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Maternal influence on diapause in a parasitic insect, Coeloides brunneri Vier. (Hymenoptera: Braconidae)
J. Insect Physic& 1965, V&. 11, pp. 1331 to 1336. Pergamn
Fress Ltd.
Printed in Great Britain
MATERNAL INFLUENCE ON DIAPAUSE IN A PARASITIC INSECT, COEL0ILE.S gR~~~~R~ VIER. (~ENOPTE~: B~~ONID~~ ROGER
B. RYAN*
Pacific Northwest Forest and Range Experiment Station, Forest Service, U.S. Department of Agriculture, Portland, Oregon
Abatract-Coekides brunneri was reared under combinations of temperature (W-85°F) and photoperiod (2-22 hr) to determine the incidence of larval diapause. Long days and high temperatures favoured unarrested development. Short days and low temperatures favoured the onset of diapause. To establish the insect stage which is sensitive to the photoperiodic influence, adult females prior to and while ovipositing into log sections and the progenies of these females while in log sections, were each exposed to diapause-inducing and diapause-averting photoperiods. The incidence of diapause in the larvae depended upon the photoperiods to which the parent females were exposed. The photoperiods to which the log sections containing the progenies were exposed had no effect. It is concluded that the parent female is sensitive to the photoperiod and transmits the determination for diapause through the egg.
PHOTOPERIODand temperature have been shown to be influential in determining the course of development of many insects with a facultative diapause (BECK, 1963 ; LEES, 1955; DEWILDE, 1962). Depending on the prevailing conditions, growth and reproduction may either proceed unchecked or may subsequently be arrested by diapause. The immediate cause for the growth inte~pt~on in all cases is not known. Experimental evidence has firmly linked neuroendocrine systems with diapause in some species (VAN DER KLOOT, 1960). It seems likely that cases of growth interruption occurring immediately prior to a moult are due to deficiencies in the normal neuroendocrine products, as has been shown in Hyalophora cecropia (L.) (WILLIS, 1956) and Cephus linctus Nort. (CHURCH, 1955). On the other hand, although the immediate cause of the egg diapause of Bombyx non’ (L.) is not known, the conditions bringing it about have been traced to neuroendocrine activity prior to the arrest (HASEGAWA,1952, 1957). * Research discussed in this paper was conducted at the Forestry Sciences Laboratory of the Pacific Northwest Forest and Range Experiment Station at Corvallis, Oregon. 1331
1332
ROGERB. RYAN
The arrest follows an often considerable period of normal development after the reception of the environmental signals. In extreme cases, the stage receiving these signals may be as much as a full generation before the stage exhibiting the arrest (DEWILDE, 1962). In such extreme cases of delayed response, the insect must possess some mechanism to retain the determination through the various stages of normal development. LEES (1955) supposed that since sensitivity to the environment is acquired only following the differentiation of the central nervous and endocrine systems, and since the sensitive and responsive stages are within the lifetime of the individual, these systems might be concerned in the retention of the induced state. This view, he stated, could be upheld only if, during the interval between the sensitive and responsive stages, the insect does not pass through the early stages of embryonic development. A number of cases have been reported, however, where this stage is apparently bridged. SIMMONDS (1948) found that in Spalangia drosophilae Ashm. both the age of the female and temperatures to which she had been exposed influenced the incidence of larval diapause in her progeny, and in C%tus inornatus Pratt both the age and the diet of the parent female had an effect. CRAGG and COLE (1952) found a greater incidence of diapause in LuciZia sericata (Meig.) larvae from egg batches obtained from females collected at the end of the season than from females collected early in the season. Similar results were obtained by DEPNER (1961) with Siphona &tans (L.). Presumably, some factor such as photoperiod had acted on the adults of both of these species prior to their collection. The age of the female (SAUNDERS, 1962), her exposure to low temperature during ovigenesis, and the deprivation of hosts in which to oviposit (SCHNEIDERMAN and HORWITZ, 1958) are reported to influence diapause in the progeny of Nasonia vitripennis (Walk.). FLANDERS(1962) reported that diapause in five species of parasitic Hymenoptera was influenced by the extent to which yolk was extracted from the unlaid egg during ovisorption. In all these, the determination for diapause apparently bridges the early stages of embryonic development. In this paper, experiments on the diapause of Coeloides brunneri Vier., a primary larval parasite of the Douglas-fir beetle, Dendroctonus pseudotsugae Hopk., are described. C. brunneri enters diapause in the last (fifth) larval instar (RYAN and RUDINSKY, 1962). Evidence is presented that photoperiod and temperature influence the diapause in this parasite and, further, that this is another case where the determination for diapause bridges the early embryogenic development. MATERIALS AND METHODS Stock cultures of Coeloides brunneri were maintained at a 16 hr photoperiod and a temperature of 75”F, using as a host the Douglas-fir beetle, Dendroctonus pseudotsugae, reared in freshly cut sections of Douglas-fir, Pseudotsuga menziesii (Mirb.) France. Parasitism was obtained by caging adult parasites together with host-infested log sections. Fluorescent lights, controlled by time-switches, were suspended above the cages. Females of the parasite penetrated the bark with their ovipositors and laid eggs on the host larvae tunelling in the inner bark region.
