Piotr KAROLEWSKI, Jacek GRZEBYTA, Jacek OLEKSYN, Marian J. GIERTYCH
1 Institute of Dendrology, Polish Academy of Sciences, Parkowa 5, 62-035 Kórnik, Poland, e-mail: pkarolew@man.poznan.pl
EFFECTS OF TEMPERATURE ON LARVAL SURVIVAL RATE AND DURATION OF DEVELOPMENT IN LYMANTRIA MONACHA (L.)
ON NEEDLES OF PINUS SYLVESTRIS (L.) AND IN L. DISPAR (L.) ON LEAVES OF QUERCUS ROBUR (L.)
Short research contribution
ABSTRACT: This study assessed the effects of temperature on survival rate and duration of development (DD), i.e., time needed by larvae to reach the pupal stage, in nun moth (Lymantria monacha L.) and gypsy moth (L. dispar L.). The larvae were raised at 15, 20 or 25°C, and fed with current-year-needles of Scots pine (Pinus sylves- tris L.) and leaves of English oak (Quercus robur L.), respectively. With increasing temperature 15, 20 and 25°C, larval survival rate (LSR) declined in L. monacha (for 35th day LSR was 0.44, 0.31 and 0.21, respectively), but increased in L. dispar (0.50, 1.00 and 0.94). In contrast, the duration of development decreased with increasing tempera- ture in larvae of both moth species (67, 52 and 39 days for L. monacha, and 66, 43 and 33 days for L. dispar, respectively). The differences in lar- val survival rate between those species at higher temperatures can affect significantly their growth and development in warmer years. Therefore, it is possible that under global climate change these differences may lead to changes in distribution of both insect species.
KEY WORDS: gypsy moth, herbivore in- sect, larval performance, Lepidoptera, nun moth, pedunculate oak, Scots pine
The global increase in air temperature became currently an important problem because of its difficult to predict the long-
term effects on the environment (Mc C ar t y 2001). Intensive research has been conducted to predict the reactions of plants and animals and their interactions to rising temperatures (Har r i ng ton et al. 2001, B a l e et al. 2002;
Wa lt h e r 2004). It can be expected that global warming will significantly change rela- tionships in the ecosystem among plants and their biotic stress factors (insects, pathogenic fungi, etc.). The direct influence of tempera- ture on insect survival is the simplest way to disturb plant-insect relationships (C ol e y and B aron e 1996, B e z e m e r et al. 1998), the distribution and occurrence frequency of various insect species (F l e m i ng 1996, 2000, Ay re s and L omb ard e ro 2000) and incidence of insect outbreaks (Te n ow et al.
1999).
Most experiments done to assess the ef- fects of elevated temperature on insect sur- vival rate and development have used tropi- cal plants (Wi l f and L ab an d e i r a 1999, A n d re w and Hu g h e s 2004, Wa lt h e r 2004, Ha l l am and R e a d 2006). Relative- ly few such studies have used plants of the temperate and boreal zones (Ås m an 2001, St i l lwe l l and Fox 2005, Ni e s e nb au m and K lu ge r 2006). Relatively large predicted temperature increases make it important to
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explore temperature effects on insects reared on native plant species in the high latitudes of the northern hemisphere. Most of scien- tific predictions indicate that global warm- ing will have a stimulating influence on the rate and range of expansion of forest insect pests (Fr it z and Si m ms 1992, Ay re s and L omb ard e ro 2000, Hó d ar and Z am or a 2004, but see: A n d e rs e n et al. 2001). In this
study, we used two tree species, Scots pine (Pinus sylvestris L.) and English oak (Quercus robur L.), which are particularly important forest-forming species in much of Europe (Gi e r t ych and Mát y á s 1991, Me u s e l et al. 1965). For our experiment, we selected insect herbivores representing the genus Ly- mantria: nun moth (L. monacha, preferring P. sylvestris), and gypsy moth (L. dispar, pre-
Fig. 1. Effects of temperature on larval survival rate (log-rank test) of Lymantria monacha and L. dispar.
The independent variable is the number of days from the beginning of the experiment.
C C C Lymantria monacha
Lymantria dispar
Survival rateSurvival rate
Time (days)
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ferring Q. robur). The former insect species is found throughout Europe, in lowlands and submontane forests, and its forage plants are mainly pines and spruces, but also nearly all other species of coniferous and broad-leaved trees (Kol k et al. 1996). L. dispar is a pest distributed not only in Europe but also in Africa, North America and Asia. It is found in forests, parks, and orchards everywhere except submontane and montane zones. It prefers leaves of oaks, but can feed also on beeches, hornbeams, maples, poplars, birch- es, and black and bird cherries (L a z are v i ć et al. 2002), and occurs sporadically on co- niferous trees such as spruces, larches, and pines (L e mpke 1980, Kol k et al. 1996). The two species differ in their latitudinal range, and consequently also in adaptation to ther- mal conditions. L. monacha is a more north- ern species, while L. dispar is more southern (Kol k et al. 1996).
This study aimed to answer two ques- tions: (1) How does temperature affect the larval survival rate of L. monacha and L. dispar? (2) Do both species react similarly to changes in temperature? The results can help us to understand the influence of cli- matic changes on the frequency of outbreaks of both insect species under different ther- mal environments. The experiments were conducted in varied thermal conditions (i.e., at 15, 20 or 25°C). Such temperatures are commonly observed within the geographic ranges of the selected insect and tree species (Wo ś 1999, Wa lte r 1973).
