Anlm. Behav., 1969, 17, 673-678

LOCOMOTORY BEHAVIOUR OF FIRST INSTAR LARVAE OF

APHIDOPHAGOUS SYRPHIDAE (DIPTERA) AFTER CONTACT

WITH APHIDS

BY A. E. F. CHANDLER* Agricultural Research Council Unit of Insect Physiology, Entomological Field Station, Cambridge

Aphidophagous Syrphidae are important natural enemies of aphids and the larvae are voracious feeders, often eating several hundred aphids each during their development. However, ovi- positing syrphid adults may lay their eggs at some distance from the nearest aphids (Chandler 1968) and, according to B/insch (1964), syrphid larval prey-locating systems are only poorly developed and aphids are first found by chance contact. The present study was therefore under- taken to investigate the distance syrphid larvae can travel before eating an aphid, and the locomotory changes consequent upon first con- tact with an aphid.

Methods Insects

Unfed first instar syrphid larvae were used throughout the experiments as their past experi- ence could be better standardized than that of second or third instars. They were obtained as follows: potted bean plants (Vicia faba L.) infested with Aphis fabae Scopoli were put in a cage of Syrphus balteatus adults for 2 hr at the same time each day. Eggs laid on these plants were then transferred to moist black filter papers in petri dishes and kept at 17 ~ to 21 ~ C. The larvae hatched 1.8 4-0-1 days later and lived for 2 to 2�89 days without food under these con- ditions. As newly-emerged larvae do not always attack the aphids they encounter, larvae approx- imately 24 hr old were used in the present experiments.

Cannibalism was a potentially serious problem in this species: one individual was reared through to a normal imago after the larva had been fed exclusively on syrphid eggs of which it consumed 414. Cannibalism was excluded as a factor in the present experiments by rearing in small groups and by rejecting large larvae and those in which the gut (visible through the integument) was coloured, indicating that in- gestion had occurred.

The aphids provided as prey during the

*Present address: Janssen Pharmaceutica, Beerse, Belt~iurrt.

experiments were adult, apterous Aphis fabae, reared on potted bean plants.

Procedure Movement of larvae was observed on an

arena of moist white filter paper, 25 • 20 cm. The filter paper rested on a sheet of glass marked with a 25-mm grid, which was visible through the paper and which allowed a separate trace of larval activity to be made by the observer on graph paper, without disturbing the larva.

The arena was confined within a large black- walled box and illuminated from below by a diffuse light source. When practicable, the arena was enclosed by a fine-mesh white terylene net, thereby providing internal light reflection and isolating outside shapes and movement from the larvae.

When larvae were transferred to the arena they usually continued to move whilst on the moist filter paper, but locomotion ceased after a few minutes or even seconds on sub- strates where the humidity was low.

Movement of the larvae on the arena was recorded continuously during each study period, each minute being marked off on the trace. The rate of forward locomotion (in cm per rain) was subsequently determined by running a map-measurer along the trace, and the rate of turn (in degrees per min) was determined by drawing tangents to the trace and measuring with a protractor the angles so formed.

The general experimental procedure was as follows: first, the larvae were placed on the arena and a 10-rain acclimatization period was allowed; secondly, the movement during the next (control) 5-min period was recorded; thirdly, the experimental situation was im- posed; finally, the movement of the larvae was recorded for a further (experimental) 10-miu period.

Aphid prey was typically consumed in 40 to 50 min by an unfed, 24-hr-old, first instar S. balteatus larva. However, the ingestion of a single aphid usually sated the larva, which then progressed directly awa)~ from 1;he r grt~[

~7~

674 A N I M A L B E H A V I O U R , 17, 4

showed little response to renewed aphid presen- tation. In the present experiments, aphid presen- tation for periods of time shorter than those permitting complete ingestion of the prey was adopted.

In preliminary trials, aphid prey was pre- sented in the path of a moving larva. On a non-veined leaf substratum, the aphids were apparently contacted by chance movement as found by B/insch (1964). However, on the moist filter paper substratum there was evidence of gustatory stimulation of the larva prior to actual aphid contact and modification of subsequent larval movements through the area. Presentation of the aphid on a small silver-foil disc prevented aphid-stimulation of the larva before actual aphid contact, but the disc itself interfered with the pattern of larval movement.

The method finally adopted was to lift the larva with a fine paintbrush and transfer it to an aphid on a separate moist filter paper. After the presentation period, the larva was returned to the centre of the arena, as also when it reached the periphery of the arena during an experiment. The effect of such mechanical interference on larval behaviour was previously found to be negligible, as described below. Experiments

1. Mechanical interference. After a 10-min acclimatization period, the movements of ten larvae were recorded during three 5-rain periods: after the first 5-min the larvae were gently returned to the centre of the arena, and after a further 5-min they were prodded and rolled over several times before being again returned to the centre.

