Individual repeatability and geographical variation in the larval behaviour of the generalist predator, Chrysopa quadripunctata

A&.

Behav.,

1995, 50, 1391-1403

Individual repeatability and geographical variation in the larval behaviour of the generalist predator, Chrysopa quadripunctata CATHERINE

A. TAUBER,

MAURICE

Department

J. TAUBER

of Entomology,

Cornell

& LINDSEY

R. MILBRATH

University

(Received I2 May 1994; initial acceptance 3 November 1994; final acreptance 25 March 199.5; MS. number: A7001R)

Abstract. Larvae from three populations of Chrysopa quadripunctata, a generalist predator with diverse habitat and prey associations, were reared under common environmental conditions and tested for patterns of variation in their feeding and defensive behaviour under two regimens. The results revealed two patterns: individual repeatability and geographical variation in specific larval activities. First, individual larvae expressed consistency in the propensity to camouflage themselves and in the priority they gave camouflaging versus feeding; they retained their characteristic levels of behaviour after two moults. Such consistency in the behaviour of a predatory arthropod has not been previously reported. Second, the larvae showed significant geographical variation in quantitative aspects of feeding and camouflaging, as well as in their responsivenessto camouflaging material (behavioural plasticity). The pattern of geographical variation in larval behaviour reflects the differential characteristics of the prey that these populations encounter in nature. We conclude that disparate prey resources can lead to the differentiation of locally adapted larval behaviour and the evolution of specialization. 0 1995 The Association

Intraspecific variation in the behavioural responsesof predators to their prey ctmstitutes a central, but relatively neglected feature in the evolution of predator-prey associations (e.g. Arnold 1986, 1992; Krebs & Kacelnik 1991). With few exceptions, analyses of genetic variation in predatory behaviour have focused on vertebrates (e.g. Ayers & Arnold 1983; Drummond & Burghardt 1983; Tully & Huntingford 1988; Arnold 1992; Osenberg et al. 1992). In contrast, investigations of geographical or individual variation in the behavioural responses of predacious arthropods to their prey are restricted to spiders (Hedrick & Riechert 1989; Riechert & Maynard Smith 1989; Riechert & Hedrick 1990; Jackson 1992). The evolution of behavioural mechanisms underlying predator-prey relations in insects, the most diverse group of animals, remains largely unknown (Tauber & Tauber 1989; Gilbert 1990). Among predatory insects, the green lacewing genus Chrysopa (Insecta: Neuroptera: Correspondence:C. A. Tauber, Department of Entomology, Comstock Hall, Cornell University, Ithaca, NY 14853-0901, U.S.A. (email: [email protected]). L. R. Milbrath is now at the Department of Entomology, Hultz Hall, North Dakota State University, Fargo, ND 58105, U.S.A. 0003-3472/95/l 11391+ 13 $12.0010

for the Study of Animal

Behaviout

Chrysopidae) provides a fine opportunity for examining the evolution of a highly specific predator-prey association. It contains a pair of sister species with disparate prey ranges: one, C slossonae, specializes on a single species of aphid, and the other, C. quadripunctata, preys on a variety of aphids and other arthropods. Comparisons of these two speciesindicated that a suite of behavioural and ecophysiological traits, in both larvae and adults, underlies the specialist’s interactions with its prey (Tauber & Tauber 1987; Milbrath et al. 1994). Many of the seemingly species-specifictraits in C. slossonae differ only quantitatively, rather than qualitatively, from those in its generalist sister species(Milbrath et al. 1993; Tauber et al. 1993). Given the phylogenetically derived position of the sister specieswithin a clade of general feeders, we proposed that a C. quadripunctata-like generalist ancestor gave rise to the specialist, C. slossonae, without the acquisition of novel behavioural, physiological or morphological traits (Tauber et al. 1993; C. A. Tauber, unpublished cladogram of Chrysopa based on larval and adult morphological traits). The above scenario assumes that the C. quadripunctata-like generalist ancestor harboured heritable variation for the behavioural

‘: 1995 The Association for the Study of Animal Behaviour

1391

Animal

1392

Behaviour,

traits that adapt the specialist to its prey. Although it is not possible to test this assumption directly, we can make inferences concerning the ancestral condition from measurements of genetic variation within the extant generalist. The finding of behavioural variation in the generalist would not signify that the ancestor haboured similar variation, but absence of such variation would cast serious doubt on our hypothesized scenario. Therefore, we used a ‘common garden’ experimental approach to examine inter- and intrapopulation variation in the behaviour of the extant generalist; our focus was on individual repeatability and geographical variability in larval feeding and defensive behaviour. Natural