MATERNAL INFLUENCE
ON DIAPAUSE IN A PARASITIC INSECT
1333
Upon completion of their immature stages, adult parasites emerged through the bark and were collected. Experiments were conducted, using adult parasites emerging from the log sections within a 24 hr period. Females were mated, then randomly assigned to 24 oz cylindrical cardboard cartons ventilated with screen and fitted with a plastic window to admit light. The cartons were then randomly assigned to the various temperature and photoperiod treatments where they were maintained until at least seven photoperiod cycles had been completed. After this initial 1 week acclimation period, females were liberated in oviposition cages where the temperature and photoperiod treatments continued until the completion of the allowed oviposition period, usually 7-14 days, after which time all females were removed from the cages. Except for the final experiment in which the photoperiod treatments of the logs containing the progenies were continued until the time of cocoon examination, photoperiod treatments were discontinued following removal of the ovipositing females. In all experiments temperature treatments were continued until cocoon examination. Adult parasites were continuously supplied with a food mixture consisting of honey, yeast, and water in small droplets on strips of waxed paper renewed daily. Following parasitism, the insects were allowed to develop until all nondiapause parasites had suflicient time to develop to the adult stage and emerge through the bark. The bark was then removed from the log sections and all parasite cocoons were examined to determine the incidence of diapause. EXPERIMENTS
AND RESULTS
Eflects of temperature and photoperiod on the indence
of &pause
To ascertain if rearing tempera~re and photope~od affected the incidence of diapause, a 3 by 2 factorial experiment in randomized blocks with two replications was conducted, using temperatures of 65”, 75”, and 85°F and photoperiods of 12 and 16 hr of light per 24 hr. The mean percentages of diapause obtained are shown in Table 1. Both temperature and photoperiod effects were significant at the 0.5 per cent level. A more detailed investigation of the effects of photoperiod was then conducted at a single temperature. A randomized block experiment with three replications of 2, 6, 10, 14, 18, and 22 hr photoperiod treatments was conducted at 75°F. Analysis of the results showed that the percentages of diapause induced by the photoperiod treatments were significantly different at the 0.5 per cent level. The percentages of diapause are shown in Fig. 1. The distinct drop in the incidence of diapause between the 14 and 18 hr photoperiods shows that the critical photoperiod at 75°F lies between those values. Since stock cultures were reared at a 16 hr photoperiod at 75”F, and under these conditions development was predominately nondiapause, indications are that the critical photoperiod actually lies between 14 and 16 hr. The mean per cent diapause in the stock cultures is also shown in Fig. 1.
1334
B. RYAN
ROGER
TABLE l-DIAPAUSE
IN
Cocloides bmmri
PERATUREAND
Rearing temperature
OF TWO
CONDITIONS REPLICATIONS)
OF TEM-
Photoperiod (hr of light per 24 hr)
65 75 85
60
RBARBDUNDER DIFFZRENT
PHOTOPWIOD(MEANVALUES
99.2 67.8 57.6
45-l 2.8 o-0
0 0
# _
0
2
4
6
8
Photopriod,
IO
12
14
16
I8
20
22
24
hr of liqht / 24 hr
FIG. 1. Diapause in Co&ides bmnneti reared under different photoperiods at 75°F.. 0, individual per cent obtained in replications of a single experiment ; 0, mean per cent obtained in stock cultures.
