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Effects of temperature and relative humidity on water loss by the Colorado potato beetle, Leptinotarsa decemlineata (Say)
Pergamon
0022-1910(94)00103-0
J. Insecr Physiol. Vol. 41, No. 3, pp. 235-239, 1995 Copyright C 1995 Elsevier Science Ltd Printed in Great Britain. All rights reserved 0022-1910/95 $9.50 + 0.00
Effects of Temperature and Relative Humidity on Water Loss by the Colorado Potato Beetle, Leptinotarsa decemlineata (Say) YVAN Received
PELLETIER* 1 I April 1994; revised 3 September
1994
Insects need to maintain an adequate level of body water and have developed adaptations to reduce losing water by diffusion. The Colorado potato beetle, Leptinotarsa decemlineata (Say), gets the water it requires from its food plant. The rate of water loss was evaluated under 12 regimes of temperature (20, 30, 35 and 4O’C) and humidity (15, 50, and 85%) during short exposure experiments (3 h) with four stages of the Colorado potato beetle. Insects lost water under all conditions tested but losses were greater at 4O’C. The critical equilibrium activity was evaluated by exposing the four stages to 30°C and 5% relative humidity (r.h.) for 2-9 days. High mortality of second instar larvae was observed after two days, when the insects had lost c. 70% of their body water. Fourth instar larvae and adults survived a reduction of c. 60 and 48% of their body water over 7 and 9 days respectively. Forty-eight percent of the pupae maintained in dry conditions for two days did not emerge as adults, after losing c. 30% of the body water.
Water balance activity
Colorado potato beetle
Leptinotarsa
INTRODUCTION
Research Canada
Station, P.O. E3B 427.
Water loss rate
Critical equilibrium
decreases. The change through time (t) of the body water mass generally follows an exponential curve described by the equation:
Insects generally require an adequate level of body water in order to keep their physiological systems functioning. Their large surface : volume ratio contributes to significant water loss. Insects possess several adaptations that reduce the rate at which water is lost. These adaptations include waterproofed cuticle with lipid; limitation of air exchange; utilization of uric acid as the major nitrogenous excretory product; and, resorption of water by the rectum (Wharton, 1985). In insects, most water loss occurs by passive diffusion through the cuticle (Wharton, 1985). The amount of water in the insect constitutes the body water mass (m) and varies with the species, the individual and with the external conditions. Diffusion of body water through the cuticle will be influenced by the water activity of the body water (a,. = ratio of the vapor pressure of water in an aqueous solution to the vapor pressure of pure water) and by the water activity of the air surrounding the insect (a, = ratio of the partial vapor pressure to the saturated vapor pressure = r.h.). The body water mass, is expressed as the percentage of the total body mass that is made of water (Wharton, 1985). Under dry conditions, insects lose water and the body water mass ratio *Agriculture and Agri-Food Canada, 20280, Fredericton, New Brunswick,
decemlineata
m, = m, exp.mkf
where m, is the initial body water mass, m, is the body water mass after time (t) and the constant k is the rate of water loss i.e. the rate at which the body water mass ratio is changing (Wharton, 1985). The value of k can be calculated from a linear regression of the net transpiration (In (m,/m,)) vs time (t). Insects need to maintain a minimum amount of body water to stay alive. The critical equilibrium activity expresses this amount of water, as a percentage of the total weight of the insect. For phytophagous insects like the Colorado potato beetle, Leptinotarsa decemlineata (Say), the water requirement is fulfilled by food intake. Food intake may be influenced by an increase in the level of body water (Wharton, 1985). Insects may also reduce water loss by diffusion by moving to a location in the plant canopy with optimal conditions of humidity and/or temperature (Weissling and Giblin-Davis, 1993). The distribution of the Colorado potato beetle within the canopy changes with its life stage (Boiteau unpublished). First instar larvae emerge from eggs which are usually located in the plant canopy and the larvae remain in the plant canopy
Box
235
236
YVAN
PELLETIER
for the duration of this stage. Larvae of the second instar migrate towards the top of the plant where they become more exposed to wind and sun. From the second to fourth instar, larvae will feed on the top of the plant. The fully grown fourth-instar larvae move to the ground, burrow themselves in, and molt to pupae. The adults emerge from the soil, return to the plant, and feed at different locations on the plant. The Colorado potato beetle overwinters as an adult in the soil. During its development, the Colorado potato beetle is exposed to diverse conditions which have different impacts on its water balance. The different stages of the Colorado potato beetle might have developed different water balance strategies in response to these diverse conditions. The objective of this study was to quantify water loss by different life stages of the Colorado potato beetle under various temperature and humidity regimes. Also, the critical equilibrium activity, the amount of water necessary to maintain life, was evaluated.
