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Determination of the lethal high temperature for the Colorado potato (Coleoptera: Chrysomelidae)

Y. PELLETIER

Agriculture andAgri-Food Canada, Research Centre, P.D. Box 20280, Fredericton, New Brunswick, Canada E3B 4Z7. Received 22 January 1998; accepted 21 August 1998.

Pelletier, Y. 1998. Determination ofthe lethal high temperature for INTRODUCTION the (Coleoptera: Chrysomelidae). Can. Agric. Eng. 40:185-189. The susceptibility of to high Temperature has a profound effect on living organisms. To a temperatures has been used to develop control methods for pests in large extend, the temperature ofan is detennined by the stored products and crops. Recently, microwaves and propane flamers temperature ofits surrounding environment (May 1985). Each were evaluated as control techniques for the Colorado potato beetle, insect species can function nonnally within a specific range of Leptinotarsa decemlineata (Say). These methods aim to increase the temperatures. Ifthe body temperature increases above the upper body temperature of the insect above its lethal threshold. The lethal limit ofthat temperature range, the insect may experience heat high temperature for six stages of the Colorado potato beetle was torpor and eventually death. The relative sensitivity ofinsects evaluated by exposing the insects to either warm water or hot air. The to high temperatures has lead to the development of control lethal temperature obtained by immersing insects in warm water was methods that wann the air or the water surrounding the insect up to 5.2 °C lower than with hot air. The difference in the LT95 above its lethal temperature. Vaporheat, hot air, and immersion between the warm water and hot air tests varied with the size ofthe insect, the larger stages showing a larger difference. When large stages in hot water have been used to treat goods against quarantined ofthe insect were placed in an oven, it took 10 to 15 minutes for the insects (Fields 1992; Heard et al. 1991; Hansen et al. 1992; body temperature to reach a value close to the air temperature. In hot lang 1991; Mangan and Ingle 1992; McFarlane 1989; Sharp air conditions, the body temperature remained several degrees below 1993; Yusofand Hashim 1992). Recently, the propane flamer the air temperature indicating heat loss by evaporation. Reducing the (Duchesne et Boiteau 1991; Duchesne et al. 1992; Moyer et al. exposure time to hot air from 20 to 10 minutes resulted in an increase 1992; Pelletier et al. 1995) and microwaves (Colpitts et al. ofup to 3.7 °C in the lethal temperature. The effect ofthe reduction of 1992, 1993; Hamid and Boulanger 1969; Hirose et al. 1975) the exposure time was more pronounced with the larger stages. The have also been evaluated as control methods for insect pests. value ofthe LT95 ranged from 50.1 to 56.2 °C depending on the stage The propane flamer utilizes combustion gases to elevate the ofthe Colorado potato beetle. Implications for the development ofnew body temperature of the insects. A proportion of the insect physical control methods are also discussed. Keywords: Colorado potato beetle, lethal temperature, physical control method. population treated with the propane flamer usually suffers extensive damage to the legs (Pelletieretal. 1995). Microwaves La susceptibilite des insectes pour les temperatures elevees a ete can also be used to transfer energy to insects. Microwaves are utilisee pour Ie developpement de methodes de controle des insectes des produits entreposes ou des cultures. Les micro-ondes et un bruleur directly absorbed by the insect which results in an increase in au propane ont ete evalues comme methode de controle du doryphore its body temperature. Data on the lethal temperature of the de la pomme de terre, Leptinotarsa decemlineata (Say). Ces methodes Colorado potato beetle, Leptinotarsa decemlineata (Say), are visent a elever la temperature corporelie de I'insecte au-dessus du required to fully evaluate and eventually improve these niveau letal. La temperature letale superieure de six stades du techniques. doryphore de la pomme de terre a ete determinee en exposant les The effect ofthe different parameters influencing the body insectes a de I'air ou de I'eau chaude. La temperature letale des insectes temperature ofthe Colorado potato beetle can be summarized immerges dans I'eau chaude etaitjusqu'a 5.2 °C moins elevee qu'avec by a generalized energy balance equation (Henwood 1975; I'air chaud. La difference entre la valeur de la TL obtenue avec les 95 Smith and Miller 1973): methodes a l'eau et a I'air chaud variait en fonction de la taille de I'insecte, les stades les plus gros demontrant une difference plus grande. Une periode de lOa 15 min etait necessaire pour stabiliser la M + S + IR;n = IRoUi - AE + K + C (I) temperature corporelle des gros insectes places dans un four. Dans ces where: conditions, la temperature corporelle de I'insecte demeurait plusieurs degres plus basse que I'air environnant ce qui suppose une perte de M = production ofmetabolic heat, chaleur par evaporation. La reduction du temps d'exposition a I'air S = absorption ofradiant energy, chaude de 20 a 10 min a resulte en une augmentation de la temperature IRin = absorption ofinfra-red radiation, letale de 3.7 0c. L'effet de la reduction du temps d'exposition etait plus IRou, = emission ofinfra-red radiation, marque avec les gros stades. La valeur de la TL 95 variait entre 50.1 et E = loss ofheat by evaporation ofwater, 56.2 °C scion les stades de developpement du doryphore. Les A= latent heat ofvaporization, implications de ces resultats sur Ie developpement de methodes K = heat transferred by conduction, and physiques de controle des ravageurs sont discutees. C = heat transferred by convection.

