- Email: [email protected]
Inhibition of the growth of an insect by fatty acids
3. Insect Phyriol.,1965, Vol. 11, pp. 1247 to 1252. Pergawwn Press Ltd.
INHIBITION
Printed in Great Britain
OF THE GROWTH OF AN INSECT BY FATTY ACIDS*
J. E. McFARLANE
and G. 0. HENNEBERRY
Departments of Entomology and Chemistry, Macdonald ColIege, McGill University, Macdonald College P.O., P.Q., Canada (Received 4 January
1965; revised 2 April 1965)
Abstract-Growth of the cricket, CryZlodes sigihtus (Walk.), was inhibited by fatty acids and their methyl esters where the route of entry appeared to be through the external body wall. The effective fatty acids were lauric, my&tic, stearic, and behenic acids. The methyl esters of palmitic acid, especially, but also of myristic, stearic, and oleic acids also inhibited growth. Methyl pahnitate drastically reduced the incidence of wing development in this polymorphic species. When lauric acid, my&tic acid, and methyl pahnitate were fed, only lauric acid at a high level (1%) inhibited growth. INTRODUCTION
MCFARLANE(1964) has reported the inhibition of growth of Gryllodes sigillatus (Walk.) by A&eta ahesticm (L.) (both Orthoptera: Gryllidae) when the two species were reared together. In one attempt to discover the cause of inhibition, an ether extract of A. domesticus was prepared which when adsorbed on filter paper and placed in the rearing jar, inhibited the growth of G. sigilhztus. However, it also inhibited the growth of A. domesticus itself. This was not, therefore, the inhibiting factor originally looked for. Isolation of the new inhibitor was pursued by the fractionation of the ether extract, and the active factor was found to be present in the saponifiable fraction (MCFARLANEand HENNEBERRY,unpublished). This led to the finding that fatty acids and their methyl esters could inhibit growth, which is the subject of the present article, All experiments to be reported were carried out with G. sigillutus as test animal. This species was chosen only for the sake of convenience. MATERIALS
AND METHODS
Groups of ten newly hatched insects were placed in ointment jars (16 oz) and were maintained on a diet of ground baby rabbit pellets contained in small boats constructed of filter paper. Water was supplied to each group in a shell vial (32 ml) filled with distilled water and stoppered with a cotton-wool plug. A chloroform solution of each fatty acid or methyl ester was prepared at a concentration of O-062 M (preliminary experiments had shown this to be an effective *This work was supported by a grant from the National Research Council of Canada. 79
1247
1248
J. E. MCFARLANE AND G. 0. HENNEBERRY
concentration of the most active chemical-methyl palmitate). One ml of the test solution was adsorbed by a piece of folded filter paper (6 x 1.5 in.), and the chloroform evaporated in a current of warm air before the paper was placed in the ointment jar. Vitamin E (0,042 mg/ml of chloroform) was added as an antioxidant except in the dietary experiment: tests showed that vitamin E had no effect on the results obtained with saturated acids. All fatty acids and esters were of the highest purity obtainable from Nutritional Biochemicals Corp., Cleveland, Ohio. Each test was carried out in replicates of six, using ten insects per treatment, for a total of sixty insects per acid or ester tested. Means and standard deviations are given. An asterisk beside the mean value denotes that there was a significant difference between the control at P = 0.01 and the mean. Larvae were weighed after about 11 days and 21 days. Only a few of these results are presented, however, to give an idea of the kind of result obtained. RESULTS
Experiment I In Experiment I, various methyl esters were tested. survival of the larvae after 21 days is as follows:
Treatment Methyl Methyl Methyl Methyl Methyl Methyl Methyl Methyl Methyl Control
caprylate caprate laurate myristate palmitate stearate oleate linoleate linolenate
No. 58 56 56 55 45 51 41 54 57 57
The average
weights
and
Mean wt. (mg) 68-l+ 63.0 + 62.0 4 47.4 + 33.3 + 48-l_+ 41.9 + 60.2 + 67.4 4 65.8 +
4.6 2.5 8.8 12.1 6.4* 6.5 9.7” 3.8 8.7 10.0
Methyl palmitate especially, and also methyl oleate, significantly retarded While methyl stearate and methyl growth, and resulted in lower survival. the results were significant only myristate certainly gave some inhibition, at the 0.02 and 0.05 levels respectively. The results for 11 days were similar, which suggests that the effect is perhaps operative only during early postembryonic life. The results for the adults are given in Table 1. Here methyl palmitate and methyl stearate gave a significantly longer female larval period ; methyl myristate and methyl oleate were similarly effective only at the 0.02 level. The weight of males treated with methyl stearate was reduced. The survival with methyl palmitate and methyl oleate was very poor.