MATERIALS
Short exposure
AND METHODS
time experiments
Second-instar larvae (L2), fourth-instar larvae (L4), pupae and adults of the Colorado potato beetle, both alive and dead, were exposed to 12 conditions of temperature and relative humidity for a period of 3 h. Food was withheld from the alive feeding stages (L2, L4 and adults) as follows: 3-4 h for L2, 4-5.5 h for L4 and 2 to 3.5 h for adults. A period of starvation was necessary to prevent defecation that would result in a weight reduction not produced by water loss. Feeding stages that were to be treated when dead were without food for 1 hour less than alive beetles and then killed by placing them in a cyanide killing jar for 1 h. Pupae were placed in the killing jar for 3 h. Insects were weighed and placed individually in 25 ml plastic cups (Solo Cup Co., Urbana, IL) with ventilated (nylon screening) snap covers except for the L2, which were placed in groups of five per cup. The cups were placed into plastic containers (Nalgene Biosafe Carrier cat. No. 7135-0001) inside a growth chamber (Conviron, Model No. 118L ). Relative humidity inside the containers was monitored with an hygro-thermometer (Omega, model RH70), and maintained by a closed system composed of two air streams, one passing through drierite and one bubbling through water, that were mixed before entering the plastic TABLE
1. Surface
estimate,
n
Stages Second-instar Fourth-instar Pupa Adult
area
larva larva
118 239 235 230
‘Calculated from the formula: Standard error of the mean.
container. The relative humidity was controlled by varying the volume of air from the humid air stream. Air exhausted from the container was fed back to the two humidity controlled air streams. The air flow through the container was 2 L/min. After 3 h, insects were removed from the set-up, weighed, dried at 60°C in a Precision oven for 48 h and weighed again. Long exposure
Living Colorado potato beetles (L2, L4, pupae and adults) were held at 30°C and low relative humidity (3-5%) in the same apparatus as described above. The air flow was 2.5-3 L/min and L2 were placed individually in ventilated cups. Food was withheld from the feeding stages as follows: 5-7 h for L2, 6 h for L4 and 6.5 h for adults. The feeding stages were weighed daily and their dry weights were determined after removal from the set-up. Pupae were placed in the system for periods of 1,2,3 and 4 days and were weighed daily. The cups containing the pupae were then removed from the system and placed in a plastic container with 2 wet filter papers to produce high relative humidity (70-90%). The container was held at room temperature in the laboratory for a week. Emerged adults were classified as: normal, slightly deformed (elytra have small dents and holes or do not lay right), deformed (elytra cover abdomen, but are misshapen) and severely deformed (elytra are not expanded or are so severely misshapen that the abdomen is exposed). Dry weights were determined for those pupae that did not emerge. Twenty-five control pupae were placed individually in ventilated cups in a plastic container with 2 wet filter papers (r.h. was 77-90%) and the container was placed in growth chamber for three days at 30°C. The container was then moved to the laboratory where it was held at room temperature until the pupae emerged and the emerged adults were classified as above. RESULTS
The surface area of the first-instar larvae was estimated to be 10 times smaller than that of the fourthinstar larvae (Table 1). The estimated surface areas of the pupae and adults were approximately the same and were one third larger than fourth-instar larvae. Water content of the pre-imaginal stages constituted c. 82% of the total body mass but dropped to 58% for the adult stage. Water mass increased during the immature stages
water mass, and water Colorado potato beetle Surface’ cm’ 0.29 2.05 3.18 3.15
time experiments
f O.OO* * 0.02 k 0.03 & 0.03
12 x fresh weight
content
Water
mass
of four
stages
Water
content %
83.59 82.93 82.34 58.05
* 0.172 * 0.15 & 0.10 & 0.33
mg 2.98 57.26 110.78 76.85
* + f. +
066 (Weissling
0.06* 0.89 1.32 1.07
of the
and Giblin-Davis
1993).