Larvae of L. monacha were fed with cur- rent-year needles of Pinus sylvestris and lar- vae of L. dispar were fed with leaves of Quer- cus robur. All the plant material was collected from trees growing within the Experimental Forest ‘Zwierzyniec’ near Kórnik, Poland (52°14’36”N and 17°05’00”E). The pine nee- dles were harvested from several 20-year-old trees and the oak leaves from trees about 30 years old of the local provenance. Needles and leaves were always collected from the sunlit part of the tree crown, at about 1/3 of its height (measured from the apex).
Eggs of the gypsy moth (Lymantria dis- par) from northeastern Poland were obtained from Ms Lidia Sukovata (Forest Research Institute, Warsaw, Poland). Eggs of the nun moth (Lymantria monacha) were collected in
a commercial forest stand in western Poland (Forest District Slawa Slaska,). Larvae of L.
monacha were reared in Petri dishes and fed with pine needles and those of L. dispar were fed with oak leaves. The dishes were kept in phytotrons at 15±1°C (low temperature), 20±1°C (medium temperature) and 25±1°C (elevated temperature). They were reared there for 2.5 months, from 17th June until 1st September, and fresh food was provided ev- ery 2 days (remnants of the uneaten portion of food were also removed at that time). One larva at the developmental stage L2–3 was placed with pine needles or an oak leaf (the needle bases or petioles were dipped in 2-ml Eppendorf tubes filled with water), in a 15 cm Petri dish. This was replicated (N = 16) for each thermal treatment and each species of larvae (total of 96 Petri dishes). The Petri dishes were checked every day to assess sur- vival rate and duration of development (DD), i.e., the number of days from the beginning of the experiment to pupation.
Analysis of variance (ANOVA) and oth- er statistical analyses were conducted using JMP software (version 4.0.4 SAS, Institute, Cary, NC, USA). Survival rates of larvae of both Lymantria species were analysed with the use of the log-rank test (Mante l 1963).
The experiments demonstrated that thermal conditions significantly affected the survival rate of L. monacha (P = 0.084) and L. dispar (P = 0.0002) larvae, although in dif- ferent ways for the two species. With increas- ing temperature, the survival rate of larvae at 20 and 25°C declined in L. monacha but in- creased in L. dispar, as compared with 15°C (Fig. 1).
In contrast, the influence of temperature on the duration of development affected the two Lymantria species similarly: for L. mo- nacha (P = 0.03) and in L. dispar (P <0.0001), the duration of development significantly de- creased with increasing temperature (Fig. 2).
Results of this study show that an in- crease in temperature caused a significant shortening of the duration of development (DD) of larvae of both these closely related Lymantria species. With increasing temper- ature, the survival rate of larvae declined in L. monacha, but increased in L. dispar. For the temperature interval of 20−25°C, our results for L. dispar are consistent with the
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results of L i n d rot h et al. (1997) and Wi l - l i ams et al. (2003). However, due to the wider range of temperatures applied in our study, we found that the survival rate of lar- vae of L. dispar reared at 15°C is about 50%
lower than at 20 and 25°C. The tempera- ture-dependent differences in larval surviv- al rate between these two insect species is reflected in their natural ranges. Our study seems to indicate that one of the reasons for the absence of L. dispar from submontane and montane areas, and its wide geographic range including parts of Africa (L a z are v i ć et al. 2002), can be the high thermal op- timum of larvae of this species and their low tolerance to lower temperatures (15°C, Fig. 1).
Higher temperatures shortened the dura- tion of development both in L. monacha and L. dispar, supporting prior observations by L i n d rot h et al. (1997) and Wi l l i ams et al.
(2003). The rearing of larvae of L. dispar at higher temperatures (25°C and 3.5°C above ambient) in these studies significantly short- ened the time needed for larvae to reach the pupal stage (by about 15 and 8 days, respec- tively). Higher temperatures have also been found to result in shortening of the larval
period in other insect species (L e at h e r and Ma c Ke n z i e 1994).
Higher temperatures can also affect in- sects indirectly by changing the physiology and metabolism of host plants. For example, Bus e et al. (1998) and D u r y et al. (1998) found that even a moderate rise in tempera- ture (by 3°C) causes changes in the chemi- cal composition of leaves of Q. robur, and the changes are unfavourable for foraging larvae, resulting in an increase in concentrations of phenolic compounds (condensed tannins in particular), and a decrease in nitrogen con- centration. Our experiments were made with the use of leaves collected in similar thermal conditions. Therefore, our results reflect the genetically determined direct effects of rising temperature on insect herbivores.
Results of this study show that because of the high tolerance to elevated temperature in respect to larval survival rate in L. dispar (a major pest on broad-leaved trees) it may be expected that in warmer years or under con- ditions of global warming this insect species will pose a greater potential threat to forests than L. monacha. If the global warming sce- narios come true, then results of this study, attesting to a greater survival rate of L. dispar Fig. 2. Effects of temperature on the duration of development (DD − number of days from the begin- ning of the experiment to pupation) in larvae of Lymantria monacha feeding on pine needles and of L.
dispar feeding on oak leaves. Means with the same letter are not significantly different (P = 0.05, Tukey test). Vertical bars are standard errors (SE).
CC C
L. monacha L. dispar
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than of L. monacha at higher temperatures, suggest that the natural range of those spe- cies can shift towards the temperate zone. As a result, ranges of distribution of both species may overlap to a larger extent.
ACKNOWLEDGEMENTS: We thank Dr. Lee E. Frelich for comments on early version of the manuscript. This work was supported by State Committee for Scientific Research (Grant no 5P04G 022 21).
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(Received after revising March 2007)
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