Adv. Odonatol. 2:169-176. December, 1984 169

The effects of prey density and

temperature on development of larvae of the sponsa (Hans.)

(Zygoptera: )

J. Pickup & D.J. Thompson

Department of Zoology, The University,

P.O. Box 147, Liverpool L69 3BX, U.K.

An was in which larvae of Lestes experiment performed sponsa (instars 6 to 10)

maintained and from were at constant prey density temperature the time they

moulted into an instar until the time they moulted out of it.

of sizes Prey were Daphnia magna appropriate for each instar. Lestes larvae

were maintained at six densities, and three The of prey temperatures. range prey

densities and used were the temperatures thought to encompass likely ranges

encountered by Lestes larvae in the field.

rates increased with and Development prey density temperature. There was no

evidence of trend of instar to any adaptation by any any prey density/temperature

combination; later instars were not better adapted to higher temperatures and

better earlier instars were not adapted to lower temperatures.

INTRODUCTION

This examines the effects of paper prey availability and temperature on the rate

of of larvae of the development damselfly (Hans.). The data are

utilized in the first is in two ways; relation to extending work on the components

of the second in which predator-prey systems; explores the ways

Lestes larvae sponsa achieve the rapid development rates typical of most lestids.

In their reviews of the components of arthropod predation, HASSELL, LAW-

TON & BEDDINGTON (1976) and BEDD1NGTON, HASSELL & LAWTON

(1976) considered that the two main components of predation were the prey 170

death and the of increase. Each of these rate predator rate components was

divided into three sub-components. The prey death rate was determinedby prey

density, predator density and the relative distributions of predator and prey; the

predator rate of increase was influencedby the survival and development rates of

the predator and its fecundity. BEDDINGTON et al. (1976) pointed out that few

in which data and able studies, were given on development rates survival, were

to provide informationon several predator instars. LAWTON, THOMPSON &

THOMPSON (1980) tried to remedy this by studying the effects of prey density

on survival and development rates of the last three instars of larvae of the

damselfly Ischnura elegans (Vander Lind.). One aim of this study was to provide

fuller of the relation between rate and for five a picture development prey density

instars of another damselfly species, Lestes sponsa.

THOMPSON (1978) has pointed out that temperature changes have profound

effects on the feeding rates of invertebratepredators. It follows then that tempera-

ture will influence the between and rate. relationship prey density development

For this have extended the work reason we earlier reported by BEDD1NGTON et

al. (1976) and LAWTON et al. (1980) by performing all our experiments at three

temperatures.

The larval of in the field first growth Lestes sponsa was described by CORBET

(1956a) for a population in a pond in Berkshire, southern England. A more

detailed analysis has been provided by PICKUP, THOMPSON & LAWTON

(1984). Larval growth is completed in about 75 days, considerably faster than

the latitude coenagrionid at same (PARR, 1970). Lestids usually

spend the winter in an egg diapause (CORBET, 1956b) so that their larvae can

expect to experience predictable relatively high water temperatures. Later Lestes

instars experience water temperatures in the region of 10°C higher than lower instars. Therefore it might be expected that larval instars ofLestes might differ in

their temperature adaptations, with later instars being better adapted (relatively

faster development rates) to higher temperatures and youngerinstars better adapted

to lower temperatures. Coenagrionid larvae on the other hand experience the

whole range of water temperatures that occur from winter to summer and no

least in of particular adaptation to temperature (at terms feeding rate) was found

by THOMPSON (1978).

METHODS

Lestes larvae collected from Cheshire sponsa were a pond at Malpas, (northern

Latitude fed in England, 53°N) and ad lib. 16°C constant temperature growth cabinets until they had moulted into the required instar. Instars have been deter- mined from field populations by plotting wing bud lengths against head widths

(PICKUP et ai, 1984). Counting the pronymph as instar 1, Lestes typically has ten larval instars. Only larvae showing little variation from the mean head width 171

for that instar were included in this experiment. The mean head widths and standard errors for each of the last five instars are given in Table I.

