Ent. exp. & appl. 13 (1970). 194---207. North-Holland Publishing Co., Amsterdam

T H E E F F E C T I V E N E S S O F A N T HOCORIS N E M O R U M A N D

A. CONFUSUS ( H E M I P T E R A : A N T H O C O R I D A E ) AS P R E D A T O R S

O F T H E S Y C A M O R E A P H I D , DREPANOSIPHUM P L A T A N O I D E S

1 T H E N U M B E R OF A P H I D S C O N S U M E D D U R I N G D E V E L O P M E N T

uY

R. J. RUSSEL Zoologisk institut, Aarhus Universitet, Aarhus, Denmark

Nymphs of Anthocoris nemorum and Anthocoris confasus were reared in the laboratory, using the sycamore aphid Drepanosiphum platanoides as food. Most of the food required for development is consumed during the last three instars. Individuals which developed in the shortest time consumed least food during development, and weighed least at maturation. Comparison of the weights of field and laboratory reared animals indicates that they consume similar amounts of food during development, and the number of sycamore aphids required for development in the field is estimated. It is suggested that under natural conditions, the number of anthocorids which reach maturity is dependent upon the number of young aphids available during the early stages of the anthocorids' development.

Anthocorids are predatory fluid-feeding insects which live on a wide variety of prey. Hill (1957, 1961, 1965, 1968) has described the biology of anthocorids in Scotland, where the present work took place. The work reported here is part of a study of the effect of predation of the anthocorids Anthocoris nemorum (L.) and Anthocoris confusus Reuter on populations of the sycamore aphid, Drepanosiphum platanoides (Schr.), which occurs on sycamore, Acer pseudoplatanus (L.). From the results of this work, an estimate is made of the quantity of sycamore aphids killed by an anthocorid during its development under natural conditions. The effect of temperature on the rate of development of anthocorids is also examined.

MATERIALS AND METHODS

Nymphs of A. nemorum were reared individually in cages similar to those described by Anderson (1962a). Humidity in the rearing areas was high, as the cages were kept in clear plastic boxes with moistened foam plastic floors. The smaller, more delicate nymphs of A. confusus were reared individually in gelatine capsules 1.6 X 0.5 cm in size, and only transferred to rearing cages after completing the second instar. All anthocorids were fed on sycamore aphid nymphs, and kept in incubators illuminated with fluorescent lights giving a photoperiod of 15 hrs. A. nemorum was reared at 14 ° and 24 ° C, whereas A. confusus was only reared

at 24 °

ANTHOCORIS S I P . AS APHID PREDATORS 195

The amount of food consumed by the anthocorid nymphs was measured as follows: every feeding-day, similar sized aphid nymphs were collected from a green- house culture and weighed, and the average weight of a live aphid found. These aphids were then fed to the anthocorid nymphs. The rearing cages were examined daily, and each aphid which had been killed was recorded. These aphids were then weighed, and their average weight found. As the high humidity in the rearing cages prevented the killed aphids from losing much water by evaporation, no allowance was made for water loss from aphid remains. The average fresh weight of the aphid tissue consumed by an anthocorid was then obtained by subtracting the average weight of an aphid which had been killed from the average weight of a five aphid, determined on the previous feeding-day. The exuviae of anthocorids were retained after moulting, and the post-moult weight of each nymph recorded. Initially the anthocorids were fed every day, but later this was altered to every second day, at which frequency not even the largest anthocorids consumed all the food they received. Thus the amount of food provided each time was considered to be adequate. The above procedure enabled the determination of the growth rate, i.e. increase in live weight, of each anthocorid during each instar, and the weight of the resulting teneral adult, together with the duration of each instar and the average amount of aphid tissue consumed during it. The method was used to measure the food consumption of A. nemorum nymphs reared at 14 ° and A. confusus nymphs reared at 24 ° .

The experiment was repeated to obtain values of the actual amounts of aphid tissue consumed by each anthocorid, and also to determine the effect of a higher temperature during rearing upon development. One group of A. nemorum was reared at 14 °, and a second group at 24 °. Both groups were fed daily, and the weight recorded of each anthocorid, of the live aphids given to it as food, and of the aphids killed by it since the previous day.