Determination of the sensitive stage Attention was next turned to determining the stage of C. brunneri receiving the photoperiodic stimulus resulting in diapause in the last larval instar. It was suspected that since the adult is the only stage which lives outside of the bark it must be photosensitive. This hypothesis was tested in a 2 by 2 factorial experiment with three replications. Two groups of females were exposed to a diapauseinducing (10 hr) photoperiod and two groups of females to a diapause-averting (16 hr) photoperiod. Following the 1 week acclimation period, each group of females was allowed to oviposit into one of four host-infested logs for l-2 weeks. These logs, thereafter containing developing parasite progenies, were at the same time exposed to either a 10 or a 16-hr photoperiod, two of the logs receiving the same photoperiod as the parent females and two receiving the alternative
MATERNAL
INFLUENCE
ON
DIAPAUSE
IN
A PARASITIC
1335
INSECT
photoperiod. To maintain different photoperiods on parents and progenies, females were maintained in oviposition cages only during the light phase of the 10 hr photoperiod where both they and the logs were illuminated. At the were removed to windowed end of this period, females in all treatments cartons where they were subjected either to immediate darkness or to 6 additional hours of light and then to darkness until the start of the next light period, when they were again liberated in the oviposition cages. Photoperiod treatment of the adults was discontinued at the ‘end of the allowed oviposition period but was continued on the logs containing the developing progenies until the time of cocoon examination. The mean percentages of diapause obtained are shown in Table 2. Statistical analysis showed that the differences obtained from the treatment of adults were TABLE
2-DIAPAUSE
PHOTOPERIOD MATERNAL
IN
TREATMENT
PARENTS
PRIOR
MATURE OF
LARVAE
THE
TO AND
LOGS DURING
OF cOehdt?S IN
WHICH
OVIPOSITION
bnrnne?i THEY
FOLLOWING
WERE
(MEAN
Photoperiod (hr of light per 24 hr) To parents
To logs containing progenies
10 10 16 16
10 16 10 16
SHORT
DEVELOPING
VALUES
OF THREE
AND
LONG
OF
THEIR
AND
REPLICATIONS)
Diapause (%)
83.7 77.0 12.7 9.1
significant at the 0.5 per cent level, but the differences from the treatment logs containing the progenies were not significant at the 5 per cent level. CONCLUSIONS
of the
AND DISCUSSION
Coeloides brunneri appears to react to photoperiod
and temperature in a manner similar to many other insects with a facultative diapause. Long days and high temperature favour unarrested development, and short days and low temperatures favour the onset of diapause. In the experiment on determining the sensitive stage, the incidence of larval diapause was not affected by the photoperiod applied to the immature stages. We may conclude that either the photoperiod is without effect during this time or that, in this experiment, the overlying bark effectively shielded the insects from its effects The incidence of diapause was affected, however, by the photoperiod to which the parent females were exposed prior to and during oviposition. The females, therefore, laid eggs which differed in their potential to diapause during the last larval stage. In this experiment, females were allowed to oviposit part of each day for at least a week so that the number of progeny would be sufficient to obtain a reliable
1336
ROGERB. RYAN