INHIBITION
OF THE GROWTH OF AN INSECT BY FATTY
1249
ACIDS
TABLE I--AVERAGE WEIGHTOF THE ADULTS( d 24 hr old), AWRAGEDURATION OF THE LARVAL STAGE,ANDPERCENTAGE SURVIVAL TO THBADULTSTAGE. EWERIMENT I Adult females obtained
Aduit males obtained
Treatment
Methyl Methyl Methyl Methyl Methyl Methyl Methyl Methyl Methyl Control
caprylate caprate laurate myristate palmitate stearate oleate linoleate linolenate
No.
19 21 25 18 17 19 8 13 16 20
Mean wt. (mg) 179.4 186.9 179.2 192.7 183.7 166.6 176.3 173.2 178.5 194.9
* 34.1 + 34.2 2 32.8 f 37.6 k49.6 + 28.6* + 42.6 f 28.1 + 40.9 + 34.6
Mean duration larval stage (days) 37.5 37.5 38.4 39.3 41.8 39.2 42.6 39.3 36.6 39.0
f 4.5 + 3.0 f 4.8 f 3.8 f 7.1 + 4.6 + 5.4 f 8.9 + 4.1 + 3.7
No.
25 24 24 27 12 27 24 28 33 23
Mean wt. (mg) 238.3 247.0 228.6 235.4 226.7 229.4 224.3 223.6 231.1 231.6
f 34.3 f 40.2 + 40-2 f 40.2 k 33.3 + 29.8 + 34.2 f 60.6 + 39.3 + 36.4
Mean duration larval stage (days) 33.5 34.1 33.8 36.3 37.4 35.4 36.5 34.9 33.8 33.5
& 3.0 + 3.3 + 2.9 f 5.1 f 4.8* + 2.0* f 5.0 + 6.7 f 3.6 f 2.8
%
Survival 73 75 82 75 48 77 53 68 82 72
Experiment II In Experiment are as follows:
II, various free fatty acids were tested. The results for 11 days Treatment Methyl palmitate Laurie acid Myristic acid Palmitic acid Oleic acid Elaidic acid Stearic acid Linoleic acid Behenic acid Control
No. 30 55 48 60 55 53 58 60 56 58
Mean wt. (mg) 5.8 + 1-o* 10.6 + 1.5* 8-O+ l-2* 13-l * 0.8 13*3 2 0.4 12.9 f O-9 14.0 * 1-o 13.5 f O-8 12.9 f 1.0 13.8 + 0.9
The results at 21 days were similar. These results differ greatly from those of Experiment I. None of the fatty acids, with the exception of myristic acid, corresponding to effective methyl esters, were inhibitory. And, conversely, free lauric acid, whose methyl ester was not inhibitory, gave reduced growth. The adult results are given in Table 2. The duration of the male larval period was prolonged with methyl palmitate and with myristic acid. It was also prolonged with steak and behenic acids, which did not significantly affect growth to 21 days. Laurie acid affected the male larval period only at the O-02 level. The female
1250
J. E. MCFARLANEm~G.0.
HENNEBERRY
larval stage was significantly lengthened only with methyl palmitate. poor with methyl palmitate and myristic acid.
Survival was
TABLE 2-AWRAGE WEIGHTOF THEADULTS( < 24 hr old), AVERAGEDURATIONOF THE LARVAL
STAGE,AND PERCENTAGESURVIVALTO THE ADULT STAGE.EXPERIMENTII Adult males obtained
Treatment
Methyl pahnitate Laurie acid Myristic acid Pahnitic acid Oleic acid Elaidic acid Stearic acid Linoleic acid Behenic acid Control
No.