WATER Second
larval
BALANCE
IN THE COLORADO 0.030
instar
POTATO BEETLE Fourth
larval
231
instar
0.045
0.025
* I
0.020 E
-i r: g 3
0
25
35
30
40
0
25
30
35
40
0.030
0.030
Adults
Pupae 0.025
0.025
0.020 F
0.020
b
c
I
0-
25
30
40
35
0
Temperature
25
30
35
40
(“C)
FIGURE 1. Rate of water loss, k (% h-‘), of dead (----) and live (-) individuals of four stages of the Colorado potato beetle at 15% r.h. (A), 50% r.h. (O), and 85% r.h. (0) as a function of temperature. Error bars are the standard error of the mean and were drawn only if larger than the data markers.
to 110 mg for pupae and dropped stage.
to 77 mg for the adult
Adults
0
Pupae
A
Fourth
larval
experiments
In long exposure experiments, second instar larvae lost c. 70% of their body water after two days and 70.2% died during this interval (Fig. 2). The survivors lost an average of 80% of their body water after three days. All fourth-instar larvae and adults survived the loss of c. 60 and 48% of their body water after 7 and 9 days respectively. Pupae lost up to 60% of their body water in four days and survived the pupal stage but most did not emerge as adults. Forty-eight percent of the pupae maintained in dry conditions for two days did not emerge as adults after losing c. 30% of their body water (Table 2). Post-treatment mortality increased to 89 and 92% after three and four days of exposure to dry conditions. Pupae that survived, emerged with severe elytra deformation. When calculated from the net transpiration data (Fig. 3), the rate of water loss for the adults was 0.0012% h-’ (F = 3058, d.f. = 7), 0.0010% h-’ (F = 590, d.f. = 5) for the fourth-instar larvae, and 0.0066% h-’ (F = 385, d.f. = 1) for the second instar larvae. The net transpiration rate for the pupae did not follow a linear relationship with time (F = 41, d.f. = 3).
Short
0
Long exposure
instar
DISCUSSION
e a
0
I
I
I
I
2
4
6
8
Time
(days)
FIGURE 2. Proportion of body water lost by second-instar larvae (O), fourth-instar larvae (O), pupae (A), and adults (0) of the Colorado potato beetle, as a function of the time spent at 30°C and 5% r.h. Error bars are the standard error of the mean and were drawn only if larger than the data markers.
The Colorado potato beetle finds the water it requires in its food. As larvae, the beetles feed continually (Lactin et al., 1993) on a food source containing c. 90% water providing the insects with supply sufficient to maintain its high water content. Pre-imaginal stages had a water content of c. 82% and water constitutes 58% of the adult weight. The average water content of an insect is c. 70%
238
YVAN TABLE
PELLETIER
2. Mortality and percentage of adult Colorado potato beetles with elytra deformation after exposure to dry conditions during pupal stage % with elytra
Exposure time Control One day Two days Three days Four days
n
(%)
Normal
Slight
Medium
Severe
25 24 25 27 24
0 0 48.0 88.9 91.7
28.0 16.7 0 0 0
52.0 33.3 0 0 0
20.0 41.7 0 0 0
0 8.3 52.0 11.1 8.3
(Edney, 1977) and is related to the amount of cuticle (Wharton, 1985). Short exposure experiments with dead larvae resulted in a value of k lower than with live larvae, indicating that the observed difference in weight might have been caused by both water loss and the metabolic activity of the larvae. Without a source of water, secondinstar larvae rapidly lost water and died when the water mass was reduced by c. 70%. More resistant to dry conditions, the fourth-instar larvae survived a reduction of almost 60% of their water mass during a seven day exposure to dry conditions. Adults have access to water in their food source but spend less time feeding than the larvae (Pelletier and Smilowitz, 1990). The rate of water loss was generally lower for adults than for larvae. Adults did survive in dry conditions for a relatively long period of time, even if its water mass was reduced by 50%. The k values obtained with live beetles were lower than with dead ones, suggesting a regulation of water loss, possibly by controlling