numbers of the captured insects (5.43 individuals per plot) were recorded in the Wielkopolska-Pomerania natural-forest region. In 2002, a distinct decline in abundance of this pest was noted in most of natural-forest regions to reach the level of 1998 or even lower in some regions. The country mean dropped to 0.11 individuals per plot.

The abundance of the pine lappet moth (Dendrolimus pini) remains low over the prevailing area of the country. The mean abundance for the country in the period 1998-2000 was notably low from 0.02 to 0.24 individuals per plot). In 2001, it drastically increased reaching the value 1.41 individuals per plot to decrease again in 2002 (0.14 individuals per plot). An increase in the 2001 mean abundance was caused by locally significant increases in abundances of the pine lappet in the Regional Directorates of the State Forests in Toruń (up to 17.22 individuals per plot), in Szczecin (up to 0.82 individuals per plot) and in ŁódŸ (up to 0.41 individuals per plot). In 2002, the increased abundances were not recorded; the values dropped to the level before 2000, or were even lower. In younger stands, the abundance of the pine lappet in most regions remained on a zero level during the measurement period. A local increase in population abundance of the pest was noted in the Wielkopolska-Pomerania natural-forest region.

The pine webworm (Acantholyda nemoralis) did not pose a major threat to forests during the last five years. The country mean in the older stands ranged from 0.28 to 0.58 individuals per plot. The abundances of this insect pest did not markedly changed. An increase in abundances of this pest was noted in the natural-forest regions: Silesian (in the period 1999-2002 from 0.80 to 1.00 individuals/plot) and Małopolska (in the years 1998, 2002 from 0.90 to0.80 individuals/plot, respectively). In 2002, the greatest number of insects of this species was trapped on the plots in the Małopolska and Silesian natural-forest regions (0.80 and 1.00 individuals/plot). In younger stands, the mean abundances for the country were not high. The abundances were significantly higher (from1.40 to 4.10 individuals/plot) during the whole measurement period in the Silesian natural-forest region. In 2000, no significant changes in the population size of the pine webworm were observed in the younger stands.

Pine-shoot beetles (Tomicus spp.) are the most frequently encountered secondary pests of pine stands. The mean numbers of shoots damaged by pine-shoot beetles collected in older stands on POPs in 1998, 2000 and 2001 was comparable (from 6.4 to 6.8 shoots/plot). In 1999 and 2002, these figures were higher (8.7 and 8.3 shoots/plot). In 1999, the greatest average number of damaged shoots was found on the POPs in the Carpathian natural-forest regions (25 shoots/plot), more than 10 shoots per plot were collected on POPs in the Baltic (11.5 shoots/plot) and Mazury-Podlasie (10.9 shoots/plot) natural-forest regions. The mean number of damaged shoots collected on POPs over all country was 6.7 shoots per plot and was higher than in the older stands. The greatest number of damaged shoots was collected in the in the Mazury-Podlasie (16.6 shoots/plot) and Mazowsze-Podlasie (14.1 shoots/plot) natural-forest regions. In 2000, the country mean was 6.3 shoots per plot and was comparable to the results for the older stands. The greatest number of damaged shoots was found in the Mazury-Podlasie Podlasie natural-forest region (10.2 shoots/plot). In 2002, the country mean notably increased reaching 8.9 shoots per plot. Great quantities of damaged shoots per plot in that year were collected in the Mazowsze-Podlasie natural-forest region (26.9 shoots/plot).

The greatest threat posed by folivorous insect pests to the older coniferous stands during the last five-year period occurred in the following natural-forest regions: Wielkopolska-Pomerania (stands threatened by 6 pest species except for pine webworm and pine-shoot beetles) and Mazowsze-Podlasie (stands threatened by 5 pest species except for pine looper, pine lappet moth and pine webworm). The lowest threat to coniferous stands was in the Mazury-Podlasie natural-forest region (no threat from 7 pest species except for nun moth) and Carpathian natural-forest region (no threat from 7 pest species except for pine-shoot beetles).

The greatest threat by folivorous insect pests to the younger coniferous stands during the last five-year period occurred in the Wielkopolska-Pomerania natural-forest region (stands threatened by 6 pest species except for pine webworm and pine-shoot beetles), the great threat - in the Mazowsze-Podlasie natural-forest region (stands threatened by 5 pest species except for pine hawk moth, pine lappet moth and pine webworm). The least threatened were stands in the natural-forest regions: Sudeten (no threat from 7 pest species except for pine webworm), Silesian (no threat from 6 pest species except for nun moth and pine webworm) and Baltic (no threat from 6 pest species except for nun moth pine looper moth).

It was possible to identify pest species most harmful to stands for each year over the five-year period. In 1998, the great threat to forest stands was posed by the nun moth and pine looper moth, in 1999 - by the pine beauty moth and pine looper moth and in 2001 - by pine lappet moth. The year 2002 saw an increase in the number of the nun moth, pine sawflies, pine looper moth and pine hawk moth.