2. Single aphid presentation period. Observa- tions were made on seventy larvae. Each larva

was presented with an aphid for an arbitrary period of 0-5, 1, 2, 5, 10, 15 and 30 min, ten larvae being studied per time interval. After a 10-min acclimatization period, the movement of each larva was recorded for 5 min before aphid presentation (period A) and for two consecutive 5-min periods afterwards (periods B and C).

3. Repeated aphid presentation. Ten larvae were studied separately during ten sequential observa- tion periods as follows: acclimatization (10 min), control (5 min), aphid presentation (0-5 min), experimental (5 min), resting (1 to 2 hr), acclimatization (10 min), control (5 min), aphid presentation (5 min) and experimental (5 min). For the 1 to 2 hr resting period, each larva was kept in a dry glass specimen tube to permit waning of the response to the first aphid presentation and to standardize as far as poss- ible (by minimizing) larval locomotion between the two experimental periods.

Results 1. Mechanical interference. Mechanical inter-

ference did not appreciably alter the turning behaviour (Table I). Moreover, the rate of turn was sufficiently constant over a period of time for large variations after experimental treatment to be considered as real changes in behaviour.

2. Single aphid presentation period. There was a highly significant increase in the mean turning rate of the larvae after the seventy aphid presen- tation periods (sixty-nine larvae increased their turning rate and one decreased it: the probability that such an extreme result could have occurred by chance is, according to binomial theory, less than 0.00001). Figure 1 shows that during period B the turning rate increased in propor- tion to the duration of aphid presentation up to a

Table I. Effect of Mechanical Interference on the Larval Turning Rate

Period studied

minute 0 - 1

1 - 2

2 - 3

3 - 4

4 - 5

0 - 5

Larval turning rate (degrees per min) after stated prior treatment (n = 10)

undisturbed moved gently moved roughly

88 :k 22 91 -j: 23 77 s 12

98 :t: 16 90 _4= 17 90 :t: 23

77 A: 15 76 :t_ 19 83 :t: 15

88 -4- 12 86 :k 16 83 :I: 14

85 4- 21 74 • 14 75 4- 14

87 :k 13 83 • 14 81 • 10

CHANDLER: LOCOMOTORY BEHAVIOUR OF SYRPHIDAE LARVAE 675

m ax i mum for the 5-rain contac t per iod, after which it progressively decreased. However , it always remained marked ly higher than dur ing pe r iod A. The turning ra te dur ing per iod C was lower for all aphid con tac t t imes than dur ing per iod B, bu t i t remained significantly higher than before aph id contact .

In add i t ion to the increased turn ing ra te

after aph id presenta t ion , there was a small bu t v e r y significant decrease in the mean speed of fo rward movement dur ing per iod B as compared with per iod A (Table H). The decrease in speed dur ing per iod B was so small that i t was not possible to de termine whether this var ied in re la t ion t o the dura t ion of aphid presenta t ion, bu t there was apparen t ly no reduct ion at all

250-

(u,

g E ~ ,_.,~

L - 0

I I J c ~

200-

150-

100-

50- Period C

~] Period A -I

1'0 1'5 2'0 25 Duration of aphid presentation [min)

Fig. 1. Increase in larval turning rate after aphid contact. Period A = 5 min immediately before aphid presentation, period B = 5 rain immediately after aphid presentation, and period C = 6 to 10 min after aphid presen- tation. Each point represents the mean value for ten larvae. Vertical lines show the standard errors of the means. A solid circle or square indicates that the mean is significantly different (P<0.05, Mann-Whitney U test, 2- tailed) from that preceding it on the plot, and an arrow tip represents a significant difference (P<0.05, Wilcoxon matched-pairs, signed-ranks test, 1-tailed) between the mean tO which it i~ attaqhe.d ~ocl the mean to which it point~,

i

3O

676 A N I M A L B E H A V I O U R , 17, 4

over the pre-contact level after the 30:min presentation period. Moreover, the mean de- crease in speed of the seventy larvae during period B was shortlived, the speed returning to approximately the pre-presentation level during the next 5 min (periOd C).

3. Repeated aphid presentation. The turning rate increased after all twenty aphid presenta- tions and, in nine of the ten larvae, the turning rate was higher during the 5 rain following the longer presentation period (P<0.05, Wilcoxon matched-pairs, signed-ranks test, 2-tailed). The mean turning rate was highest during the 1st min after both the 0.5-min and 5-min presen- tation periods, whereafter it declined progress- ively with time (Table III).