Anthocorids were also collected in the field, to enable comparison of the weights of animals developing under natural conditions with those of laboratory reared animals. Collection took place once a week, or more often in favourable weather, by beating sycamore trees in a mixed deciduous wood at Drymen, Stirlingshire (grid reference 472874). The instar and weight of each anthocorid were determined on the day of collection.

R E S U L T S

Food consumption, development time, and weight at maturity Mean values of food consumption, development time, and weight of teneral

adult were determined from the results for anthocorids of each sex of each species which were reared in the laboratory from eclosion to maturity (Table I).

For A. nemorum, individuals of either sex reared at 14 ° are significantly heavier than those of the same sex reared at 24 °. Only in males is this associated with a significantly higher food consumption during development at the lower temperature.

196 R . J . R U S S E L

1.4

1.2

1.0

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t"-

. _

~ 0"

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Q -

E

x 0.4

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IV

~. t, A. con fusus , 24*

o .... o A. nemorum, 24*

; :. A. nemorum, 14"

0 10 20 30 40 50

T i m e ( d a y s )

Fig. 1. The mean increase in the live weight of an anthocorid, reared from eclosion to maturation in the laboratory (Instars denoted by Roman numerals)

ANTHOCORIS SPP. AS APHID PREDATORS 197

Females are significantly heavier at maturity than males reared at the same temperature, although they do not consume significantly more food during develop- ment than males. There is a significant difference between the development times of individuals of the same sex reared at different temperatures, but none between individuals of opposite sexes reared at the same temperature.

A comparison of the mean values of food consumption, development time at 24 °, and weight of teneral adults, obtained for each sex of A. confusus, shows that the difference between the amounts of food consumed by males and females during development is significant, although the difference between development times of males and females is not. The difference between weights of teneral males and females is significant.

The weight of a nymph prior to feeding at the beginning of each instar is shown in Fig. 1, for both A. nemorum and A. confusus. The lines join points which are mean values of the results for all individuals which completed a given instar: this allowed the use of more results than would have been possible if only results from those individuals which were reared from eclosion to maturity were used. No differentiation for sex was made in deriving the mean values.

Nymphs of A. confusus develop more rapidly to maturity at 24 ° than nymphs of A. nemorum at the same temperature, but their mean weight at maturity is less. Nymphs of A. nemorum develop much more rapidly at 24 ° than at 14 °, but also their mean weight at maturity is less than that of the slower developing nymphs.

The mean food consumption of a nymph in each instar during the development of A. confusus and A. nemorum is shown in Fig. 2. The mean values for each instar are derived from results for each individual which completed the relevant instar, and no differentiation was made for sex. The mean values of the total amounts of food consumed during development are in the same order as the mean values of the development time of anthocorids and the weight of teneral adults (Fig. 1), that is, individuals which consume the least amount of food in the shortest period are lightest at maturity (and vice versa).

The amount o[ food consumed during each instar

For nymphal instars of A. nemorum and A. con[usus, the mean value of the amount of food consumed in an instar (cf. Fig. 2) was calculated as a percentage of the mean value of the total amount of food consumed during development.

Although nymphs of A. nemorum reared at 24 ° develop 1.9 times as fast as those reared at 14 °, and consume a smaller quantity of food during development, similar proportions of the total amount of food required during development are consumed during corresponding instars at each temperature. Also, during any instar, nymphs of A. confusus consume a similar proportion of the total food required during development to that consumed during the corresponding instar of A. nemorum at 24 ° or 14 °. Of the total food required during the development of an anthocorid, about 35% is consumed in the 3rd and 4th instar, and 50% in the fifth instar alone.

198

10'0 1

R . J. R U S S E L

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09 O~

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E

A. confusus, 240

A. nemorum, 240

A. nemorum, 140

I 1 I I 0 10 20 30 40 50

Time (days)

Fig. 2. The mean amount of aphid tissue consumed during each instar of an anthocorid, reared from eclosion to maturation in the laboratory (Instars denoted by Roman numerals).

ANTHOCORIS SPP. AS APHID PREDATORS 199

The quantity of tissue removed from the prey

Anthocorids are fluid feeders, sucking out the contents of their prey to leave an empty skeleton. For nymphs of A. nemorum reared'at 24 °, the weight of each aphid was recorded when provided as prey and again when killed, and it was possible to determine a mean value of the percentage of the live weight of an aphid removed by a given instar of .4. nemorum during feeding. On average, 75% of the live weight of an aphid is removed by a nymph during feeding, when the food supply is adequate.