History

Our experiments focused on three widely separated, geographical populations of the generalist C. quadripunctata that have diverse habitat and prey associations. Below, we summarize the traits of the various generalist populations; to provide a meaningful basis for comparison, we begin the summary with reference to the specialist sister species(see also Table I). The specialist, C. slossonae, occurs primarily on alder trees, Alnus incana ssp. rugosa, in association with colonies of a single species of robust, ant-tended aphids, Prociphilus tesselatus, the woolly alder aphid. Chrysopa slossonae larvae appear to be well adapted to preying on the woolly alder aphid: they have enlarged heads and mouthparts that apparently aid in attacking and subduing the large aphids (Tauber et al. 1995), they are efficient feeders (Milbrath et al. 1993), and they circumvent ants by covering themselves thoroughly with the aphids’ flocculent secretions and by hiding within the aphid colonies (Eisner et al. 1978; Milbrath et al. 1994). In contrast, the generalist occurs on a variety of annual plants, trees and shrubs, including oak, maple, hickory, apple, elm, rose and sorghum (Banks 1903; Smith 1922; Tauber & Tauber 1987; L. R. Milbrath, M. J. Tauber & C. A. Tauber, unpublished data). Its broad range of prey includes various speciesof aphids and other softbodied insects that may or may not be tended by ants (Smith 1926; Throne 1971). In the field, first instar C. quadripunctata frequently camouflage themselves by placing aphid exuviae, flocculent secretions from aphids, or other plant or animal

50, 5

debris, on their dorsa; second and third instars vary in the expression of this trait (Smith 1922, 1926; L. R. Milbrath, M. J. Tauber & C. A. Tauber, unpublished data). Remaining motionless on the undersides of leaves or twigs appears important to their defence (Smith 1922; Milbrath et al. 1993). We examined two geographical populations of C. quadripunctata from eastern U.S.A. (Ithaca, Tompkins County, New York and Monticello, Jefferson County, Florida); these populations inhabit deciduous, hardwood trees with relatively large leaves, where they feed on aphids and other non-aphid prey: (Ithaca, New York: Myzocallis occultus and M. granovsky on red oak, Quercus rubra, and Monellia sp. poss. caryella on hickory, Carya sp.; Monticello, Florida: Melanocallis caryaefoliae, Monelliopsis pecanis, and Monellia caryella on pecan, Carya illinoensis). Second and third instars from these populations usually do not cover themselves with camouflaging material. Although these populations only occasionally associate with flocculence-secreting aphids, larvae from the Ithaca population thoroughly cover themselves when they encounter the woolly alder aphid’s flocculent material in the field or the laboratory (Milbrath et al. 1993, 1994). In comparison, the C. quadripunctata population from western U.S.A. (Davis, Yolo County, California) is associated with mixed populations of generally small-bodied aphids, Stegophylla mugnozae, S. essigi, and Tuberculatus californicus, scales (undetermined, Eriococcidae), and phylloxerans (undetermined, Phylloxeridae) on the undersides of the leaves of valley oak, Quercus lobata. Several of these homopteran speciesproduce flocculent secretions, and spider webbing usually covers the aphid-infested leaves. These coverings may provide the aphid colonies as well as the C. quadripunctata larvae with some protection from predation or parasitization. Chrysopa qudripunctata larvae (all instars) typically occur under the spider webbing, amid the aphids’ secretions; a light dusting of flocculence adheres to their bodies.

METHODS

We exposed larvae from each population to identical rearing and handling conditions (details in Milbrath et al. 1993). Our experiment consisted of

quadripuncata

slossonae

tesselatus

(woolly alder aphid)

Prociphilus

Myzocallis occultus M. granovskyi Monellia sp. pass. caryelia Moneffia curyella Monelliopsis pecunis Melanocallis caryaefoliae Stegophylla mugnozae S. essigi Tuberculatus californicus

Prey association (Aphididae)+

Yes**

Yes5 Yes8 ?

No% No% No% No% No% No%

Colonial

4’O

1.54.6 ?’ 1.5* 1.I9

23.4 1434.5

22 2.52 2’

-Size (mm; adult male)

Yes

Nu No No No No No No No No

Ant tended

Characteristics of prey

larvae and their prey

Yes

No No No No No No Yes Yes ?

Flocculent secretions

1

Feeding efficiencyt

1

Loading (camouflaging)

1

4

3

2

Response to flocculence

1 (I-2)

3 (3)

Not measured

2 (l-2)

Tibia1 length 1st (3rd) instar’

Traits of predatory larvae (rank: 1=highest, 4=lowest)

References: 1: Tauber et al. 1995; 2: Richards 1968; 3: Bissell 1978; 4: Tedders 1978; 5: Bissel 1983; 6: Davis 1910; 7: Remauditre & Quednau 1985; 8: Hille Ris Lambers 1966; 9: Baker 1917; 10: Pergande 1912; Smith 1974. *Occasional non-aphid prey may be taken. tHere defined as the amount of time required to consume one aphid. IDispersed on relatively large leaves. &mall to large colonies on undersurface of relatively small leaves. **Large colonies on branches and trunks.