Mean wt. (mg)
13 22 17 22 20 19 22 29 21 23
200.3 + 35.6 197.6 zk27.9 192.1 + 55.6 187.7 f 47.4 187.8 f 24.3 213.8 f 34.1 190.5 f 34.8 187.8 f 35.7 206*3+36.9 199.6 f 36.7
Adult females obtained
Mean duration larval stage (days) 45.2 f 7*9* 37.9 f 4-l 43.7 f 6.7* 36.8 +_8.7 36.8 f 4.7 38.5 + 7.5 39.0 + 4.0* 36.2 + 4.4 38.7+5-l* 35.2 + 2.5
No.
Mean wt. (mg)
Mean duration larval stage (days)
15 20 19 28 31 27 30 25 28 28
219.2k31.4 255.1 574.7 232.9f65.5 228.0 + 39.6 244.3 f 28.9 228.2f36.4 238.6f43.4 230.9 + 45.0 233.9 f 44.2 233.Ok42.1
37.2+3.1* 36.6 k7.6 36.5k7.8 33.0+ 3.7 31.4 f 3.1 32.4k4.0 32.8k2.5 32.6 f 7.3 32.6 + 2.5 32.3 &-5.1
% Surviva1 47 70 60 83 85 77 87 90 82 85
TABLE ~-AVERAGE WEIGHTOFTHJZADULTS(~~~~~ ~~~),A~~GED~~TIoN OFTHELARVAL STAGE,
AND
PERCENTAGESURVIVALTOTHE ADULT STAGE.EXPERIMENTIII Adult males obtained
Adult females obtained
No.
Mean wt. (mg)
Mean duration larval stage (days)
39.9 + 5.0 37.3 f 3.5 38.4 f 3.9
29 33 25
229.8 + 40.9 242.4 f 36.5 238.1 k 45.7
33.9 + 3.0 33.3 + 2.9 32.5 + 2.5
87 88 83
183.6 f 32.3
36.6 + 4.6
28
244.5 f 34.5
33.9 f 2.9
82
15
169.2 f 34.9
39.7 + 6.0
33
226.1 + 49.8
33.4 f 2.4
80
19 10 26 21 20
180.3 179.3 185.0 184.1 180.6
38.3 f 42.1 + 38.0 + 38.8 + 37.2 +
34 33 28 30 33
225.9 234.0 242.6 238.7 239.9
33.2 36.2 33.8 33.1 33.3
88 72 90 85 88
Diet
No.
Mean wt. (mg)
1% Myristic acid 0.1% Myristic acid 0.01% Myristic acid 1% Methyl palmitate 0.1% Methyl palmitate O-01% Methyl palmitate 1% Laurie acid 0.1% Laurie acid 0.01% Laurie acid Control
23 20 25
165.1 f 30.4 187.5 + 31.4 189.0 f 32.8
21
f f k f f
20.5 36,4 33.6 31.1 27.4
Mean duration larval stage (days)
3.7 5.3* 5.0 3.3 3.0
+ 33.9 f 37.5 + 53.8 + 40.7 f 37.8
f + + + f
2.7 2,6* 3.6 3.3 3.1
% Surviva1
INHIBITION
OF THE GROWTH OF AN INSECT BY FATTY
ACIDS
1251
Experintent III In this experiment, different levels of three hitherto effective chemicals were The adult results are presented in Table 3. Laurie acid at the 1% level gave prolonged male and female developmental periods. Laurie acid at the 1o/o level also inhibited growth significantly when weights were taken at 11 and 21 days. It was the only effective chemical. Repetition of this experiment gave similar results, but lauric acid at the 1% level was less effective than in Experiment III.
fed to the insects.