The relationship of the increase in weight of an anthocorid to the food consumed during development

It was observed earlier that the lightest adults at maturation had also consumed the least food during development. Fewkes (1960) used the term 'conversion ratio' to describe the ratio of the increase in the live weight of a predator to the fresh weight of the food consumed by it. This ratio can be determined for anthocorids, using the results from feeding studies in the present work, as is explained below. The weight increase of an anthocorid during development is the difference between its weight at eclosion and maturation (prior to feeding, in each case), plus the weight of the five exuviae shed during development. The mean weight of a nymph at eclosion is 0.05 mg (n----24) for A. nemorum, and 0.04 mg (n = 7) for .4. confusus, and the mean weight of the exuviae shed is 0.06 mg (n ---- 10) for A. nemorum and 0.04 nag (n = 5) for .4. confusus. The weight increase of A. nemorum and .4. confusus during development is calculated by using the above data in conjunction with the weights of teneral adults given in Table I, and the mean values of food consumed given in the same table are used together with the values of weight increase during development to determine conversion ratios (Table I).

Within the temperature range 14°--24 ° the conversion ratio of .4. nemorum is 0.18. The method of measuring the food consumption of A. nemorum at 24 ° was more accurate in approach than the average weight method used for A. nemorum at 14 ° and .4. confusus at 24 ° but much more time-consuming. In view of the fact that no significant difference was found (P > 0.05), either between the mean conversion ratios of males reared at each temperature, of females reared at each temperature, or of males and females reared at the same temperature, it appears that the method used of recording average weights of live and killed aphids gives sufficiently reliable results on which to base estimates of the food consumption of anthocorids. The conversion ratio of ,4. confusus, the instars of which are smaller than the corresponding instars of `4. nemorum, is 0.16 at 24 °.

The conversion ratio of a predator is not a suitable term for use when comparing the development of different predators on various prey, as the water contents of the animals compared may be dissimilar, and have an unequal effect on the live weight values involved. The growth efficiency of a predator (increase in dry weight of predator X 100 divided by the dry weight of the prey minus the

200 R . . I . R U S S E L

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ANTHOCORIS SPP. AS APHID PREDATORS 201

dry weight of the food remains) is a better term to use for the comparison of the conversion of prey by predators into tissue, as the effect of dissimilar water contents is eliminated by using dry weights only. An estimate of the growth efficiency of A. n e m o r u m is made in the following way. During feeding-experi- ments, five females of ,4. n e m o r u m reared from eclosion to maturation at 24 ° each consumed an average weight of 10.31 mg of live aphids, and the average increase in weight of the females during development was 1.41 mg. Experiments with D. pla-

tanoides showed that the average dry matter content of first- to third-instar aphids is 24.0% of the live weight (Russel 1968), and of first- to third-instar aphids killed by adult anthocorids is 2.7%. The dry weight to live weight ratio of ,4. n e m o r u m

nymphs was calculated from weighings of 10 individuals of each of the third, fourth, and fifth instars, and for these three instars, the average value of the dry weight was found to be 33.5% of the live weight.

Much of the weight increase during development occurs in these instars (Fig. 1), and the calculation of the dry weight of the total weight increase is made using the value for the percentage of dry matter found in nymphs of these three instars. As- suming that the percentage dry weight of the prey killed by nymphs is the same as that of prey killed by adult anthocorids, the growth efficiency of A. n e m o r u m

during development at 24 ° is estimated as:

0.335 X 1.41 X 100

(0.240 X 10.31) - - (0.027 X 10.31) = 21.5%

The relationship o f the deve lopmen t t ime o f A. nemorum to temperature

It is well-known that the rate of development of insects is affected by temper- ature. Therefore the significant difference between the duration of development of individuals of A . n e m o r u m reared at 14 ° and 24 ° (Table I) is not unexpected. However, the nymphs of A . n e m o r u m spend similar percentages of their total duration of development in a given instar, whether development takes place at 14 ° or 24 ° (Table II). The percentages are based on mean values of results for all individuals which completed the instar concerned, and no differentiation is made for sex. For nymphs of A. confusus, the percentage of the duration of development