Ithaca, NY (alder)

Chrysopa

Davis, CA (valley oak)

Monticello, FL (pecan)

Ithaca, NY (oak and hickory)

Chrysopa

Predator Population (Plant host of larval prey)

Table I. Physical and behavioural characteristics of Chrysopu

5 a

%

i

R

2 2. E? F’ 3_.

2 R ,.

i

2 sm

1394

Animal

Behaviour,

observing and quantifying larval activity in the presence of one type of prey (Myzus persicae, green peach aphids) and one of two types of aphid-derived camouflaging material (flocculent secretions from woolly alder aphids or exuviae of green peach aphids). We examined larvae from each of the three populations of C. quadripunctata in the presence of the two types of aphid-derived camouflaging material; in addition, we observed each larva four times during its development (twice, early and late, in the first instar and twice, early and late, in the third instar). Thus the entire experiment comprised a 3 (population) x 2 (treatment) x 4 (instar and age) factorial design. The insects used in our tests originated from field-collected females: (Davis, California: N= 7; Monticello, Florida: N=3; Ithaca, New York: N=4). Each population was repiesented by 10 first-generation, laboratory-reared larvae per treatment. During the four observation periods, we exposed each larva to only one type of aphidderived material (either Myzus exuviae or flocculent secretions from woolly alder aphids). In all cases, assignment of larvae to treatments was paired on the basis of female parent, with one larva from a pair receiving one treatment (either exuviae or flocculent secretions), and the other larva receiving the alternate treatment. This experimental design controlled for conditioning, maternal effects, and genetic variation within populations. Specimens from all populations were deposited in the Cornell University Insect Collection, lot number 1205. We took data from the specialist and one population of the generalist (both from Ithaca) from an earlier study (Milbrath et al. 1993); our subsequent tests (reported here) with the other C. quadripunctata populations used identical procedures and food (see Milbrath et al. 1993 for details). The prey species(green peach aphids) is one that C. quadripunctata larvae frequently encounter on apple, maple and rose; it is suitable for C. quadripunctata’s development and reproduction (Tauber & Taubcr 1987). In nature, these aphids may or may not be tended by ants. In all tests, larvae were held without food for about 4 h (early first instars) or 12 h (late first, early and late third instars) before each observation; this procedure resulted in consistent but moderate levels of searching (i.e. moderately ‘hungry’ larvae). Larvae were introduced into the arena without debris on their dorsa (i.e. ‘naked’

50, 5

larvae). The above approach, which used larvae that were both moderately hungry and naked pitted feeding and defence (camouflaging) aga& each other. Each observation (four per larva) lasted 45 min. During this time we recorded all larval activity under the following five categories: feeding, loading (i.e. camouflaging; placing or rearranging material on the dorsum), cleaning, mobility (i.e. movement other than feeding, loading or cleaning), and immobility. We noted the sequence of activities, the number of times each behaviour was performed, and the amount of time spent on each activity per 45-min observation; in addition, we considered the number of prey eaten and the amount of time spent feeding per prey item (see Milbrath et al. 1993 for details). In conjunction with the 45-min observations, we also quantified the initial activity of hungry, naked larvae when they were first placed in an arena and given a choice between feeding and camouflaging themselves. To increase the sample size for this one-time behavioural event, we observed 10 additional larvae, for a total of 20 larvae per treatment (offspring of the original field-collected females). We treated these larvae as above, except that each observation stopped when the larvae initiated feeding or loading, or when 5 min had elapsed, whichever came first. Statistics

Our primary interest focused on the variation within specific behavioural traits (primarily feeding, loading and immobility), not on interactions or trade-offs among the traits. Moreover, our experimental design (e.g. observation periods fixed at 45 min, larvae restricted to an artificial arena) precluded the use of multivariate analysis on the full data set. Therefore, we analysed the data on feeding, loading, cleaning and immobility with univariate analysis of variance with a nested design; population, family (nested within population), instar, age within instar, and treatment were treated as factors, with repeated measures on instar and age within instar (SAS 1985). Family was designated as a fixed effect, and we used the Bonferroni correction to adjust P-values for the number of tests run. Mobility included several different activities, such as walking, running, searching and probing. Because we often could not distinguish these