Experiment IV In a preliminary experiment, using a strain selected for the winged form of this polymorphic species, methyl palmitate not only prolonged the male and female larval periods significantly, but also reduced the incidence of wing development. Methyl palmitate gave 9 per cent winged forms (survivors = 46/60), whereas the control gave 36 per cent winged forms (survivors = 55/60). DISCUSSION
AND CONCLUSIONS
It seems likely that the effective chemicals entered the body through the cuticle of the tarsi as the larvae walked over the filter paper. This is concluded because lauric acid was the only effective chemical when presented in the diet (and at a high level), and because the filter paper with adsorbed chemicals was not eaten in any experiment. As the effective chemicals very likely entered the haemolymph, the gut must play a role in their detoxification when they are presented in the diet. With methyl palmitate, of course, hydrolysis in the gut would probably be sufficient to detoxify it. The effect seems to have been mainly on the young larvae, except in the case of stearic and behenic acids, where inhibition did not show up until after 21 days of growth. Either the chemicals were only effective on young larvae, or the young larvae absorbed relatively more than older ones. These possibilities could be investigated by determining just how much of the chemicals actually enters the body, and by testing the effect of injecting known amounts into the body. Speculation on other results is probably not useful at the present time. All the effective free fatty acids were saturated. Wing development was inhibited, as well as growth, by methyl palmitate. Both males and females were affected by methyl palmitate, and by lauric acid when fed. Otherwise only the males were affected by free fatty acids, and only the females by the esters except for methyl stearate. Adult weight was usually not affected. All these results suggest that fatty acids and/or their esters might play a role in the control of growth and development, and not be merely a passive reserve of energy. Any or all of the fatty acids might be involved. This is in addition to the polyunsaturated fatty acids-especially linoleic and linolenic-which have been shown to be absolutely required by a number of insects (FAST, 1964). While none of the polyunsaturated acids is required in the diet of Gryllodes sigillatus,
1252
J. E. MCFARLANEANDG. 0. HENNEBERRY
they probably are essential in a metabolic sense. Changes in the fatty acid composition of insect fat during growth and development have been shown to occur in a number of insects (FAST, 1964), but no meaning has as yet been attributed to them. The changes in the depot fat may not, however, be as meaningful as changes in the circulating fat. The work of SLAMA (1962) on the effect of fatty acids when topically applied to the wing pads of larval Pyrrhocoris apterus is interesting in this connexion. SlQma found that certain fatty acids inhibited wing growth whereas others stimulated it. His primary interest was in demonstrating the juvenile hormonelike activity of these compounds. However, the effects he obtained were restricted to the area of application, rather than being systemic as in our work. Further, we make no claim as to the specific endocrinological relationships of the fatty acids. We intend to follow carefully the changes in depot fat and circulating fat The amount of effective chemical which peneduring growth and development. trates into the insect and its fate will also be determined. The effects of injecting fatty acids and their esters will be investigated. REFERENCES FAST P. G. (1964) Insect lipids: a review. Mem. mt. Sot. Canada No. 37, 50 pp. MCFARLANEJ. E. (1964) Interaction between two species of crickets reared together. Canad. J. Zool. 42, 689-691.
SLAMAK. (1962) The juvenile hormone-like effect of fatty acids, fatty alcohols and some Acta Sot. mt. &chosIov. 59, 323-340. other compounds in insect metamorphosis.