TABLE II

The percentage o/ the duration ol development of A. nemorum and A. confusus spent in a given instar

Species Temp. Percentage of duration of development per instar Instar

1 2 3 4 5

A. nemorum 14 ° 18.1 12.2 13.4 19.9 36.7 24 ° 16.0 10.0 13.0 22.6 36.3

A. confusus 24 ° 21.6 12.2 14.1 16.9 34.5

202 R. J, RUSSEL

from eclosion until maturation which is spent in a given instar (Table II) is similar to that spent in the corresponding instar by nymphs of A. nemorum.

T A B L E I I I

The occurrence of each instar of A. nemorum on, and weights of individuals of successive instars from sycamores at Drymen in 1966

Instar n Mean First Last weight Range appearance Appearance (rag)

1 23 0.08 0.05--0.11 5 June 11 July 2 34 0.18 0.11--0.25 9 June 21 July 3 30 0.32 0.15--0.60 20 June 1 August 4 62 0.72 0.35--1.05 1 July 17 August 5 80 1.25 0.85--1.80 4 July 16 Sept.

Male 82 1.51 1.05--1.80 21 July 12 Oct. Female 36 1.97 1.35--2.60 21 July 12 Oct.

The weight of anthocorids in the field

Anthocorids were collected in the field to determine the average weight of in- dividuals of each instar developing under natural conditions. A. confusus was not present in sufficient numbers on sycamores at Drymen in 1966 or 1967 to give information on the average weight of each instar of: this species. The average

weight of each instar of A. nemorum in the field in 1966, and the approximate duration of occurrence of each instar, are given in Table III. Occasionally teneral animals, identifiable by their bright red colouration, were collected. They were kept in a humid environment, and not weighed until the cuticle had hardened and darkened. The average weights of teneral individuals from the field and laboratory rearing study are compared in Table IV. No significant difference (P > 0.05) was found between the weights of teneral males and females from the field, but since both the results of Anderson (1962a) and the present study (Table I) indicate that females are significantly heavier than males at maturation, the unexpectedly high weight of teneral males in the field is considered to be due to natural variation in

T A B L E IV

The average weights of teneral specimens of A. nemorum collected from Drymen in 1966, and of teneral individuals of corresponding instars reared in the laboratory at 14 °

Instar

1 •

2 3 4 5

Male Female

Field

0.12 0.24 0.44 0.91 1.55

1.65

Weight of teneral specimens (mg) n Laboratory n

- - 0.05 24 2 0.14 24 5 0.26 25 6 0.51 25

13 0.96 14 13 1.35 6

8 1.64 6

ANTHOCORIS SPP. AS APHID PREDATORS 203

the sample collected. With the exception of the weight of teneral males, the weights of teneral individuals of field and laboratory reared animals are otherwise similar, and from this it is considered that anthocorids developing in the field consume a quantity of food during their development similar to that consumed by anthocorids reared in the laboratory at 14 ° .

The quantity of aphids consumed by an anthocorid during its development

The results of the foregoing work on the growth of anthocorids in the laboratory can be used in conjunction with the data on the weight of anthocorids in the field, to estimate the number of sycamore aphids required for the development of an anthocorid in the field. Firstly, the average weight increase of an anthocorid is determined from the average weight of teneral individuals in the field at maturity, as explained on p. 199. By assuming that the conversion ratio of anthocorids reared in the laboratory and in the field is the same, the amount of aphid tissue consumed during development can be calculated by dividing the weight increase by the con- version ratio. Finally, making the further assumption that anthocorids in the field consume the same percentage of the live weight of their prey as anthocorids reared in the laboratory, the live weight of aphids required during development in the field is calculated, and converted to the nearest whole number of aphids of known weight. Since all instars of anthocorid are only capable of capturing small sycamore aphids (Russel, 1968), the weight of aphids required is expressed as a number of first- and second-instar aphids.