Tuuher et al.: Variation

activities from one another, and to provide independence for our statistical treatment of the proportion of time spent on the other activities, we &uded this category from the analysis. Neither untransformed data nor transformed data [log(number events+ 1) or arcsine squareroot (proportion of time)] displayed equal variances; therefore, we used untransformed data in the four-factor ANOVA. This did not present a problem because inequality of variances has little effect in a bdanoed design (Scheffe 1959). Preliminary tests showed that the presence or absence of flocculent secretions did not significantly influence the duration of feeding. Therefore, we combined data on the time required for larvae to consume individual aphids from the two treatments, and we assessedthe remaining factors [population, family (population), instar, and age within instar] with a four-factor ANOVA on the log-transformed data. Means were separated with the Tukey-Kramer test. We tested variation in the choice of an initial activity (feeding versus loading) by hungry, naked larvae for independence from population and treatment (each combination of population and type of aphid-derived material) with G-tests having a simultaneous test procedure (Sokal & Rohlf 1981). Because of the repeated-measures design, we tested each instar and age separately. We used two methods to examine individual repeatability in the various behavioural traits. First, we correlated the amount of time or number of events per 45-min observation period for individual larvae across first and third instars (product-moment correlations; P=O.O5; Falconer 1981; Arnold & Bennett 1984). Second, we examined the consistency of larvae in their first activity (feeding versus loading) over the four observation periods with a chi-squared test. Becausethe expression of camouflaging behaviour varies significantly between populations, we analysed the results from each population separately. To correct for inter-population variation in hunger levels and overall propensity to engage in feeding versus loading, we calculated expected values from the binomial distribution of the observed frequencies of feeding and loading for each population. We classified ‘consistent’ behaviour as four out of four occurrences of either feeding or loading; any combination of feeding and loading constituted ‘inconsistent’ behaviour. Thus our test provides a very conservative

in Chrysopa hehaviout

1395

assessment of the repeatability of larval behaviour. Finally, we used the Fisher method for combining the probabilities from independent tests to assess the general characteristics of C. yuadripunctafa larvae from the three geographical populations (Sokal & Rohlf 1981). RESULTS Individual lnstars

Repeatability

in Rebaviour

across

Although C. yuadripuncta~a larvae varied in all the components of behaviour we measured, individuals were consistent, across two moults, in the expression of two activities, both of which involve camouflaging (Table II). First, the incidence of loading by individuals in the first instar was significantly and positively correlated with their performance in the third instar. Although the relationship was not statistically significant for individual populations (probably a function of sample size), it was consistently positive for each population, and when the probabilities from the independent tests on all populations were combined (Fisher method), the result was statistically significant. Second, individual larvae tended to be consistent in their initial choice of feeding versus camouflaging (i.e. loading). Although the incidence of larvae that engaged in loading before feeding ranged from 0 to 70%. the behaviour of individuals over their lifetimes was more consistent than expected from the binomial distribution of observed frequencies of the two behavioural traits. The deviation from expected was significant for the Ithaca larvae, as well as for larvae from all populations combined (Fisher method for combining probabilities from independent tests of significance; Table II). Geographical

Variation

in Lkhaviour

Larvae from the C. quadripunctata populations expressed significant geographical variation, as well as striking similarities in the four behaviourdl traits that we tested (feeding, loading, cleaning and immobility; Table III). There was also significant variation in the overall level of larval activity and in larval responsiveness to the flocculent secretions of the woolly alder aphid (Table IV, Fig. I).

Animal

1396

Table II. Individual consistency in C. quadripunctuta first and third instars) Geographical population of C. quadripunctata

Camouflaging material

Davis, CA Monticello, FL Ithaca, NY C. slossonae (Ithaca, NY) - 2ZlnP (C. quadripunctata P (k=ll)

Flocculence Exuviae Flocculence Exuviae Flocculence Exuviae Flocculence Exuviae

0.369 0.596 0.182 0.220 0.353 0.393 0.262 0.028

19.748 0.05

0045 - 0.114 0.592 0.035 0.528 0.272 - 0.332 - 0.060

(0.483) (0.500) (0.109) (0.462) (0.125) (0.241) (0.500) (0.500)

Feeding (% time) - 0.142 0.051 0.567 0.343 - 0.148 0.320 0.346 - 0.588

15.822 NS

Cleaning (% time)

(0.500) (0.395) (0.115) (0.196) (0.500) (0.208) (0.194) (0.500)

- 0.168 (0.500) - 0.067 (0,500) 0.034 (0.463) 0.017 (0.482) 0.215 (0.289) - 0.598 (0.500) 0.166 (0.334) - 0.375 (0.500)

15.356

964

NS

NS

Consistent

Variable

Gadj

P

10 (8.7) 8 (5.6) 8 (3.3)

10 (11.2) 12 (14.4) 12 (16.7)

0.41 1.29 6.26

0.541 0.352 0.013

8 (2.5)

12 (17.5)

9.37

0.003

Davis, CA Monticello, FL Ithaca, NY C. slossonae (Ithaca, NY) P (k=5)