The effects of aphid contact on the turning rates studied in this experiment were closely similar to those obtained in the previous experi- ment (Table III , last line).

Discussion Of the ninety aphid presentations considered in this study, eighty-nine evoked an increase in the turning rate of unfed, first instar Syrphus

balteatus larvae. Evidence for increased turning was obtained by two independent methods: parallel experiments on different larvae (Fig. 1) and sequential experiments on the same larvae (Table III).

Increased turning after a successful encounter is a feature common to other aphid predators - - Coccinellidae and Chrysopidae (Fleschner 1950; Banks 1957; B~insch 1964, 1966). However, Bgnsch (1964) found that f o r S y r p h i d a e the searching behaviour of hungry or sated larvae differed only in the speed of movement, and that once a direction had been adopted the larvae rarely deviated from it.

The difference from the present findings may be explained by our study of different species and different instars: B~insch reported primarily upon the behaviour of third instar Scaeva pyrastri (L.) larvae, whereas the present study was~ made upon first instar Syrphus balteatus larvae, I t seems likely that the eating of a single a p h i s is a more important event to the latter, as first instar larvae eat fewer aphids per day than third instars, and as Seaeva species have

Table H. Effect of Aphid Contact on the Speed of Larval Movement

Duration of aphid contact

(min)

Speed of larval movement (degrees per min)

Period A Period B Period C

0"5 (n=10) 2.8 • 0.1 2.6 • 0.2 2"7 4- 0.2

1 (n=10) 3'1 4- 0.1 2"7 -V 0"1 3'1 • 0"1

2 (n=10) 2-9 • 0-2 2-5 • 0.2 2.8 • 0.2

5 (n=10) 3.0 • 0.3 2"7 4- 0"2 3"0 • 0"3

10 (n=10) 2-7 i 0"2 2'2 • 0.2 2"4 ~ 0"3

15 (n=10) 3"1 4- 0.2 2"6 4- 0.2 2.7 4- 0"3

30 (n=lO) 3'0 4- 0.2 3.0 • 0.2 3"1 4- 0.3

Mean (n=70) 2.9 • 0'1 2.6 4- 0'1 2'8 4- 0.1

Comparison A to B B to C A to C

change in mean 12 ~ -t- 9 ~ - 3

Speed unchanged 5 6 6

increased 12 50 27

decreased 53 16 37

Probability* <0.0001 <0.0001 0.13

*Probability according to binomial theory of obtaining as extrenle ~, ~lis, tribution bY chanr

CHANDLER: LOCOMOTORY BEHAVIOUR OF SYRPHIDAE LARVAE

Table HI. Effect of Aphid Contact on the Larval Turning Rate

677

Experiment (see text)

Mean rate of turn in degrees per min (n=10)

Period �89 presentation studied before after

5-min presentation before after

minute 0-1 62:5 8 315 4- 31

1-2 60:5 14 249 ~ 44

2-3 61:5 10 175:5 30

3-4 62:5 18 142:5 19

4-5 27:5 5 114:5 15

57 • 12 594 • 61

59 • 11 309:5 46

49:5 11 292:5 47

70:5 11 228 -t- 28

71:5 14 196:5 27

0-5 54:5 7 199 ~ 26

0-5 57 ~ 8 213 ::h 30

61 -4- 7 324 ~ 28

60 • 9 287 • 27

particularly voracious larvae even for Syrphidae (Fluke 1929; Yakhontov 1966).

A further factor is the tendency of syrphid larvae to 'cast ' for their prey, i.e. to lift up the anterior portion of the body from the substratum and to strike out forwards or laterally without progression of the hinder part of the body. Casting occurs in all instars, but is likely to be more immediately effective in the third instars because of their greater size. Perhaps, therefore, it also occurs more frequently in third than first instars. In the present study casting was not investigated in detail, as the length of cast varied and could not conveniently be quantified for recording at the same time as forward progression which occurred far more frequently.

Casting was most apparent immediately after removal of the aphid stimulus and it was probably the main reason for the lower speed of forward movement at this time (Table II). The similarity of the speed during period A and period C (when casting occurred much less frequently) supports this view. It therefore seems likely that, f rom the prey-finding stand- point, the temporary reduction in the mean speed of forward travel was compensated for by broader scanning of the area through which the larva was moving.