their respiration. Second-instar larvae exposed to dry conditions (30°C and 5% r.h.) for a long period of time, showed a value of k (0.0066% h-‘) similar to the one calculated from a short exposure (O.O07O/, h-‘) to similar conditions (30°C and 15% r.h.). The rates of water loss calculated from the long exposure experiment for the fourth instar larvae (0.0010% h-‘), and the adults (0.0012% h-‘) were lower than the k value calculated from the short exposure experiment (0.0060% h-’ for the fourth instar larvae, and 0.0035% h-’ for the adults). The k value for the dead fourth instar larvae (0.0022% h-‘) calculated from
^o E
_o.:c Rqy-%?+
short exposure experiment was lower than with alive beetles but higher for the adults (0.0065% h-‘). It is possible that adults maintain water loss at a minimum by changing their respiration, when food is not available. The difference observed in the value of k suggests that deprived of food, fourth instar larvae can modify their physiology in order to reduce water loss. Such a modification could not take place with dead larvae and resulted in a higher value of k for dead beetles. A similar physiological change may also take place during the pre-pupal stage when the larvae stop feeding and burrow into the soil. The pupae had a high water mass and were very susceptible to desiccation. A reduction of their water mass by 25% resulted in mortality at emergence near 50% and adults with wings severely deformed. However, pupae are usually not exposed to dry conditions since air in soil is probably almost saturated with water. The rate of water loss by pupae was relatively low at low temperature but increased rapidly above 30°C with low humidity. These conditions are rarely found in the soil. The similarity of k values between dead and live pupae suggest that water loss depends on the physical properties of the pupae. As the water mass decreases, the rate of water loss is affected resulting in a curvilinear relationship between water loss and time. The k value increased with time suggesting a profound effect on the physiological processes of the pupae. The different stages of the Colorado potato beetle appear to have developed different adaptations and strategies to limit water loss. The information gained during this study should increase our understanding of the behavior of this insect in its natural environment.
-0.4
Adults Pupae Fourth larval instar CI Second larval instar
-0.6
-0.8
-1.0
REFERENCES
0 0 a
2 5
deformation
Mortality
t
I 0
'
50
100
150
200
250
Time (h) FIGURE 3. Net transpiration of second-instar larvae (O), fourth-instar larvae (O), pupae (A), and adults (0) of the Colorado potato beetle, as a function of the time spent at 30°C and 5% r.h. Error bars are the standard error of the mean and were drawn only if larger than the data markers.
Edney E. B. (1977) Water balance in land arthropods. In Zoophysiology and Ecology. Vol 9. Berlin, Heidelberg, New York. Lactin D. J., Holliday N. J. and Lamari L. L. (1993) Temperature dependence and constant-temperature die1 aperiodicity of feeding by Colorado potato beetle larvae (Coleoptera: Chrysomelidae) in shortduration laboratory trials. Environ. Entomol. 22, 784-790. Pelletier Y. and Smilowitz Z. (1990) Feeding behavior of the adult Colorado potato beetle, Leptinotarsa decemlineata (Say) on Solanum berthaultii. Can. Ent. 123, 219-230 Weissling T. J. and Giblin-Davis R. (1993) Water loss dynamics and humidity preference of Rhychophorus cruentatus (Coleoptera: Curculionidae) adults. Environ. Entomol. 22, 93-98.
WATER Wharton In.wcr
G. W. (1985) Physiolog.,
G. A. and Gilbert nutrition.
Water
balance
Biochc,misrry
and
L. I.). Vol. 4 pp. 565
excretion.
Pergamon
BALANCE
IN THE
COLORADO
POTATO
BEETLE
239
of insects. In Comprehensiw Pharntocolog~.
(Eds
6001. Regulation:
Press, Oxford.
Kcrkut
Digestion.
Acknow/edg~mm~.s thank
I
thank
Cathy
Clark
Dr G. Boiteau and Dr W. Coleman
early version of this manuscript.
for her technical for their comments
help.