INHIBITION
Printed in Great Britain
OF THE GROWTH OF AN INSECT BY FATTY ACIDS*
J. E. McFARLANE
and G. 0. HENNEBERRY
Departments of Entomology and Chemistry, Macdonald ColIege, McGill University, Macdonald College P.O., P.Q., Canada (Received 4 January
1965; revised 2 April 1965)
Abstract-Growth of the cricket, CryZlodes sigihtus (Walk.), was inhibited by fatty acids and their methyl esters where the route of entry appeared to be through the external body wall. The effective fatty acids were lauric, my&tic, stearic, and behenic acids. The methyl esters of palmitic acid, especially, but also of myristic, stearic, and oleic acids also inhibited growth. Methyl pahnitate drastically reduced the incidence of wing development in this polymorphic species. When lauric acid, my&tic acid, and methyl pahnitate were fed, only lauric acid at a high level (1%) inhibited growth. INTRODUCTION
MCFARLANE(1964) has reported the inhibition of growth of Gryllodes sigillatus (Walk.) by A&eta ahesticm (L.) (both Orthoptera: Gryllidae) when the two species were reared together. In one attempt to discover the cause of inhibition, an ether extract of A. domesticus was prepared which when adsorbed on filter paper and placed in the rearing jar, inhibited the growth of G. sigilhztus. However, it also inhibited the growth of A. domesticus itself. This was not, therefore, the inhibiting factor originally looked for. Isolation of the new inhibitor was pursued by the fractionation of the ether extract, and the active factor was found to be present in the saponifiable fraction (MCFARLANEand HENNEBERRY,unpublished). This led to the finding that fatty acids and their methyl esters could inhibit growth, which is the subject of the present article, All experiments to be reported were carried out with G. sigillutus as test animal. This species was chosen only for the sake of convenience. MATERIALS
AND METHODS
Groups of ten newly hatched insects were placed in ointment jars (16 oz) and were maintained on a diet of ground baby rabbit pellets contained in small boats constructed of filter paper. Water was supplied to each group in a shell vial (32 ml) filled with distilled water and stoppered with a cotton-wool plug. A chloroform solution of each fatty acid or methyl ester was prepared at a concentration of O-062 M (preliminary experiments had shown this to be an effective *This work was supported by a grant from the National Research Council of Canada. 79
1247
1248
J. E. MCFARLANE AND G. 0. HENNEBERRY
concentration of the most active chemical-methyl palmitate). One ml of the test solution was adsorbed by a piece of folded filter paper (6 x 1.5 in.), and the chloroform evaporated in a current of warm air before the paper was placed in the ointment jar. Vitamin E (0,042 mg/ml of chloroform) was added as an antioxidant except in the dietary experiment: tests showed that vitamin E had no effect on the results obtained with saturated acids. All fatty acids and esters were of the highest purity obtainable from Nutritional Biochemicals Corp., Cleveland, Ohio. Each test was carried out in replicates of six, using ten insects per treatment, for a total of sixty insects per acid or ester tested. Means and standard deviations are given. An asterisk beside the mean value denotes that there was a significant difference between the control at P = 0.01 and the mean. Larvae were weighed after about 11 days and 21 days. Only a few of these results are presented, however, to give an idea of the kind of result obtained. RESULTS
Experiment I In Experiment I, various methyl esters were tested. survival of the larvae after 21 days is as follows:
Treatment Methyl Methyl Methyl Methyl Methyl Methyl Methyl Methyl Methyl Control
caprylate caprate laurate myristate palmitate stearate oleate linoleate linolenate
No. 58 56 56 55 45 51 41 54 57 57
The average
weights
and
Mean wt. (mg) 68-l+ 63.0 + 62.0 4 47.4 + 33.3 + 48-l_+ 41.9 + 60.2 + 67.4 4 65.8 +
4.6 2.5 8.8 12.1 6.4* 6.5 9.7” 3.8 8.7 10.0
Methyl palmitate especially, and also methyl oleate, significantly retarded While methyl stearate and methyl growth, and resulted in lower survival. the results were significant only myristate certainly gave some inhibition, at the 0.02 and 0.05 levels respectively. The results for 11 days were similar, which suggests that the effect is perhaps operative only during early postembryonic life. The results for the adults are given in Table 1. Here methyl palmitate and methyl stearate gave a significantly longer female larval period ; methyl myristate and methyl oleate were similarly effective only at the 0.02 level. The weight of males treated with methyl stearate was reduced. The survival with methyl palmitate and methyl oleate was very poor.
INHIBITION
OF THE GROWTH OF AN INSECT BY FATTY
1249
ACIDS
TABLE I--AVERAGE WEIGHTOF THE ADULTS( d 24 hr old), AWRAGEDURATION OF THE LARVAL STAGE,ANDPERCENTAGE SURVIVAL TO THBADULTSTAGE. EWERIMENT I Adult females obtained
Aduit males obtained
Treatment
Methyl Methyl Methyl Methyl Methyl Methyl Methyl Methyl Methyl Control
caprylate caprate laurate myristate palmitate stearate oleate linoleate linolenate
No.