percentage. Therefore, different temperatures on adult and progeny were not possible. Although temperature was shown to influence diapause (Table l), no information was obtained on whether temperature exerted its effect on the parent female or on her progeny, or both. The physiological basis for photoperiodic action and the persistence of its effects until the next generation is unknown. Since at the time of oviposition no embryonic organ systems have been differentiated (RYAN, 1963), it is concluded that the photoperiod acts on the maternal physiology. The break in continuity of the central nervous and endocrine systems from one generation to the next precludes the possibility that these systems, per se, are the means for retaining the determination for diapause. Possibly a hormone is produced in the mother in response to photoperiod, becomes incorporated in the unlaid egg, and persists to act on a target in a later stage. REFERENCES BECK S. D. (1963) Physiology and ecology of photoperiodism. Bull. ent. Sot. Amer. 9, 8-16. CHURCHN. S. (1955) Hormones and the termination and reinduction of diapause in Cephus cinctus Nort. (Hymenoptera: Cephidae). Can. J. Zool. 33, 339-369. CRAGGJ. B. and COLE P. (1952) Diapause in Lucilia sericata (Mg.) (Diptera). J. eJep. Biol. 29, 600-604. DEPNER K. R. (1961) The effect of temperature on development and diapause of the horn fly, Siphona irritans (L.) (Diptera: Muscidae). Can. Ent. 93, 855-859. FLANDJIRSS. E. (1962) The parasitic Hymenoptera: Specialists in population regulation. Can. Ent. 94, 1133-1147. HASBGAWA K. (1952) Studies on the voltinism in the silkworm, Bombyx mori L., with special reference to the organs concerning determination of voltinism. J. Fat. Agric., Tottori lJniv. 1,83-124. ~EGAWA K. (1957) The diapause hormone of the silkworm, Bombyx mori. Nature, Lond. 179,1300-l 301. LEES A. D. (1955) The PhysioZogy of Dzizpause in Arthropods. Cambridge University Press, London. RYAN R. B. (1963) Contribution to the embryology of Coeloides bnmne7i (Hymenoptera: Braconidae). Ann. ent. Sot. Am. 56, 639-648. RYAN R. B. and RIJIXNSKYJ. A. (1962) Biology and habits of the Douglas-fir beetle parasite, Coeloides brunnti Viereck (Hymenoptera: Braconidae), in western Oregon. Can. Ent. 94, 748-763. SAUNDERS D. S. (1962) The effect of the age of female Nasoti vitr$wnnis (Walker) (Hymdnoptera, Pteromalidae) upon the incidence of larval diapause. J. Insect Physiol. 8,309-3 18. SCHNEIDERMAN H. A. and HORWITZJ. (1958) The induction and termination of facultative diapause in the chalcid wasps Mormoniella vitripemtis (Walker) and Tritneptis klugii (Ratzeburg). J. exp. Biol. 35, 520-551. SIMMONDSF. J. (1948) The influence of maternal physiology on the incidence of diapause. Phil. Trans. R. Sot. (B) 233, 385414. VAN DER KLOOT W. G. (1960) Neurosecretion in insects. Annu. Rev. Ent. 5, 35-52. WILDE J. DE (1962) Photoperiodism in insects and mites. Annu. Rev. Ent. 7, l-26. WILLIAMS C. M. (1956) Physiology of insect diapause. X. An endocrine mechanism for the influence of temperature on the diapausing pupa of the cecropia silkworm (Hyalophora cecropia). Biol. Bull., Woods Hole 110,201-218.
Fress Ltd.
Printed in Great Britain
MATERNAL INFLUENCE ON DIAPAUSE IN A PARASITIC INSECT, COEL0ILE.S gR~~~~R~ VIER. (~ENOPTE~: B~~ONID~~ ROGER
B. RYAN*
Pacific Northwest Forest and Range Experiment Station, Forest Service, U.S. Department of Agriculture, Portland, Oregon
Abatract-Coekides brunneri was reared under combinations of temperature (W-85°F) and photoperiod (2-22 hr) to determine the incidence of larval diapause. Long days and high temperatures favoured unarrested development. Short days and low temperatures favoured the onset of diapause. To establish the insect stage which is sensitive to the photoperiodic influence, adult females prior to and while ovipositing into log sections and the progenies of these females while in log sections, were each exposed to diapause-inducing and diapause-averting photoperiods. The incidence of diapause in the larvae depended upon the photoperiods to which the parent females were exposed. The photoperiods to which the log sections containing the progenies were exposed had no effect. It is concluded that the parent female is sensitive to the photoperiod and transmits the determination for diapause through the egg.