I
on an
0022-1910(94)00103-0
J. Insecr Physiol. Vol. 41, No. 3, pp. 235-239, 1995 Copyright C 1995 Elsevier Science Ltd Printed in Great Britain. All rights reserved 0022-1910/95 $9.50 + 0.00
Effects of Temperature and Relative Humidity on Water Loss by the Colorado Potato Beetle, Leptinotarsa decemlineata (Say) YVAN Received
PELLETIER* 1 I April 1994; revised 3 September
1994
Insects need to maintain an adequate level of body water and have developed adaptations to reduce losing water by diffusion. The Colorado potato beetle, Leptinotarsa decemlineata (Say), gets the water it requires from its food plant. The rate of water loss was evaluated under 12 regimes of temperature (20, 30, 35 and 4O’C) and humidity (15, 50, and 85%) during short exposure experiments (3 h) with four stages of the Colorado potato beetle. Insects lost water under all conditions tested but losses were greater at 4O’C. The critical equilibrium activity was evaluated by exposing the four stages to 30°C and 5% relative humidity (r.h.) for 2-9 days. High mortality of second instar larvae was observed after two days, when the insects had lost c. 70% of their body water. Fourth instar larvae and adults survived a reduction of c. 60 and 48% of their body water over 7 and 9 days respectively. Forty-eight percent of the pupae maintained in dry conditions for two days did not emerge as adults, after losing c. 30% of the body water.
Water balance activity
Colorado potato beetle
Leptinotarsa
INTRODUCTION
Research Canada
Station, P.O. E3B 427.
Water loss rate
Critical equilibrium
decreases. The change through time (t) of the body water mass generally follows an exponential curve described by the equation:
Insects generally require an adequate level of body water in order to keep their physiological systems functioning. Their large surface : volume ratio contributes to significant water loss. Insects possess several adaptations that reduce the rate at which water is lost. These adaptations include waterproofed cuticle with lipid; limitation of air exchange; utilization of uric acid as the major nitrogenous excretory product; and, resorption of water by the rectum (Wharton, 1985). In insects, most water loss occurs by passive diffusion through the cuticle (Wharton, 1985). The amount of water in the insect constitutes the body water mass (m) and varies with the species, the individual and with the external conditions. Diffusion of body water through the cuticle will be influenced by the water activity of the body water (a,. = ratio of the vapor pressure of water in an aqueous solution to the vapor pressure of pure water) and by the water activity of the air surrounding the insect (a, = ratio of the partial vapor pressure to the saturated vapor pressure = r.h.). The body water mass, is expressed as the percentage of the total body mass that is made of water (Wharton, 1985). Under dry conditions, insects lose water and the body water mass ratio *Agriculture and Agri-Food Canada, 20280, Fredericton, New Brunswick,
decemlineata
m, = m, exp.mkf
where m, is the initial body water mass, m, is the body water mass after time (t) and the constant k is the rate of water loss i.e. the rate at which the body water mass ratio is changing (Wharton, 1985). The value of k can be calculated from a linear regression of the net transpiration (In (m,/m,)) vs time (t). Insects need to maintain a minimum amount of body water to stay alive. The critical equilibrium activity expresses this amount of water, as a percentage of the total weight of the insect. For phytophagous insects like the Colorado potato beetle, Leptinotarsa decemlineata (Say), the water requirement is fulfilled by food intake. Food intake may be influenced by an increase in the level of body water (Wharton, 1985). Insects may also reduce water loss by diffusion by moving to a location in the plant canopy with optimal conditions of humidity and/or temperature (Weissling and Giblin-Davis, 1993). The distribution of the Colorado potato beetle within the canopy changes with its life stage (Boiteau unpublished). First instar larvae emerge from eggs which are usually located in the plant canopy and the larvae remain in the plant canopy
Box
235
236
YVAN
PELLETIER
for the duration of this stage. Larvae of the second instar migrate towards the top of the plant where they become more exposed to wind and sun. From the second to fourth instar, larvae will feed on the top of the plant. The fully grown fourth-instar larvae move to the ground, burrow themselves in, and molt to pupae. The adults emerge from the soil, return to the plant, and feed at different locations on the plant. The Colorado potato beetle overwinters as an adult in the soil. During its development, the Colorado potato beetle is exposed to diverse conditions which have different impacts on its water balance. The different stages of the Colorado potato beetle might have developed different water balance strategies in response to these diverse conditions. The objective of this study was to quantify water loss by different life stages of the Colorado potato beetle under various temperature and humidity regimes. Also, the critical equilibrium activity, the amount of water necessary to maintain life, was evaluated.