Table I

Mean head widths (with standard errors) for

Lestes instars 6 10 sponsa to

Instar No. 6 7 8 9 10

Mean Head width (mm) 1.638 2.068 2.589 3.237 3.803

± SE 0.010 0.014 0.018 0.017 0.021

N 55 61 48 44 41

Larvae were fed on laboratory cultured Daphnia magna Straus, sieved to standard sizes. The sizes of used varied Daphnia with larval instar, and were chosen to within the size so as provide prey rangeof naturally taken prey. Instar

6 Lestes larvae were fed size-class “E” Daphnia which are of a size which will

600 mesh Endecott pass through a (xm test sieve but not through a 500

mesh. Instars 7 and 8 fed size-class pun were on “D” which pass through

850 mesh but not 710 an pirn a pun mesh. Instars 9 and 10 were fed on size-class “C” which through a 1000 mesh but an 850 pass pun not (xm mesh.

after each larva was Immediately moulting, placed into a plastic cup contain-

75 ml of filtered and small ing pond water a cocktail stick to provide a “fishing site”. of Three 12, 16 and 20°C were chosen. These temperatures cover the range of be temperatures most likely to encountered by Lestes larvae in northern

six used in England. Typically prey densities were order to ensure that maximum

rates were as well as a lower of down feeding achieved, range feeding rates to a

where encountered. point mortality was However, for instars 9 and 10 at 12°C and 20°C the lower end of the of densities used curtailed range prey was so that only the maximum development rates were covered. At least three replicates

used for each were prey density/temperature combination.The larvae were kept at 16 hour dark constant a light:8 hour photoperiod which approximates to the average natural photoperiod during Lestes larval life in northern England.

The Lestes larvae were checked every 24 hours. Those larvae which had not moulted were transferred to cups of fresh pond water containing the appropriate number of Control Daphnia. cups containing Daphnia but not Lestes larvae

deaths indicated that Daphnia from “natural causes” were negligible and there- fore affect unlikely to the values of prey densities used. Larvae which had moulted were removed from the study and the duration of the instar (in days) recorded. 172

The between the duration and the 3 Fig. I. relationship development rate (1/instar (D)) prey density at

12°C I6°C and 20°C for 3 instars of Lestes temperatures, (squares), (circles) (triangles) sponsa; (a)

instar 8. densities Size “E” instar 6, (b) instar 7, (c) The prey are numbers of DaphniaJ75 ml. class

“D” in Daphnia were used in (a) and size class (b) and (c). Means and standard errors are plotted.

RESULTS

= The effect of prey density on the development rate (development rate

1/instar duration (D)) at the three temperatures used is shown in Fig/ 1(a), (b)

and (c) for instars 6, 7 and 8 respectively. The development rate increases with

each until is reached. level of prey density at temperature a plateau The this

plateau is determined by the temperature. Maximum development rates have been obtained fromestimates ofthe plateau levels by eye. For example, in instar

6 (Fig. 1(a)), these are 0.24 at 20°C, 0.13 at 16°C and 0.08 at 12°C. A similar

is exhibited instars 7 and 8 in and pattern by Figs. 1(b) 1(c). 173

A series of similar curves rise showing a decelerating to an asymptote is found for consecutive instars when development rates are plotted against prey density for one temperature, 16°C 2). Since different sizes of (Fig. prey are used for different instars the is in of prey density expressed dry weight prey per 75 ml rather than Daphnia numbers. As would be expected this figure also shows that larger instars reach maximum development rates at higher prey densities. Fig. 3 shows maximum for each ofthe five development rates instars and three tempera- ture combinations.

2. The Fig. between the and the for instars 6 relationship development rate prey density (open

circles), 7 (solid 8 (squares), 9 (solid circles) and 10 of Lesles triangles), (open triangles) sponsa at

I6°C. The is in prey density expressed mg dry weight of Duphnia/75 ml. Means and standard errors

are plotted. 174

If later instars are to show adaptation towards higher temperatures early instars would be expected to develop relatively faster at the cooler temperature of 12°C

when compared with the 20°C rate. Similarly later instars should show a greater difference between 20°C and 12°C development rates than should earlier instars.

Fig. 3 shows that the maximum development rate against instar curves at 16°C and 12°C are similar to the shape of the curve at 20°C. In other words the maximum development rate at 16°C is constant at between 50 and 60% of the

20°C rate for all 5 instars, and at 12°C is 20-30% of the 20°C rate for each instar.

This can be seen in Fig. 3 by looking at the constant nature of the maximum

for instar. there is development rate against temperature curve each Clearly no evidence of adaptation by any instar to the 3 temperatures used in this study.