(0.184) (0.108) (0.319) (0.284) (0.192) (0.174) (0.247) (0.470)

Immobile (% time)

Frequency of behavioural patterns (expected)

C. quadripunctuta

- 2ZlnP (C. quadripunctata

50, 5

larval behaviour after two moults (two observations each per

Loading (no. of events)

only)*

Geographical population of

Behaviour,

only)*

11.94 co.05

Performance by first instars correlated against performance by third instars; r,2= correlation coefficient (productmoment correlation), P=level of significance. G-test on initial behaviour of larvae confronted with a choice between food and camouflaging material. Consistent behaviour=either feeding or loading in all four observations (no combinations); variable behaviourzany combination of feeding and loading. Expected frequencies tested against the binomial distribution of the observed frequencies of the two types of behaviour (feeding versus camouflaging). Data from Chrysopa slossonae are presented for comparison. N=20 larvae/population. *Fisher’s test for combining probabilities.

Degree

of activity

Among the three C. yuadripunctata populations, larvae from Ithaca generally had the highest overall level of activity (as measured by the average number of times, per observation period, that larvae changed from one activity to another, excluding mobility; Table IV). In this trait, the Ithaca population was the one that most closely resembled the specialist predator, C. slossonae. Within all populations, the presence of flocculent secretions from the woolly alder aphid increased the overall level of larval activity; this increase largely reflected higher levels of loading behaviour.

Feeding

There was significant geographical variation among C. quadripunctata larvae in the amount of time required to consume an aphid; instar and age also had significant effects on this trait (Tables III, V). There were no significant interaction terms, however, and in all age classes, larvae from the Ithaca population consumed prey more quickly than those from the other two populations. Only third instars from Ithaca approached C. slossonae’s speed of feeding (Table V). Flocculent secretions from the woolly alder aphid did not significantly affect the number of aphids eaten, the rate of feeding or the proportion

Tauber

et al.:

Variation

in

Chrysopa

behaviour

1397

Table III. Results of repeated-measure ANOVAs on the behavioural traits of C. quadripunctata larvae (first and third instars with two age classes each) from three geographical populations when provided with prey and either one of ,wo types of aphid-derived camouflaging material

-

Behavioural traits Feeding

ANOVA Terms population Family (POP) Cam0 material Instar Age

Camouflaging

No. aphids Proportion eaten time

NS

co.01 NS

NS NS NS

Time to consume No. Proportion an aphid events time

co.01 co.01 NS co.01 co.01

co.01 co.01

co.01

NS NS

NS NS NS NS NS

NS

NS NS

NS NS NS

NS

NS NS

NS NS

co.01 co.01 co.05


co.01 co.04

NS NS

NS NS

NS NS NS

NS NS

Cleaning

Immobility

No. Proportion events time

Proportion time


NS NS

NS NS

NS NS

NS NS NS

NS co.01 co.01 co.02 NS NS NS NS NS NS NS

NS NS

NS NS

co.03

co.01

NS NS

NS NS

NS NS

NS

0.01

~teractions

pop-cam0 mat Pop-instar Pop-age Family (poptinstar Family (pop)-age Family (poptinstar-age Cam0 mat-instar Cam0 mat-age Instar-age Pop-cam0 mat-instar Cam0 mat-instar-age

NS

co.01 NS

0.03 NS


co.01

NS NS NS NS

NS NS NS NS

co.01 NS

NS NS NS

CO.01 NS NS

NS NS NS

NS

-=O.Ol

NS NS

NS

NS

NS NS NS NS

NS

co.01 NS

co.04

Level of significance calculated from the Bonferroni correction for multiple tests; ~s=P>O.05.

Table IV. Geographical variation between C. quadripunctata populations in the overall level of larval activity (i.e. total number of changes in behaviour, excluding mobility) under two conditions Geographical population of C. quadripunctata

Davis, CA Monticello, FL Ithaca, NY C. slossonae (Ithaca, NY)

Type of camouflaging material

Early

Late

Exuviae Flocculence Exuviae Flocculence Exuviae Flocculence Exuviae Flocculence

5.1 f 5.5 14.3 f 12.8 5.3 rt4.4 14.6 f 12.2 6.5 f 4.6 16.0 f 13.5 16.7 f 10.1 35.6 f 20.1

7.4 zk6.2 25.3 f 13.0 4.8 f 4.2 16.7 f 14.9 IO.9 f 8.2 30.9 f 18.9 31.7 f 12.9 52.9 f 26.6