MATERIALS
Short exposure
AND METHODS
time experiments
Second-instar larvae (L2), fourth-instar larvae (L4), pupae and adults of the Colorado potato beetle, both alive and dead, were exposed to 12 conditions of temperature and relative humidity for a period of 3 h. Food was withheld from the alive feeding stages (L2, L4 and adults) as follows: 3-4 h for L2, 4-5.5 h for L4 and 2 to 3.5 h for adults. A period of starvation was necessary to prevent defecation that would result in a weight reduction not produced by water loss. Feeding stages that were to be treated when dead were without food for 1 hour less than alive beetles and then killed by placing them in a cyanide killing jar for 1 h. Pupae were placed in the killing jar for 3 h. Insects were weighed and placed individually in 25 ml plastic cups (Solo Cup Co., Urbana, IL) with ventilated (nylon screening) snap covers except for the L2, which were placed in groups of five per cup. The cups were placed into plastic containers (Nalgene Biosafe Carrier cat. No. 7135-0001) inside a growth chamber (Conviron, Model No. 118L ). Relative humidity inside the containers was monitored with an hygro-thermometer (Omega, model RH70), and maintained by a closed system composed of two air streams, one passing through drierite and one bubbling through water, that were mixed before entering the plastic TABLE
1. Surface
estimate,
n
Stages Second-instar Fourth-instar Pupa Adult
area
larva larva
118 239 235 230
‘Calculated from the formula: Standard error of the mean.
container. The relative humidity was controlled by varying the volume of air from the humid air stream. Air exhausted from the container was fed back to the two humidity controlled air streams. The air flow through the container was 2 L/min. After 3 h, insects were removed from the set-up, weighed, dried at 60°C in a Precision oven for 48 h and weighed again. Long exposure
Living Colorado potato beetles (L2, L4, pupae and adults) were held at 30°C and low relative humidity (3-5%) in the same apparatus as described above. The air flow was 2.5-3 L/min and L2 were placed individually in ventilated cups. Food was withheld from the feeding stages as follows: 5-7 h for L2, 6 h for L4 and 6.5 h for adults. The feeding stages were weighed daily and their dry weights were determined after removal from the set-up. Pupae were placed in the system for periods of 1,2,3 and 4 days and were weighed daily. The cups containing the pupae were then removed from the system and placed in a plastic container with 2 wet filter papers to produce high relative humidity (70-90%). The container was held at room temperature in the laboratory for a week. Emerged adults were classified as: normal, slightly deformed (elytra have small dents and holes or do not lay right), deformed (elytra cover abdomen, but are misshapen) and severely deformed (elytra are not expanded or are so severely misshapen that the abdomen is exposed). Dry weights were determined for those pupae that did not emerge. Twenty-five control pupae were placed individually in ventilated cups in a plastic container with 2 wet filter papers (r.h. was 77-90%) and the container was placed in growth chamber for three days at 30°C. The container was then moved to the laboratory where it was held at room temperature until the pupae emerged and the emerged adults were classified as above. RESULTS
The surface area of the first-instar larvae was estimated to be 10 times smaller than that of the fourthinstar larvae (Table 1). The estimated surface areas of the pupae and adults were approximately the same and were one third larger than fourth-instar larvae. Water content of the pre-imaginal stages constituted c. 82% of the total body mass but dropped to 58% for the adult stage. Water mass increased during the immature stages
water mass, and water Colorado potato beetle Surface’ cm’ 0.29 2.05 3.18 3.15
time experiments
f O.OO* * 0.02 k 0.03 & 0.03
12 x fresh weight
content
Water
mass
of four
stages
Water
content %
83.59 82.93 82.34 58.05
* 0.172 * 0.15 & 0.10 & 0.33
mg 2.98 57.26 110.78 76.85
* + f. +
066 (Weissling
0.06* 0.89 1.32 1.07
of the
and Giblin-Davis
1993).