Fig. 3. The relationship between the maximum development rate and the temperature and instar

number for Lestes sponsa.

Fig. 3 also enables the maximum development rates to be compared between

instars. For instars 7 and 8 the maximum 6, development rates at all 3 tempera-

tures are essentially similar though there is a slight trend towards a reduction of

maximum development rate in later instars. Instar 9 is of noticeably longer

duration than instars 6 to 8 at each of the 3 temperatures and the final instar,

instar 10, which has to the of to incorporate period metamorphosis prior emergence, is about twice as long as instar 9 at all 3 temperatures. 175

The effects of low densities be temperature on development rates at prey can

in seen Figs. 1(a), (b) and (c). The data here are not sufficiently comprehensive for detailed a analysis but 2 observations can be made. Firstly, instar durations tend to be at lower of 12°C longer temperatures irrespective prey density; devel- opment rates, for example, are lower than those at 16°C and 20°C of each instar/prey density combination. Secondly survivorship is higher at lower

instars 7 and 8 survive at the lowest 12°C. temperatures; prey density only at

DISCUSSION

The rate follow development against prey density curves the same pattern as those of LAWTON al. ef (1980). Whether these curves are best described by the basic model of BEDDINGTON ef al. (1976) or the more complex version that

LAWTON ef al. found for Ischnura cannot be determinedwithout examining the development rate as a function ofthe actual feeding rate. However this difference between the models is two not thought to make a significant difference to the

dynamics of the predator-prey interaction(LAWTON ef al., 1980). The contribu- tion that this study makes to modelling predator-prey interactions is in describing such curves over five consecutive instars at three temperatures and showing that the form of the curves is constant for each instar/temperature combination.

The level for each plateau curve (i.e. the maximum development rate), is determined the instar number by two parameters, and the temperature. The instar number determines the “base level” for the development rate upon which the

There is temperature acts. no apparentinteraction between instar and temperature which would for present problems the modeller; that is, similar increases in temperature cause similar percentage increases in development rates.

The increase in maximum development rate with temperature reflects the increase in maximum feeding rates in coenagrionids found by THOMPSON

for Ischnura and LAWTON (1978) elegans, by (1971a) for Pyrrhosoma nymphula

(Sulz.), and in the libellulidCelithemis fasciataKirby by GRESENS, COTHRAN

& THORP (1982). Although our study provides only 3 temperature points, the development rate almost doubles with both of the 4°C rises suggesting that the curve of maximum follows development rate against temperature an exponential 20°C pattern over the 12 to range. This contrasts with the feeding rate studies where LAWTON (1971a) found a linear relationship between maximum feeding rate and temperature between 4 and 15°C. THOMPSON (1978) found maximum

rates to increase 16°C feeding exponentially up to and then level off, and GRESENS ef al. found the (1982) maximum feeding rate to increase from 10°C to 15°C, level off between 15°C and 20°C, and then increase again between 20°C and

25°C. The ability of development rates to almost double with a 4°C rise in

that Lestes is sensitive temperature suggests extremely to temperatures within this range. THOMPSON (1978) found a similar rate of increase in maximum 176

feeding rate in Ischnurabetween 5°C and 16°C. Clearly slight changes in tempera- ture can profoundly affect feeding and development rates of damselfly larvae.

The absence of trend of towards a adaptation any temperature/prey density combination indicates that such fine tuning of the physiology of different instars

different to the temperatures that they can predictably encounter does not explain the rapid growth rate exhibited by Lestes. In this way this species is similar to

Ischnura elegans (THOMPSON, 1978) and probably most other odonate larvae.

It be that Lestes survive lower densities might expected can prey better at lower from the observation of LAWTON temperatures (1971b), on Pyrrhosoma, that increased incurred respiratory costs are at higher temperatures. Furthermore, lower respiratory costs at lower temperatures might enable larvae to complete an instar by making more of a limited energy intake available for growth. Thus, at low densities where survival all prey occurs at three temperatures, larvae at lower temperatures may be expected to develop faster. That this is not the case is an observation which will require investigation in terms of energetics and the qual- ity of the next instar.

ACKNOWLEDGEMENTS

thank for We SALLY COWLEY assistance with the experiment and endless encouragement. JP was supported by a Studentship from the Natural Environment Research Council.

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