1st instar

3rd instar Early 17.5 f 28.3 f 23.6 f 52.4 f 20.2 f 61.6 f 45.7 f 84.1 f

7.6 18.2 10.4 21.9 7.6 21.8 11.2 26.7

Late 32.8 f 38.2 f 26.8 f 59.1 f 36.2 f 73.4 h 42.2 f 89.6 h

7.2 14.2 9.9 14.9 13.6 26.5 20.9 19.7

Values are given as the total number of activities per observation (xk SD; N= 10 larvae/cell): camouflaging material (P
1398

Animal

Behaviour,

30

50, 5 40

Early first instar

Late first instar

30 20

20

3 10 2 E z

10

9 .!z o 5 .2 E4 2 50

50

Early third instar

2 ‘% g

0

Late third star

40-

40

30 -

30

20 -

20

10 -

10

i 0”

0

Exuviae (Myzus)

Flocculence (Prociphilus)

0

Exuviae (Myzus)

Flocculence (Prociphilus)

Figure 1. Norm of reaction curves depicting geographical variation in age-specific responsiveness of C. quudripunctutu larvae to aphid-derived camouflaging material (N= 10 larvae/data point). See Table I for statistical analysis of the variation. Note differences in scale between graphs. 0: C. slossonae (Ithaca); 0: C. quadripunctatu (Ithaca); q : C. quadripunctaru (Davis); n : C. quudripunctuta (Monticello).

of time spent feeding (Table III). There was a significant interaction among camouflaging material, instar and age in determining the number of aphids eaten, but any biological significance of this interaction is not apparent. Camoujlaging

Aphid-derived material (exuviae versus flocculence) had a large and significant effect on

camouflaging behaviour (Table III, Fig. 1). In the presence of ~Vyzusexuviae, larvae of all ages and from all populations expressed relatively low incidences of loading (e.g. individuals engaged in fewer than 10 loading events per observation period). By contrast, when the woolly alder aphids’ flocculent secretions were available, larvae loaded frequently and spent relatively large amounts of time in loading. Moreover, in the

Tauber et al:

Variation

in Chtysopa

behaviour

1399

Table V. Geographical variation in the time taken for C. quadripunccara larvae to consume one M. persicae (first or second instar) Geographical population of C. quadripunctata Davis, CA Monticello, FL Ithaca, NY C. slossonae (Ithaca, NY)

1st instar

3rd instar

Early

Late

25~49 f 11:48” (17) 24:30 3~9:45”

17147f 12:54= (21) 1625 f 7153”

(16)

21:05 3~ 12:21b (13) IO:55 rt 5:03 (34)

(18)

IO:28 f .5:46b

(20)

627 I+C3:03 (54)

Early

Late

2:18 f 2~23~ (127) 1:55 f 1:0X? (151) 1:15 f 0:42b (73) 1:19Ito:46 (192)

1:15 f 1:03” (246) 1:19 f 0:47” (207) 0:52 f 0:36b (213) 0:59 zk 0:40

(254)

Values are given as mean min:s. N values in parentheses (no. of individuals). Values within columns followed by different letters are significantly different (ANOVA with log-transformed data, followed by Tukey-Kramer test for separating means, P~0.01). Data from C. slossonae are presented for comparison (not included in statistical analysis).

presence of the flocculence, third instars generally expressed higher levels of loading behaviour than first instars (significant interaction terms in the ANOVA on the number of loading events; Table III). Because loading is usually a very quick hehaviour, the proportion of time spentdoading was a poor indicator of geographical variation, but it did reflect the influence of camouflaging material (Table III). The expression of geographical variation in loading behaviour and the effect of the woolly alder aphids’ secretions on C. quudripunctata

between populations in a consistent manner. The degree of variation in these age classes was intermediate (between early first and late third instars), however, and it was not statistically significant.

responsiveness

nificant

Feeding versus camouflaging

When larvae of C’. quadripunctata were given both prey and aphid-derived camouflaging material, their choice of behaviour (feeding versus loading) was strongly influenced by the type of larval camouflaging behaviour were highly stage- material present (Table VI). Overall, larvae in all specific (Table III; significant populationage classes and in all populations gave higher camouflaging material-instar effect; also Fig. 1). priority to loading when they were in the presence Early first instars did not vary geographically in of flocculent secretions, than they did in the their responsivenessto the secretions, nor did they presence of Myzus exuviae. However, the degree approach the level of loading behaviour expressed to which the flocculent secretionsinfluenced larval by C. slossonae first instars. In contrast, late third choice of an initial behaviour varied between instars showed significant geographical variation populations. The effects of the secretions was ‘in the degree to which the flocculent secretions significant in three of four age classesof Ithaca influenced camouflaging behaviour. Moreover, in larvae and in two of four age classesof Monticello the presenceof the flocculent secretions, late third larvae; it was not significant for any age class in instars from the Ithaca population covered them- the Davis population. selvesto a degree approaching that of C slossonae larvae. The secretions also greatly increased the -expression of camouflaging by late third instars Cleaning from the Monticello population, but they had The type of aphid-derived material in the arena little effect on larvae from the Davis population. did not significantly influence cleaning behaviour, Among late first instars and early third instars, the nor was there a clear effect of population on level of larval behaviour, as well as the degree of cleaning. The frequency of cleaning showed sigto

flocculent

secretions

varied

interaction

terms involving

population

1400

Animal

Behaviour,

50,

5

Table VI. Priority that hungry, naked C. quadripuncfutu larvae gave to camouflaging (i.e. loading) versus feeding, when provided with prey and camouflaging material u/oCamouflaging prior to feeding 1st instar