For A. nemorum, the average weight of a teneral adult in the field in 1966 was 1.60 mg (Table IV). Such an anthocorid is estimated as outlined above to have consumed 8.94 mg of aphid tissue, during its development. Since an average of 75% of the live weight of an aphid is removed by a nymph of A. nemorum on feeding, (cf. p. 199), the estimated weight of aphids required during development is there- fore 11.92 mg of live aphids. Dixon (1966) has shown that the average weight of individuals of a given instar of the sycamore aphid varies during the year, and thus aphid weights measured over a short period are not likely to be representative of the average aphid weight over the whole season. Bearing this in mind, the average weight of first-instar sycamore aphids in May was found to be 0.12 mg (n = 27), and of second instars, 0.24 mg (n-----25). Thus a teneral adult of A. nemorum weighing 1.60 mg is estimated to have killed 99 first-instar or 50 second-instar sycamore aphids, during its development.

A. confusus was only reared in the laboratory at 24 °, and the number of aphid nymphs required during development in the field is estimated indirectly, by drawing certain parallels with the results obtained for A. nemorum. Firstly, it is assumed that individuals of A. conlusus reared in the laboratory at 14 ° are 1.2 times heavier than those reared at 24 °, as was found for A. nemorum (Table I). Secondly, it is assumed as for A. nemorum that the weights of teneral adults of A. conlusus reared at 14 ° are similar to those of individuals which develop in the field. Finally, as the conversion ratio of A. ,nemorum did not vary significantly within the range 14°--24 °

2 0 4 R . J . RUSSEL

(Table I), the same is assumed to be true for the conversion ratio of A. con[usus, which was found to be 0.16 at 24 ° . On the basis of the above assumptions, indi- viduals of A. confusus developing in the field are estimated to weigh 1.16 mg on reaching maturity, and to have consumed 7.33 mg of aphid tissue during develop- ment. On average, nymphs of A. confusus reared in the laboratory consumed 88% of the live weight of their prey, and the live weight of aphids required during development is estimated as 8.33 mg. This weight represents 69 first-instar or 35 second-instar sycamore aphid nymphs.

D I S C U S S I O N

The effectiveness of aphid predators has been visualised by van Emden (1966) as the result of the interplay of three major factors: the voracity of the predators, the rate of reproduction of their prey, and the synchronisation of the predator and prey populations. Van Emden also points out that as effectiveness is judged on different criteria in different situations, it is perhaps best considered as a scalar concept. Each of the major factors mentioned is a composite of other minor factors: for instance, amongst other things, the voracity of a predator is affected by its mobility and by the number of prey it can consume. Estimates of the number of sycamore aphids required for the development of anthocorids indicate that their food requirements are small, compared with those of larger predators. As a result, it may be supposed that anthocorids can survive at lower prey population densities than larger predators. However, for reasons explained below, it is queried that predation by anthocorids can have the effect of restraining the numbers of their prey population.

A. nemorum is polyphagous, and the number of overwintered females occurring in the spring is therefore independent of the reproductive success on any one prey species in the previous year. The overwintered A. nemorum females disperse in the spring, (Anderson, 1962b) and are to be found on various plants, their occurrence being largely determined by the presence or absence of prey species (Hill, 1957). One such prey species is the sycamore aphid, which in the Glasgow area first appears in April, and rapidly increases in numbers during the following weeks (Dixon, 1963, 1969). During May 1966, overwintered females of A. nemorum, A. conJusus, and Anthocoris nemoralis (Fab.) were found on sycamores at Drymen, and the presence of anthocorid eggs on the leaves showed that these females were able to find enough small aphids for oviposition to take place. The average monthly temperature for May and June is about 14 ° in the Glasgow area, and thus in the field anthocorid eggs take about 16 days to develop (Russel, 1968), a further 53 days being required for nymphs to reach maturity at this temperature (Table I). There is only one generation of A. nemorum each year in Scotland (Hill 1957), in contrast to at least two in southern England (Southwood & Scudder, 1956). Whereas the overwintered females are able to fly from plant to plant in search of prey and oviposition sites, the nymphs which subsequently hatch from their eggs are much

ANTHOCORIS SPP. AS APHID PREDATORS 205

less mobile, being confined by their poor mobility to the plant on which eggs were laid, and their choice of prey is restricted to that available on that particular plant.