CANADIAN AGRICULTURAL ENGINEERING Vol. 40, No.3 July/August/September 1998 185 The relative importance ofthese factors will vary with the and 56 °C in an oven (Precision). The microprobes were control method used. For example, methods such as the connected to a microprocessor thermometer (Model HH21, propane flamer transfer heat mainly by convection, while Omega Engineering Inc., Stamford, CT) and the internal body microwaves depend on radiation absorption for increasing the temperature was recorded once a minute until it remained body temperature of the insect. As the body temperature stable. Air temperature was measured as above and was increases, the importance ofheat loss by evaporation increases recorded o~ce a minute. (Pelletier 1994). This causes the body temperature to remain lowerthan the surrounding airtemperature. Immersion in warm 100 water prevents heat loss by evaporation and the body temperature increases rapidly up to the water temperature. The ~80 effect of exposure time to warm conditions and the cooling effect of evaporation are important factors to consider in the ~ 60 evaluation and development ofcontrol methods. E ....c: ~AduJlS The objective ofthis work was to determine the lethal high Q) 40 -D-Fourth Inslar ~ -6-Third Instar temperature for six stages (adult, pupae, and four larval instars) Q) Q. 20 -0-Second Inslar of the Colorado potato beetle. Immersion in warm water was _Firstlnslar used to quickly transfer energy to the insects and to eliminate -A-Pupac heat loss by convection and evaporation. Since convection and 0 evaporation are significant heat loss factors for most control 35 40 45 50 55 60 65 70 Air temperature rC) methods tested with the Colorado potato beetle, the lethal A temperature was also evaluated in hot air conditions. The effect of varying the length of the exposure to high air temperature 100 was evaluated. Finally, the change in body temperature ofthe insect in time under high air temperature conditions was ~80 measured. as ~ 0 60 MATERIAL and METHODS E ....c: ~AdullS Six stages (adults, pupae, and four larval instars) of the Q) 40 -D-Founh IllS1lIr ~ -6-Third Instar Colorado potato beetle (field-collected) were exposed to air Q) Q. -o-Second IllS1lIr temperatures between 36 and 64 °C for periods of 10 and 20 20 _Firstlnstar minutes. The exposure times were kept as short as possible -A-Pupac while allowing enough time for the insect body temperature to 0 rise to a level close to the air temperature. Groups often first 35 40 45 50 55 60 65 70 instars were placed in 25 mL plastic containers with screened Air temperature CC) tops. The other larger stages were placed (in groups often) in B plastic Petri dishes (90 mm diameter) with screened tops and lined with dry filter papers. Each combination of stage, Fig. 1. Percent mortality of six stages of the Colorado temperature, and exposure time was replicated five times. A potato beetle exposed for 10 min (A) and 20 min hybridization incubator (Model 400, Robbins Scientific Corp., (B) to different temperatures ofhot air. Error bars Sunnyvale, CA) was used to expose the insects to the different are the standard errors. air temperatures and an electronic thermometer (Tri-Sense meter, Model No.37000-00, Barnant Co., Barrington, IL) and a K thermocouple were used to monitor the temperature. After The lethal temperatures for six stages (adult, pupae, and the test, the adults or larvae were placed with potato leaves in four larval instars) of the Colorado potato beetle were also Petri dishes with regular covers and damp filter papers. Pupae measured by immersing the insects in warm water (40 to 56 were placed in Petri dishes filled with damp vermiculite. The °C) for one minute. Ten insects (field-collected) were caged in number of living larvae or adults was recorded immediately a plastic tube (20 mm in diameter and 50 mm long) with after the test. An insect was determined to be dead ifit failed to screening glued on one end. A foam stopper was used to plug move after being gently poked with a brush. No mortality was the other end. This arrangement allowed the water to rapidly observed after manipulating the insects but keeping them at fill the tube but prevented the insect from floating at the room temperature. Survival of pupae was measured as the surface. The tube containing the insects was immersed for one proportion that emerged during the week following the test. No minute in 150 mL ofwarm water in a 200 mL beaker. The test mortality was observed with control pupae set up at the same was replicated ten times for adults and five times for the other time as the test insects. stages for each temperature. The water temperature was monitored using an electronic thermometer equipped with a K The actual body temperatures ofthe adult, third instar, and thermocouple. After immersion in warm water the insects were fourth instar Colorado potato was measured by impaling handled as they were after exposure to hot air and the mortality individuals dorsally on hypodermic needle microprobes (Type was determined. Water at 20 to 25 °C (room temperature) was MT-29/1B, Physitemp Instruments Inc., Clifton, NJ). Each used as a control to evaluate the effect ofthe manipulation on combination ofstage and temperature was replicated ten times. the insect mortality. Insects were exposed to temperatures ofapproximately 45, 50,