Geographical population of

Type of camouflaging material

C. quadripunctata

Davis, CA

Myzus exuviae P. tesselatus

Monticello, FL

flocculence

Myzus exuviae P. tesselatus

Ithaca, NY

flocculence

Myzus exuviae P. tesselatus

flocculence

Myzus exuviae

C. slossonae

(Ithaca, NY) P. tesselatus

flocculence

Early (N)

Late WI

26.3ab 20.0”b (19) (20) 350”b 40.0b (20) (20) 12.5”

12.5”b

(16) 27.gab (18) 29.4”b (17) 7@6b (17)

(16) 26.3”b (19) 0.0” (20) 57.9b (19) 20.0 (15) 66.7 (15)

14.3

(14) 80.0 (15)

3rd instar Early m

Late (N)

0.0” (20) 20.0ab (20) 6.3” (16) 70.6b (17) 5.0a (20) 47.4b (19) 20.0 (15) 80.0 (15)

5.0” (20)

1O.O”b

(20) ;i; 37.5b (16) 0.0” (18) 58.9b (17) 20.0 (15) 86.7 (15)

Values within columns followed by the same letter are not significantly different (G-tests with simultaneous test procedure, P=O.O5). Data from C. slossonae are presented for comparison (not included in statistical analysis).

and population-age); there was also a significant family (population)-instar effect (Table III). As with the other traits, the population from Ithaca generally showed the greatest degree of similarity to C. slossonae. Instar and population-age were the only significant terms in the analysis of variance for the proportion of time spent cleaning.

(population-instar

Immobility

Age was the most prominent determinant of larval time spent in an immobile state; it appeared as a significant main effect and in several interaction terms, including population-age (Table III). In general, older larvae were less active than younger ones. The biological significance of the population-age interaction is unknown. DISCUSSION Individual

Repeatability

in Larval

1981). Behaviour

Both the degree to which individual C. quadrilarvae engage in camouflaging and the

punctata

priority that each gives to camouflaging vis-a-vis feeding were consistent throughout larval life (Table II). Given the large range of variation in the expression of these traits, the significant level of repeatability is remarkable on two accounts. First, the tendency to repeat persisted over two moults; second, it involved two prominent aspects of behaviour that affect larval interactions with their prey. Other researchershave demonstrated individual repeatability in the foraging and mating behaviour of insects (Sokowlowski 1986; Boake 1989), but to our knowledge this is the first experimental evidence for consistency in the larval behaviour of a predatory arthropod that is expressed across moults. When combined with the demonstration of genetic (e.g. geographical) variation, these findings indicate that the behavioural traits in question can be subject to natural selection at any time during the predator’s larval life (e.g. Falconer Behavioural

Variability

The pattern of variability between the larvae we observed is consistent in a number of ways with

Tuuber et al.: Variation

the proposal that the generalist C. quadripunctata harbours heritable variation for major behavioural traits that characterize larvae of the specialist, C. slossonae (see Milbrath et al. 1993; Tauber et al. 1993). First, the C. quadripunctuta larvae showed significant levels of geographical variation in feeding efficiency, camouflaging behaviour (i.e. loading), the priority they gave to feeding versus camouflaging, and the level of behavioural plasticity (responsiveness to flocculence from the specialist’s prey). Each of these traits is an important component of C. slossonae’s specialization (Tauber et al. 1993). Second, these traits vary quantitatively across C. quadripunctatu populations. That is, larvae from each of the populations express the full repertoire of behavioural traits, and they perform each of the activities in a similar manner, but they do so to significantly different degrees. Third, the pattern of geographical variation in C. quadripunctata appears to be related adaptively to geographically variable prey resources(seebelow). Such a pattern of variability indicates genetic variation that is under natural selection, and it supports our proposal that a C. quadripunctata-like general predator harboured heritable variation for traits that determine prey specialization in C. slossonae (Milbrath et al. 1993; Tauber et al. 1993). Adaptive sign$cance