Dixon (1963) has shown that the sycamore aphid undergoes a summer reproduc- tive diapause in June and part of July, during which time the number of aphid nymphs drops to a very low level. This event is important, as it is the small aphid nymphs which form the food supply of the anthocorids (cf. p. 203). Thus an abundance of small aphids during the oviposition period of the overwintered female anthocorids changes to a scarcity during much of the development period of the anthocorid nymphs of the following generation. Under these conditions, it is possible that the earlier instars of the anthocorids have difficulty in finding enough food, and that the number which survive to maturity depends upon the number of young aphids present in the aphid population. Consequently, the numbers of sycamore aphids will not be regulated by the anthocorid population: on the con- trary, the success of the anthocorid population will be dependent upon the numbers of their small aphid prey, during the reproductive diapause of the adult aphids. Dempster (1968) gives evidence for the same conclusion regarding the predation of Anthocoris sarothamni on the psyllid drytaina spartff.

In May 1966, three species of anthocorid were found on sycamores at Drymen, but later in the year only adults of A. nemorum were found. The absence of new adults of the other two species is explainable by postulating that a poor food supply, combined with inter-specific differences in the ability of anthocorids to seek for and capture their prey, prevented the successful reproduction of A. confusus and A. nemoralis on sycamore that year. I intend to describe the searching behaviour of A. nemorum and A. confusus, and their efficiency of capture of sycamore aphids, in a future paper.

The selection of prey by anthocorids

Anderson (1962a) demonstrated that the food requirements o~ some anthocorids may be more specific than previously believed. He also showed that individuals of A. con[usus reared on three species of unnatural prey had a higher growth rate index and lower mortality rate than individuals of A. nemorum reared on the same prey species, which are their natural prey. In the present work, A. con[usus and A. nemorum were reared on D. platanoides, which is a natural prey of both species. The growth rate index and survival of A. confusus reared on this prey were superior to those of A. nemorum.

A. nemorum feeds on a very wide variety of prey (Collyer, 1967), whereas A. confusus mainly feeds on callaphidid aphids (Anderson, 1962b), Some other species of anthocorid also show a tendency towards specialised feeding on a few prey species, an extreme example being Anthocoris gaIlarum-ulmi, which as a nymph feeds exclusively on the aphid Eriosoma ulmi, although Anderson (1962a) success- fully reared it on other prey. The tendency towards specialisation in feeding may be coincident with an increased ability of the anthocorid species concerned to thrive on their prey, compared with A. nemorum which has remained a more general

206 R.J. RUSSEL

predator, F o r A . n e m o r u m , a possible lower developmental success on some prey

is offset by an ability to develop on m a n y different species of prey, a factor which

contr ibutes to the wide dis tr ibut ion of this species.

Par t of a Ph.D. thesis accepted by the Universi ty of Glasgow, and done at the

Universi ty Field Station, where facilities were provided by Pro~essor D. R. Newth.

Professor H. M. Thamdrup , Zoologisk Insti tut , Arhus , gave facilities to complete

the thesis. The work was supported by an S.R.C. Research Studentship, from

1965--1968. I thank Dr. A. F. G. Dixon for supervising the work, providing advice

and encouragement, and criticising the manuscript . The help of Dr. A. R. Hill is

also appreciated.

ZUSAMMENFASSUNG

DIE W 1 R K S A M K E I T V O N ANTHOCORIS NEMORUM UND ANTHOCORIS CONFUSUS ( H E M I P T E R A : A N T H O C O R I D A E ) A L S R A U B F E 1 N D E D E R B L A T T L A U S

DREPANOSIPHUM PLATANOIDES. 1. DIE A N Z A H L DER BLATTLA'USE, DIE W X H R E N D D E R E N T W 1 C K L U N G V E R Z E H R T W E R D E N

Anthocoris nemorum und Anthocoris canfusus sind Raubinsekten, u.a. ffir die Blattlaus Drepanosiphum platanoides. Als Teil einer Untersuchung fiber die Wirkung dieser Prgdatoren auf die Populationen dieser Blattlaus, wurde in Laborexperimenten die Anzahl der w'fihrend der Entwicklung verzehrten Blatff~iuse ermittelt. Die Larven wurden im Laboratorium mit Blattl~iusen geftittert, A. nemorum bei 24 ° und 14 °, tun die Einwirkung der Temperatur auf die Entwicklung zu untersuchen, und A. confusus nut bei 24 °.