186 PELLETIER Table I. Parameters oflinear regression curves of mortality (Probit Based on the calculated LT 95 (Table I), the lethal transformed) versus temperature for six life stages ofthe temperature generally increased with the size ofthe Colorado potato beetle subjected to three conditions. insect. The pupal stage was the least sensitive to high air temperature with a LT95 of61.3 and 56.2 °C 2 for a 10 and 20 min exposure, respectively. Stage Origin S1. Error Slope S1. Error R LT95 Increasing the exposure time from 10 to 20 min Warm air, 20 minute exposure decreased the LT95 by 3.7 °C for the adults, 3.1 °C for the pupae, 2.3 °C for the fourth instar, 1.2 °C for Adult -12.56 2.78 0.33 0.05 0.94 57.5 the third instar and 0.9 °C for the second instar and increased the LT by 0.7 °C for the first instar. -30.83 5.88 0.61 0.10 0.98 61.3 95 Under conditions of high air temperature, the 41b Instar -21.75 0.33 0.50 0.01 1.00 56.9 body temperature ofthe larger insect stages required around 10 to 15 min to reach a stable value near the 3rd Instar -18.10 2.64 0.47 0.05 0.98 52.4 air temperature (Fig. 2). At that point, the body

nd temperature was 2 to 10°C lower than the air 2 Instar -22.02 6.46 0.58 0.14 0.90 49.3 temperature. This difference was generally greater 1st Instar -13.60 6.27 0.39 0.13 0.82 51.5 for the higherairtemperatures. The small increase of the difference between air and body temperature Warm air, 10 minute exposure during the first few minutes is an artefact caused by the increasing temperature ofthe oven after closing Adult -16.41 3.37 0.38 0.06 0.96 61.2 the door. Pupa -16.79 5.42 0.36 0.09 0.94 64.4 Mortality ofthe different stages ofthe colorado potato beetle immersed in warm water is presented 41b Instar -6.61 12.89 0.22 0.22 0.50 59.2 in Fig. 3. No mortality was observed when 20°C water was used as control. Ranging from 50.1 to rd 3 Instar -12.71 7.69 0.36 0.15 0.75 53.6 56.2 °C (Table I), the LT95 varied less between stages than with the hot air tests. The LT 95 of the 2nd Instar -18.79 0.51 1.00 50.2 larger stages (adult, pupae, and fourth instar) from 1st Instar -25.12 15.24 0.63 0.31 0.80 50.8 the warm water test were 4.1 to 5.2 °C and 6.8 to 8.9 °C lower than the 20 and 10 min hot air tests, Hot water respectively. This difference was reduced to 1.7 to 2.9 °C with the third instar. The LT95 obtained with Adult -8.06 1.91 0.28 0.04 0.88 52.3 the second and first instar varied by less than 1 °C between the three tests. Pupa -13.40 2.23 0.36 0.04 0.99 56.2

41b Instar -40.31 10.11 0.90 0.20 0.91 52.4 DISCUSSION

3rd Instar -17.67 2.04 0.48 0.04 0.97 50.7 The difference in the LT95 calculated from the warm water test and the hot air tests revealed the 2nd Instar -34.46 7.71 0.82 0.15 0.93 50.1 importance ofcertain factors. With the warm water test, the LT95 varied between 50.1 and 52.4 °C for st 1 Instar -39.12 2.37 0.88 0.05 1.00 51.8 the different life stages except for the pupae with 56.2 DC. A larger difference between the LT95 ofthe different stages was observed with both exposure time of the For each insect life stage/temperature/test combination, the warm air test compared to the warm water test (Table I). In the mortality observed in each replicate was used to calculate the hot air tests, the LT 95 increased with body size. Heat exchange average mortality and the standard error of the mean. As the between the insect and the air or water depends on several mortality ofthe controls were always zero except for one stage factors and is difficult to calculate precisely. However, it is in the warm water test, the average mortality was not adjusted. expected that the temperature of the insect will reach a level A linear regression ofthe "probit" transformed mortality mean close to its surroundings faster in water than in air. This is versus temperature was calculated for each insect life stage/test supported by the data presented here. Similarly, to increase the combination. The temperatures producing95% mortality (LT 95) body temperature ofa larger insect to a certain level, during the were calculated from the regression lines. same exposure time, the temperature of the surrounding air needs to be higher than with a smaller insect. An insect with a larger body size requires more heat to increase the body RESULTS temperature by one degree. These principles have been used to The percent mortality ofthe six stages ofthe Colorado potato reduce the exposure time ofthe control method for insects in beetle exposed to hot air for 10 or 20 minutes is given in Fig 1. stored products by using hot water (Heard et al. 1991) instead In most stages, mortality increased from 0 to close to 100% ofhot air (Mangan and Ingle 1992). An exposure time shorter over a temperature range of 10°C. than the time required to equilibrate the system results in a lower body temperature and lowermortality. More importantly,