The behavioural differences among the C. populations appear to represent adaptations to local environmental conditions, particularly to the characteristics of local prey. For example, of the three populations tested, larvae from Ithaca are associated with slightly larger prey (Table I) and they may be the most likely to encounter aggressive ants (M. J. Tauber & C. A. Tauber, unpublished data). They are also the most efficient feeders and the most prone to engage in loading behaviour. In contrast, Davis larvae, which are associated with relatively small-bodied prey under protective layers of spider webbing, are slow feeders and the least likely to engage in loading behaviour. The above correlation between prey type and larval behaviour within the generalist predator holds true when the specialist is included; the specialist’s prey are large and well guarded by ants, and their larvae are efficient feeders and

quadripunctatu

in Chrysopa hehaviour

1401

highly prone to camouflaging (Eisner et al. 1978: Milbrath et al. 1993). As a consequence, we infer that the type of prey normally encountered in nature can constitute a significant selective force on the behavioural repertoire of predatory larvae and that differences in prey resources can play a role in the differentiation of locally adapted predator populations. Our previous comparisons between C. slossonae and C quadripunctatu (Ithaca population) indicated that the size of larval head capsules and mouthparts may be a target of natural selection for feeding on large prey. For example, C. sfossonae first instars, with their relatively large heads and mouthparts, showed greater feeding efficiency than C. quadripunctatu first instars, but the third instars of the two species did not differ (Milbrath et al. 1993; Tauber et al. 1995). Our current results reveal that a similar relationship between size and feeding efficiency may also pert&n to the variation between populations of C’. quadripunctata. Variability in feeding efficiency was highly accentuated in first instars of C. quadripunctutu but was virtually absent in third instars, probably because first instars (but not third instars) are small relative to the green peach aphids used in our bioassay. In nature, where the size-range of prey varies considerably, we expect the feeding efficiency of all instars to be an important trait under natural selection. Behaviourul

plasticity

The low level of responsivenessto flocculence in the Davis population of C. quudripunctatu was unexpected and initially seemed anomalous. Although the Davis population is the only one of the three tested that normally associates with flocculence-secreting aphids, its larval behaviour was the least affected by the presenceof the woolly alder aphid’s flocculent secretions (Table Ill, Fig. I). At least two explanations for this apparent anomaly come to mind. First, Davis larvae may not perceive the woolly alder aphid’s flocculence as a potent chemical or physical stimulus for camouflaging (i.e. loading). Alternatively, Davis larvae may have evolved a generally lower propensity for loading behaviour than conspecific larvae from Ithaca or Monticello. For example. in their microhabitat (i.e. on leaves, amid flocculent prey and under a layer of spider webbing). loading

Animal

1402

Behaviour,

behaviour may not be advantageous. Indeed, under these conditions camouflaging behaviour (loading) and responsiveness to flocculence may be disadvantageous; for example, these activities may attract the attention of natural enemies, such as spiders, that respond to tactile or visual cues. In either case, our results hint at a genotype-environment interaction, and they are consistent with the proposal that behavioural plasticity (larval responsiveness to flocculence) is genetically variable and subject to natural selection. Finally, our findings above raise the question: why do Ithaca and Monticello larvae, which do not usually encounter flocculence-producing prey in nature, show any response to flocculence? Again, two explanations come to mind. First, occasional encounters with woolly alder aphids or other ant-tended flocculent prey may be sufficient to maintain a low level of responsiveness to flocculence in these populations. Woolly alder aphids occur throughout eastern and midwestern U.S.A. Indeed, C. quadripunctata larvae sometimes associate with woolly alder aphids on alder in Ithaca (Milbrath et al. 1994), and they may also encounter them on the aphid’s primary host, silver maple, during spring. Second, responsiveness to flocculence may be an ancestral trait that was maintained with little or no measurable benefit or cost. These two suggestions are testable, but not mutually exclusive.

ACKNOWLEDGMENTS We thank D. C. Wiernasz, University of Houston, D. R. Papaj, University of Arizona, W. T. Wcislo, Cornell University, and the anonymous referees for their thoughtful comments. We also thank C. E. McCulloch, T. C. Dorcey, W. K. NewsomStewart and S. J. Schwager (Cornell University) for assistance with the statistical analysis; M. B. Stoetzel (USDA-ARS, Beltsville) and W. L. Brown, Jr (Cornell University) for determinations and information on the aphids and ants, respectively; B. Gollands, G. S. Albuquerque, J. R. Ruberson (all of Cornell University), and R. F. Mizell III (University of Florida, Monticello) for their help; L. E. Ehler and M. Kinsey (University of California, Davis) for their cooperation. This work was supported, in part, by NSF grant BSR 88-17822 (M.J.T. and C.A.T.), Western Regional

50, 5

Project 84, and Hatch Project 408. We dedicate this paper to the memory of George C. Eickwort (1940-1994), colleague, friend and outstanding teacher.

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