PHOTOPERIODand temperature have been shown to be influential in determining the course of development of many insects with a facultative diapause (BECK, 1963 ; LEES, 1955; DEWILDE, 1962). Depending on the prevailing conditions, growth and reproduction may either proceed unchecked or may subsequently be arrested by diapause. The immediate cause for the growth inte~pt~on in all cases is not known. Experimental evidence has firmly linked neuroendocrine systems with diapause in some species (VAN DER KLOOT, 1960). It seems likely that cases of growth interruption occurring immediately prior to a moult are due to deficiencies in the normal neuroendocrine products, as has been shown in Hyalophora cecropia (L.) (WILLIS, 1956) and Cephus linctus Nort. (CHURCH, 1955). On the other hand, although the immediate cause of the egg diapause of Bombyx non’ (L.) is not known, the conditions bringing it about have been traced to neuroendocrine activity prior to the arrest (HASEGAWA,1952, 1957). * Research discussed in this paper was conducted at the Forestry Sciences Laboratory of the Pacific Northwest Forest and Range Experiment Station at Corvallis, Oregon. 1331
1332
ROGERB. RYAN
The arrest follows an often considerable period of normal development after the reception of the environmental signals. In extreme cases, the stage receiving these signals may be as much as a full generation before the stage exhibiting the arrest (DEWILDE, 1962). In such extreme cases of delayed response, the insect must possess some mechanism to retain the determination through the various stages of normal development. LEES (1955) supposed that since sensitivity to the environment is acquired only following the differentiation of the central nervous and endocrine systems, and since the sensitive and responsive stages are within the lifetime of the individual, these systems might be concerned in the retention of the induced state. This view, he stated, could be upheld only if, during the interval between the sensitive and responsive stages, the insect does not pass through the early stages of embryonic development. A number of cases have been reported, however, where this stage is apparently bridged. SIMMONDS (1948) found that in Spalangia drosophilae Ashm. both the age of the female and temperatures to which she had been exposed influenced the incidence of larval diapause in her progeny, and in C%tus inornatus Pratt both the age and the diet of the parent female had an effect. CRAGG and COLE (1952) found a greater incidence of diapause in LuciZia sericata (Meig.) larvae from egg batches obtained from females collected at the end of the season than from females collected early in the season. Similar results were obtained by DEPNER (1961) with Siphona &tans (L.). Presumably, some factor such as photoperiod had acted on the adults of both of these species prior to their collection. The age of the female (SAUNDERS, 1962), her exposure to low temperature during ovigenesis, and the deprivation of hosts in which to oviposit (SCHNEIDERMAN and HORWITZ, 1958) are reported to influence diapause in the progeny of Nasonia vitripennis (Walk.). FLANDERS(1962) reported that diapause in five species of parasitic Hymenoptera was influenced by the extent to which yolk was extracted from the unlaid egg during ovisorption. In all these, the determination for diapause apparently bridges the early stages of embryonic development. In this paper, experiments on the diapause of Coeloides brunneri Vier., a primary larval parasite of the Douglas-fir beetle, Dendroctonus pseudotsugae Hopk., are described. C. brunneri enters diapause in the last (fifth) larval instar (RYAN and RUDINSKY, 1962). Evidence is presented that photoperiod and temperature influence the diapause in this parasite and, further, that this is another case where the determination for diapause bridges the early embryogenic development. MATERIALS AND METHODS Stock cultures of Coeloides brunneri were maintained at a 16 hr photoperiod and a temperature of 75”F, using as a host the Douglas-fir beetle, Dendroctonus pseudotsugae, reared in freshly cut sections of Douglas-fir, Pseudotsuga menziesii (Mirb.) France. Parasitism was obtained by caging adult parasites together with host-infested log sections. Fluorescent lights, controlled by time-switches, were suspended above the cages. Females of the parasite penetrated the bark with their ovipositors and laid eggs on the host larvae tunelling in the inner bark region.
MATERNAL INFLUENCE
ON DIAPAUSE IN A PARASITIC INSECT
1333
Upon completion of their immature stages, adult parasites emerged through the bark and were collected. Experiments were conducted, using adult parasites emerging from the log sections within a 24 hr period. Females were mated, then randomly assigned to 24 oz cylindrical cardboard cartons ventilated with screen and fitted with a plastic window to admit light. The cartons were then randomly assigned to the various temperature and photoperiod treatments where they were maintained until at least seven photoperiod cycles had been completed. After this initial 1 week acclimation period, females were liberated in oviposition cages where the temperature and photoperiod treatments continued until the completion of the allowed oviposition period, usually 7-14 days, after which time all females were removed from the cages. Except for the final experiment in which the photoperiod treatments of the logs containing the progenies were continued until the time of cocoon examination, photoperiod treatments were discontinued following removal of the ovipositing females. In all experiments temperature treatments were continued until cocoon examination. Adult parasites were continuously supplied with a food mixture consisting of honey, yeast, and water in small droplets on strips of waxed paper renewed daily. Following parasitism, the insects were allowed to develop until all nondiapause parasites had suflicient time to develop to the adult stage and emerge through the bark. The bark was then removed from the log sections and all parasite cocoons were examined to determine the incidence of diapause. EXPERIMENTS
AND RESULTS
Eflects of temperature and photoperiod on the indence
of &pause
To ascertain if rearing tempera~re and photope~od affected the incidence of diapause, a 3 by 2 factorial experiment in randomized blocks with two replications was conducted, using temperatures of 65”, 75”, and 85°F and photoperiods of 12 and 16 hr of light per 24 hr. The mean percentages of diapause obtained are shown in Table 1. Both temperature and photoperiod effects were significant at the 0.5 per cent level. A more detailed investigation of the effects of photoperiod was then conducted at a single temperature. A randomized block experiment with three replications of 2, 6, 10, 14, 18, and 22 hr photoperiod treatments was conducted at 75°F. Analysis of the results showed that the percentages of diapause induced by the photoperiod treatments were significantly different at the 0.5 per cent level. The percentages of diapause are shown in Fig. 1. The distinct drop in the incidence of diapause between the 14 and 18 hr photoperiods shows that the critical photoperiod at 75°F lies between those values. Since stock cultures were reared at a 16 hr photoperiod at 75”F, and under these conditions development was predominately nondiapause, indications are that the critical photoperiod actually lies between 14 and 16 hr. The mean per cent diapause in the stock cultures is also shown in Fig. 1.