WATER Second
larval
BALANCE
IN THE COLORADO 0.030
instar
POTATO BEETLE Fourth
larval
231
instar
0.045
0.025
* I
0.020 E
-i r: g 3
0
25
35
30
40
0
25
30
35
40
0.030
0.030
Adults
Pupae 0.025
0.025
0.020 F
0.020
b
c
I
0-
25
30
40
35
0
Temperature
25
30
35
40
(“C)
FIGURE 1. Rate of water loss, k (% h-‘), of dead (----) and live (-) individuals of four stages of the Colorado potato beetle at 15% r.h. (A), 50% r.h. (O), and 85% r.h. (0) as a function of temperature. Error bars are the standard error of the mean and were drawn only if larger than the data markers.
to 110 mg for pupae and dropped stage.
to 77 mg for the adult
Adults
0
Pupae
A
Fourth
larval
experiments
In long exposure experiments, second instar larvae lost c. 70% of their body water after two days and 70.2% died during this interval (Fig. 2). The survivors lost an average of 80% of their body water after three days. All fourth-instar larvae and adults survived the loss of c. 60 and 48% of their body water after 7 and 9 days respectively. Pupae lost up to 60% of their body water in four days and survived the pupal stage but most did not emerge as adults. Forty-eight percent of the pupae maintained in dry conditions for two days did not emerge as adults after losing c. 30% of their body water (Table 2). Post-treatment mortality increased to 89 and 92% after three and four days of exposure to dry conditions. Pupae that survived, emerged with severe elytra deformation. When calculated from the net transpiration data (Fig. 3), the rate of water loss for the adults was 0.0012% h-’ (F = 3058, d.f. = 7), 0.0010% h-’ (F = 590, d.f. = 5) for the fourth-instar larvae, and 0.0066% h-’ (F = 385, d.f. = 1) for the second instar larvae. The net transpiration rate for the pupae did not follow a linear relationship with time (F = 41, d.f. = 3).
Short
0
Long exposure
instar
DISCUSSION
e a
0
I
I
I
I
2
4
6
8
Time
(days)
FIGURE 2. Proportion of body water lost by second-instar larvae (O), fourth-instar larvae (O), pupae (A), and adults (0) of the Colorado potato beetle, as a function of the time spent at 30°C and 5% r.h. Error bars are the standard error of the mean and were drawn only if larger than the data markers.
The Colorado potato beetle finds the water it requires in its food. As larvae, the beetles feed continually (Lactin et al., 1993) on a food source containing c. 90% water providing the insects with supply sufficient to maintain its high water content. Pre-imaginal stages had a water content of c. 82% and water constitutes 58% of the adult weight. The average water content of an insect is c. 70%
238
YVAN TABLE
PELLETIER
2. Mortality and percentage of adult Colorado potato beetles with elytra deformation after exposure to dry conditions during pupal stage % with elytra
Exposure time Control One day Two days Three days Four days
n
(%)
Normal
Slight
Medium
Severe
25 24 25 27 24
0 0 48.0 88.9 91.7
28.0 16.7 0 0 0
52.0 33.3 0 0 0
20.0 41.7 0 0 0
0 8.3 52.0 11.1 8.3
(Edney, 1977) and is related to the amount of cuticle (Wharton, 1985). Short exposure experiments with dead larvae resulted in a value of k lower than with live larvae, indicating that the observed difference in weight might have been caused by both water loss and the metabolic activity of the larvae. Without a source of water, secondinstar larvae rapidly lost water and died when the water mass was reduced by c. 70%. More resistant to dry conditions, the fourth-instar larvae survived a reduction of almost 60% of their water mass during a seven day exposure to dry conditions. Adults have access to water in their food source but spend less time feeding than the larvae (Pelletier and Smilowitz, 1990). The rate of water loss was generally lower for adults than for larvae. Adults did survive in dry conditions for a relatively long period of time, even if its water mass was reduced by 50%. The k values obtained with live beetles were lower than with dead ones, suggesting a regulation of water loss, possibly by controlling their