Bei 24 ° entwickelte sich A. confusus schneller, verbrauchte weniger Nahrung und wog weniger als A. nemorum. Auch A. nemorum entwickelte sich bei 24 ° schneller, verbrauchte weniger, und wog bei der Reife weniger als Exemplare der Art, die bei 14 ° aufgewachsen waren. Die Urnsatzrate der Blattl~iuse betrug fiir A. nemorum sowohl bei 24 ° wie bei 14 ° 0.18, und flu" ,4. confusus bei 24 ° 0.16.

Ein Vergleich der Gewichte der Anthocoriden nach Aufzucht im Laboratorium bzw. ira Freiland zeigt, daB diese Insekten im Freiland quantitativ eine ~hnliche Nahrungsaufnahme wie im Laboratorium haben. Unter Verwendung der gefundenen Umsatzraten werden Ober- schlagsberechnungert fiber das ffir die Entwieklung der Anthocoriden bis zur Reife ben~itigte Blattlaus-Lebendgewicht aufgestellt. In der Natur werden nur kleine Blatff~inse angegriffen; und das ffar A. nemorum erforderliche Gesamt-Lebendgewicht der Beute wird auf 99 Blatt- liiuse des ersten oder 50 des zweiten Larvenstadiums gesch~itzt; und flit A. confusus ent- sprechend 69 bzw. 35.

Dixon (1963, 1966, 1969) hat gezeigt, dab die Anzahl der Ahorn-Blattl~iuse schon im Mai grol3 ist, jedoch im Juni stark abnimmt, da die Erwachsenen dann in eine reproduktive Diapause eintreten. Es wird bier verrnutet, dab die Anzahl yon iiberlebenden Anthocoriden yon der Anzahl der kleinen Blattl~iuse des friihesten Sommers abh~ngig ist. Die Beuteauswahl der Anthocoriden wird im Hinblick auf die Spezialisierung ihrer Nahrungsgewohnheiten besprochen.

ANTHOCORIS SPP. AS APHID PREDATORS 207

R E F E R E N C E S

ANDERSON, N. H. (1962a). Growth and fecundity of Anthocoris spp. reared on various prey (Heteroptera : Anthocoridae). Ent. exp. & appl. 5 : 40--52.

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COLLYER, E. (1967). On the ecology of Anthocoris nemorum (L.) (Hemiptera-Heteroptera). Proc. R. ent. Soc. Lond. (.4) 42: 107--118.

DEMPSTER, J. P. (1968). Intra-specific competition and dispersal: as exemplified by a Psyllid and its anthocorid predator. In : Symposia o/ the Royal Entomological Society of Lon- don : N u m b e r 4, Insect Abundance. (lEd. by T. R. E. Southwood): 8--17. Oxford and Edinburgh. Blackwell.

DIXON, A. F. G. (1963). Reproductive activity of the sycamore aphid Drepanosiphum plata- noides (Schr.) (Hemiptera, Aphididae). J. Anim. Ecol. 32: 33---48.

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EMDEN, H. F. VAN (1966). The effectiveness of aphidophagous insects in reducing aphid populations. In : Ecology o[ Aphidophagous Insects (Ed. by I. Hodek): 227--235. Prague, Academia.

FEWKES, D. W. (1960). The food requirements by weight of some British Nabidae (Hetero- ptera). Ent. exp. & appl. 3 : 2 3 1 - - 2 3 7 .

HILL, A. R. (1957). The biology of Anthocoris nemorum (L.) in Scotland (Hemiptera : An- thocoridae). Trans. R. ent. Soc. Lond. 109 : 379--394.

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(1965). The bionomics and ecology of Anthocoris con/usus Reuter in Scotland. 1. The adult and egg-production. Trans. Soc. Br. Ent. 16: 245--256.

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RUSSEL, R. J. (1968). Certain aspects of the ecology of Anthocoris nemorum (L.). and Antho- coris conJusus Router (Hemiptera : Anthocoridae). Unpublished Ph. D. thesis, University o/ Glasgow.

SOUTnWOOD, T. R. E. & SCUDDER, G. G. E. (1956). The immature stages of the Hemiptera- Heteroptera associated with the stinging nettle (Urtica dioica L.). Entomologist's mon. Mag. 92: 313--325.

R e c e i v e d / o r publication : 19 November 1969.