1334
B. RYAN
ROGER
TABLE l-DIAPAUSE
IN
Cocloides bmmri
PERATUREAND
Rearing temperature
OF TWO
CONDITIONS REPLICATIONS)
OF TEM-
Photoperiod (hr of light per 24 hr)
65 75 85
60
RBARBDUNDER DIFFZRENT
PHOTOPWIOD(MEANVALUES
99.2 67.8 57.6
45-l 2.8 o-0
0 0
# _
0
2
4
6
8
Photopriod,
IO
12
14
16
I8
20
22
24
hr of liqht / 24 hr
FIG. 1. Diapause in Co&ides bmnneti reared under different photoperiods at 75°F.. 0, individual per cent obtained in replications of a single experiment ; 0, mean per cent obtained in stock cultures.
Determination of the sensitive stage Attention was next turned to determining the stage of C. brunneri receiving the photoperiodic stimulus resulting in diapause in the last larval instar. It was suspected that since the adult is the only stage which lives outside of the bark it must be photosensitive. This hypothesis was tested in a 2 by 2 factorial experiment with three replications. Two groups of females were exposed to a diapauseinducing (10 hr) photoperiod and two groups of females to a diapause-averting (16 hr) photoperiod. Following the 1 week acclimation period, each group of females was allowed to oviposit into one of four host-infested logs for l-2 weeks. These logs, thereafter containing developing parasite progenies, were at the same time exposed to either a 10 or a 16-hr photoperiod, two of the logs receiving the same photoperiod as the parent females and two receiving the alternative
MATERNAL
INFLUENCE
ON
DIAPAUSE
IN
A PARASITIC
1335
INSECT
photoperiod. To maintain different photoperiods on parents and progenies, females were maintained in oviposition cages only during the light phase of the 10 hr photoperiod where both they and the logs were illuminated. At the were removed to windowed end of this period, females in all treatments cartons where they were subjected either to immediate darkness or to 6 additional hours of light and then to darkness until the start of the next light period, when they were again liberated in the oviposition cages. Photoperiod treatment of the adults was discontinued at the ‘end of the allowed oviposition period but was continued on the logs containing the developing progenies until the time of cocoon examination. The mean percentages of diapause obtained are shown in Table 2. Statistical analysis showed that the differences obtained from the treatment of adults were TABLE
2-DIAPAUSE
PHOTOPERIOD MATERNAL
IN
TREATMENT
PARENTS
PRIOR
MATURE OF
LARVAE
THE
TO AND
LOGS DURING
OF cOehdt?S IN
WHICH
OVIPOSITION
bnrnne?i THEY
FOLLOWING
WERE
(MEAN
Photoperiod (hr of light per 24 hr) To parents
To logs containing progenies
10 10 16 16
10 16 10 16
SHORT
DEVELOPING
VALUES
OF THREE
AND
LONG
OF
THEIR
AND
REPLICATIONS)
Diapause (%)
83.7 77.0 12.7 9.1
significant at the 0.5 per cent level, but the differences from the treatment logs containing the progenies were not significant at the 5 per cent level. CONCLUSIONS
of the
AND DISCUSSION
Coeloides brunneri appears to react to photoperiod
and temperature in a manner similar to many other insects with a facultative diapause. Long days and high temperature favour unarrested development, and short days and low temperatures favour the onset of diapause. In the experiment on determining the sensitive stage, the incidence of larval diapause was not affected by the photoperiod applied to the immature stages. We may conclude that either the photoperiod is without effect during this time or that, in this experiment, the overlying bark effectively shielded the insects from its effects The incidence of diapause was affected, however, by the photoperiod to which the parent females were exposed prior to and during oviposition. The females, therefore, laid eggs which differed in their potential to diapause during the last larval stage. In this experiment, females were allowed to oviposit part of each day for at least a week so that the number of progeny would be sufficient to obtain a reliable