respiration. Second-instar larvae exposed to dry conditions (30°C and 5% r.h.) for a long period of time, showed a value of k (0.0066% h-‘) similar to the one calculated from a short exposure (O.O07O/, h-‘) to similar conditions (30°C and 15% r.h.). The rates of water loss calculated from the long exposure experiment for the fourth instar larvae (0.0010% h-‘), and the adults (0.0012% h-‘) were lower than the k value calculated from the short exposure experiment (0.0060% h-’ for the fourth instar larvae, and 0.0035% h-’ for the adults). The k value for the dead fourth instar larvae (0.0022% h-‘) calculated from
^o E
_o.:c Rqy-%?+
short exposure experiment was lower than with alive beetles but higher for the adults (0.0065% h-‘). It is possible that adults maintain water loss at a minimum by changing their respiration, when food is not available. The difference observed in the value of k suggests that deprived of food, fourth instar larvae can modify their physiology in order to reduce water loss. Such a modification could not take place with dead larvae and resulted in a higher value of k for dead beetles. A similar physiological change may also take place during the pre-pupal stage when the larvae stop feeding and burrow into the soil. The pupae had a high water mass and were very susceptible to desiccation. A reduction of their water mass by 25% resulted in mortality at emergence near 50% and adults with wings severely deformed. However, pupae are usually not exposed to dry conditions since air in soil is probably almost saturated with water. The rate of water loss by pupae was relatively low at low temperature but increased rapidly above 30°C with low humidity. These conditions are rarely found in the soil. The similarity of k values between dead and live pupae suggest that water loss depends on the physical properties of the pupae. As the water mass decreases, the rate of water loss is affected resulting in a curvilinear relationship between water loss and time. The k value increased with time suggesting a profound effect on the physiological processes of the pupae. The different stages of the Colorado potato beetle appear to have developed different adaptations and strategies to limit water loss. The information gained during this study should increase our understanding of the behavior of this insect in its natural environment.
-0.4
Adults Pupae Fourth larval instar CI Second larval instar
-0.6
-0.8
-1.0
REFERENCES
0 0 a
2 5
deformation
Mortality
t
I 0
'
50
100
150
200
250
Time (h) FIGURE 3. Net transpiration of second-instar larvae (O), fourth-instar larvae (O), pupae (A), and adults (0) of the Colorado potato beetle, as a function of the time spent at 30°C and 5% r.h. Error bars are the standard error of the mean and were drawn only if larger than the data markers.
Edney E. B. (1977) Water balance in land arthropods. In Zoophysiology and Ecology. Vol 9. Berlin, Heidelberg, New York. Lactin D. J., Holliday N. J. and Lamari L. L. (1993) Temperature dependence and constant-temperature die1 aperiodicity of feeding by Colorado potato beetle larvae (Coleoptera: Chrysomelidae) in shortduration laboratory trials. Environ. Entomol. 22, 784-790. Pelletier Y. and Smilowitz Z. (1990) Feeding behavior of the adult Colorado potato beetle, Leptinotarsa decemlineata (Say) on Solanum berthaultii. Can. Ent. 123, 219-230 Weissling T. J. and Giblin-Davis R. (1993) Water loss dynamics and humidity preference of Rhychophorus cruentatus (Coleoptera: Curculionidae) adults. Environ. Entomol. 22, 93-98.
WATER Wharton In.wcr
G. W. (1985) Physiolog.,
G. A. and Gilbert nutrition.
Water
balance
Biochc,misrry
and
L. I.). Vol. 4 pp. 565
excretion.
Pergamon
BALANCE
IN THE
COLORADO
POTATO
BEETLE
239
of insects. In Comprehensiw Pharntocolog~.
(Eds
6001. Regulation:
Press, Oxford.
Kcrkut
Digestion.
Acknow/edg~mm~.s thank
I
thank
Cathy
Clark
Dr G. Boiteau and Dr W. Coleman
early version of this manuscript.
for her technical for their comments
help.
I
on an