CANADIAN AGRICULTURAL ENGINEERING Vol. 40, No.3 July/August/September 1998 187 factors such as evaporation can maintain the insect temperature temperature (Pelletier et al. 1995; Wharton 1985) and as the below air temperature. As demonstrated here, the temperature water evaporates the temperature ofthe insect decreases (May ofthe surrounding air provides a poor indication ofthe body 1985). The difference in temperature between the air and the temperature ofthe insect. insect body is influenced by the insect's propensity to lose water and the amount ofwater available. The rate ofwater loss _20 increases rapidly at temperatures above 40°C (Pelletier et al. ~e16 1995). Heat loss by evaporation is not an important factor in :::;, the heat balance at normal ambient temperature (Henwood e ~ 12 OJ&: 1975), but it probably becomes significant at high c.~ 8 temperatures. These data demonstrated that in developing a ~~ 4 field control method based on hot air, any reduction in the ..... " 0+----__---...... ---_---__. exposure time has to be compensated for by an increase in the o 5 10 15 20 air temperature in order to increase the insect body temperature to its lethal level.

100

_20 ~ 80 ~e16 ~ :::;, t:: e ~ 12 0 60 OJ &: 8 E c.~ c -o-Adulls - 40 E~ 4 ~ -<)-Fourth Instar Q): "- OJ -6-Third Instar ..... " O+-----r-----r-----r----..... a. 20 ~Sccond Instar o 5 10 15 20 -Firstlnstar .....Pupac 0 40 42 44 46 48 50 52 54 56 58 Water temperature rC)

_20 Third Instar Fig. 3. Percent mortality of six stages of the Colorado ~p 16 potato beetle exposed for 1 min to different :::;,- temperature of warm water. Error bars are the ~ ~ 12 standard errors. a.!OJ &: 8 E~ 4 OJ: The insect body temperature rapidly reaches the ..... " O+----...,..----,.------y-----, temperature level of the surrounding water. The LT95 of the o 5 10 15 20 first and second instars obtained during the hot air test was very Time (min) similar to the one obtained in warm water. Because of their large surface/volume ratio, heat transfer as well as evaporation Fig. 2. Changes through time in the difference (OC) is more rapid. The small size of these stages prevented the between air temperature (45, 50, or 56°C) and recording of their body temperature, but it is likely that the body temperature of adults, fourth, and third change in internal temperature appended faster and remained instars of the Colorado potato beetle. Error bars at a temperature closer to the air temperature compared with are the standard errors. the larger stages. With the larger stages (third instar, fourth instar, and adult), the absence of evaporation decreased the The reduction of the exposure time from 20 to 10 min observed LT95. A proportion ofthe adults died at relatively low resulted in an increase of up to 3.7 °C in the LT95. The LT95 temperature (between 40 and 50°C). This might have been due was calculated with the air temperature and not the body to variability in the ability of adults to tolerate high temperature. Since the reduction of the exposure time had a temperatures. This variability could be linked to genetic or larger effect on the LT95 oflarger stages (Table I) it is probable physiological factors. that the body temperatures of insects exposed for a different These results provide basic information that can be used in time to hot air did not reach the same level. the development ofnew control methods. The transfer ofheat The rate of change in body temperature ofthe insect was from hot air to the insect is affected by several variables such reduced after a few minutes in the oven (Fig. 3) but remained as the exposure time, temperature, and evaporative cooling. 4 to 10°C below the air temperature. This difference between These factors must be considered in the development ofcontrol the air and the body temperature increased with the air methods, such as the propane flamer or the use ofmicrowaves, temperature, possibly due to heat loss by evaporation. A large that aim to heat the insect above its lethal temperature. The proportion ofthe body mass of the Colorado potato beetle is LT95 calculated from warm air and hot water tests were very composed ofwater (Pelletier et al. 1995). The rate ofdiffusion similar in spite of the exposure time varying from I to 20 of water through the cuticle into the air increases with body minutes. The difference between the body and the air

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CANADIAN AGRICULTURAL ENGINEERING Vol. 40, No.3 July/August/September 1998 189