1336
ROGERB. RYAN
percentage. Therefore, different temperatures on adult and progeny were not possible. Although temperature was shown to influence diapause (Table l), no information was obtained on whether temperature exerted its effect on the parent female or on her progeny, or both. The physiological basis for photoperiodic action and the persistence of its effects until the next generation is unknown. Since at the time of oviposition no embryonic organ systems have been differentiated (RYAN, 1963), it is concluded that the photoperiod acts on the maternal physiology. The break in continuity of the central nervous and endocrine systems from one generation to the next precludes the possibility that these systems, per se, are the means for retaining the determination for diapause. Possibly a hormone is produced in the mother in response to photoperiod, becomes incorporated in the unlaid egg, and persists to act on a target in a later stage. REFERENCES BECK S. D. (1963) Physiology and ecology of photoperiodism. Bull. ent. Sot. Amer. 9, 8-16. CHURCHN. S. (1955) Hormones and the termination and reinduction of diapause in Cephus cinctus Nort. (Hymenoptera: Cephidae). Can. J. Zool. 33, 339-369. CRAGGJ. B. and COLE P. (1952) Diapause in Lucilia sericata (Mg.) (Diptera). J. eJep. Biol. 29, 600-604. DEPNER K. R. (1961) The effect of temperature on development and diapause of the horn fly, Siphona irritans (L.) (Diptera: Muscidae). Can. Ent. 93, 855-859. FLANDJIRSS. E. (1962) The parasitic Hymenoptera: Specialists in population regulation. Can. Ent. 94, 1133-1147. HASBGAWA K. (1952) Studies on the voltinism in the silkworm, Bombyx mori L., with special reference to the organs concerning determination of voltinism. J. Fat. Agric., Tottori lJniv. 1,83-124. ~EGAWA K. (1957) The diapause hormone of the silkworm, Bombyx mori. Nature, Lond. 179,1300-l 301. LEES A. D. (1955) The PhysioZogy of Dzizpause in Arthropods. Cambridge University Press, London. RYAN R. B. (1963) Contribution to the embryology of Coeloides bnmne7i (Hymenoptera: Braconidae). Ann. ent. Sot. Am. 56, 639-648. RYAN R. B. and RIJIXNSKYJ. A. (1962) Biology and habits of the Douglas-fir beetle parasite, Coeloides brunnti Viereck (Hymenoptera: Braconidae), in western Oregon. Can. Ent. 94, 748-763. SAUNDERS D. S. (1962) The effect of the age of female Nasoti vitr$wnnis (Walker) (Hymdnoptera, Pteromalidae) upon the incidence of larval diapause. J. Insect Physiol. 8,309-3 18. SCHNEIDERMAN H. A. and HORWITZJ. (1958) The induction and termination of facultative diapause in the chalcid wasps Mormoniella vitripemtis (Walker) and Tritneptis klugii (Ratzeburg). J. exp. Biol. 35, 520-551. SIMMONDSF. J. (1948) The influence of maternal physiology on the incidence of diapause. Phil. Trans. R. Sot. (B) 233, 385414. VAN DER KLOOT W. G. (1960) Neurosecretion in insects. Annu. Rev. Ent. 5, 35-52. WILDE J. DE (1962) Photoperiodism in insects and mites. Annu. Rev. Ent. 7, l-26. WILLIAMS C. M. (1956) Physiology of insect diapause. X. An endocrine mechanism for the influence of temperature on the diapausing pupa of the cecropia silkworm (Hyalophora cecropia). Biol. Bull., Woods Hole 110,201-218.