Not only was the reduction in speed tempor- ary, it was also small--only 12 per cent lower than during period A and 9 per cent lower than during period C (Table II). Thus, although the turning response is expressed in the present paper as degrees per unit time rather than per unit distance, the conclusions to be drawn concerning the larval turning behaviour and its spatial significance are substantially the same

by whichever method the response is expressed. It seems probable that the three components

of the locomotory behaviour studied after aphid contact--slowing, turning and scann ing ,a l l helped to concentrate prey-searching in areas where aphids were most likely to be encountered.

After contact with an aphid, syrphid pro- gression usually ceased and the mouthparts began rasping or piercing, with or without striking movements of the head. Penetration and ingestion were typically evident after 2.5 to 4 min, although the moment of first intake of aphid contents was difficult to determine. There was clearly, however, increased turning after contact with an aphid even when this period of contact was of insufficient duration for any ingestion to occur. Indeed, the briefest of encounters (0-5 min) gave a marked rise in turning rate and one that was significantly higher than after a 30-min presentation ( P < 0.01, Mann-Whitney U test, 1-tailed).

The following tentative hypothesis is put forward to account for these observations, although other factors are doubtless involved: increased sensory contact with an aphid, without the intervention of ingestion activity, promotes increased turning movements by hungry syrphid larvae, but the action of ingestion progressively suppresses this turning behaviour. For a pre- viously unfed first instar Syrphus balteatus larva, ingestion of the body contents of one adult Aphis fabae is sufficient to suppress it more or less completely and there is no evident slowing or increase in turning in response to immediate presentation of another aphid.

Unfed first instar syrphid larvae were able to travel considerable distances, certainly well in

678 A N I M A L B E H A V I O U R , 17, 4

excess of 1 m, under suitable conditions. Al- though this potentiality may not always be realized in the field, it should not be overlooked when assessing the true importance of the proximity of syrphid oviposition to aphid colonies2

Summary The locomotory behaviour of aphidophagous syrphid larvae was studied on 24-hr-old larvae of Syrphus balteatus (Degeer). The unfed larvae moved ~tlmost continuously on a moist filter paper arena and were able to travel distances well in excess of 1 m. Their rate of turn per min- ute was approximately constant in the absence of aphids, even after limited mechanical inter- ference.

Aphids (adult, apterous Aphis fabae Scopoli) were presented individually to eighty larvae for periods of time less than those permitting com- plete ingestion of the contents of the aphid. After loss of contact with the aphid, the larval turning rate increased with the duration of previous aph id presentation up to 5 min, beyond which it progressively declined. I t was more than doubled after a 0.5-min aphid presen- tation period during which no ingestion occurred. The speed of forward movement was lower immediately after the period of aphid presen- tation than immediately before it, and this slowing was apparently associated with greater lateral 'casting' by the larvae. These three com- ponents of the locomotory behaviour after aphid contact--slowing, turning and scanning--

probably all help to concentrate prey-searching in areas where aphids are most likely to be encountered.

Acknowledgments It is a pleasure to thank Professor J. S. Kennedy, F.R.S. for making this study possible and for his help, interest and stimulating discussion throughout the project. Thanks are also due to Professor Sir Vincent Wigglesworth, F.R.S. and the Agricultural Research Council for the provision of facilities at the Entomological Field Station, Cambridge.

REFE~RENCES Banks, C. J. (1957). The behaviour of individual coccinel-

lid larvae on plants. Br. J. Anita. Behav., 5, 12-24. Bansch, R. (1964). Vergleichende Untersuchungen zur

Biologie und zum Beutefangverhalten aphidivorer Coccinelliden, Chrysopiden und Syrphiden. ZooL Jb. Syst., 91, 271-340.

B~insch, R. (1966). On prey-seeking behaviour of aphid- ophagous insects. EcoL aphidoph, lnsects, Proc. Syrup. Prague, 1965, t13-115.

Chandler, A. E. F. (1968). Some factors influencing the occurrence and site o f oviposition by aphido- phagous Syrphidae (Diptera). Annls appL Biol., 61, 435--446.

Fleschner, C. A. (1950). Studies on searching capacity of the larvae of three predators of the citrus red mite. Hilgardia, 20, 233-265.

Fluke, C. L. (1929). The known predacious and parasitic enemies of the pea aphid in North America. Bull. Wis. agric. Exp. S(n, 93, 1-47.

Yakhontov, V. V. (1966). Coccinellidae and Surphidae as predators of aphids in Uzbekistan. Ecol. aphidoph. Insects, Proc. Symp. Prague, 1965, 267-269.

(Received 19 December 1968 ; revised 14 June 1969; Ms. number: 865)