19 21 25 18 17 19 8 13 16 20
Mean wt. (mg) 179.4 186.9 179.2 192.7 183.7 166.6 176.3 173.2 178.5 194.9
* 34.1 + 34.2 2 32.8 f 37.6 k49.6 + 28.6* + 42.6 f 28.1 + 40.9 + 34.6
Mean duration larval stage (days) 37.5 37.5 38.4 39.3 41.8 39.2 42.6 39.3 36.6 39.0
f 4.5 + 3.0 f 4.8 f 3.8 f 7.1 + 4.6 + 5.4 f 8.9 + 4.1 + 3.7
No.
25 24 24 27 12 27 24 28 33 23
Mean wt. (mg) 238.3 247.0 228.6 235.4 226.7 229.4 224.3 223.6 231.1 231.6
f 34.3 f 40.2 + 40-2 f 40.2 k 33.3 + 29.8 + 34.2 f 60.6 + 39.3 + 36.4
Mean duration larval stage (days) 33.5 34.1 33.8 36.3 37.4 35.4 36.5 34.9 33.8 33.5
& 3.0 + 3.3 + 2.9 f 5.1 f 4.8* + 2.0* f 5.0 + 6.7 f 3.6 f 2.8
%
Survival 73 75 82 75 48 77 53 68 82 72
Experiment II In Experiment are as follows:
II, various free fatty acids were tested. The results for 11 days Treatment Methyl palmitate Laurie acid Myristic acid Palmitic acid Oleic acid Elaidic acid Stearic acid Linoleic acid Behenic acid Control
No. 30 55 48 60 55 53 58 60 56 58
Mean wt. (mg) 5.8 + 1-o* 10.6 + 1.5* 8-O+ l-2* 13-l * 0.8 13*3 2 0.4 12.9 f O-9 14.0 * 1-o 13.5 f O-8 12.9 f 1.0 13.8 + 0.9
The results at 21 days were similar. These results differ greatly from those of Experiment I. None of the fatty acids, with the exception of myristic acid, corresponding to effective methyl esters, were inhibitory. And, conversely, free lauric acid, whose methyl ester was not inhibitory, gave reduced growth. The adult results are given in Table 2. The duration of the male larval period was prolonged with methyl palmitate and with myristic acid. It was also prolonged with steak and behenic acids, which did not significantly affect growth to 21 days. Laurie acid affected the male larval period only at the O-02 level. The female
1250
J. E. MCFARLANEm~G.0.
HENNEBERRY
larval stage was significantly lengthened only with methyl palmitate. poor with methyl palmitate and myristic acid.
Survival was
TABLE 2-AWRAGE WEIGHTOF THEADULTS( < 24 hr old), AVERAGEDURATIONOF THE LARVAL
STAGE,AND PERCENTAGESURVIVALTO THE ADULT STAGE.EXPERIMENTII Adult males obtained
Treatment
Methyl pahnitate Laurie acid Myristic acid Pahnitic acid Oleic acid Elaidic acid Stearic acid Linoleic acid Behenic acid Control
No.
Mean wt. (mg)
13 22 17 22 20 19 22 29 21 23
200.3 + 35.6 197.6 zk27.9 192.1 + 55.6 187.7 f 47.4 187.8 f 24.3 213.8 f 34.1 190.5 f 34.8 187.8 f 35.7 206*3+36.9 199.6 f 36.7
Adult females obtained
Mean duration larval stage (days) 45.2 f 7*9* 37.9 f 4-l 43.7 f 6.7* 36.8 +_8.7 36.8 f 4.7 38.5 + 7.5 39.0 + 4.0* 36.2 + 4.4 38.7+5-l* 35.2 + 2.5
No.
Mean wt. (mg)
Mean duration larval stage (days)
15 20 19 28 31 27 30 25 28 28
219.2k31.4 255.1 574.7 232.9f65.5 228.0 + 39.6 244.3 f 28.9 228.2f36.4 238.6f43.4 230.9 + 45.0 233.9 f 44.2 233.Ok42.1
37.2+3.1* 36.6 k7.6 36.5k7.8 33.0+ 3.7 31.4 f 3.1 32.4k4.0 32.8k2.5 32.6 f 7.3 32.6 + 2.5 32.3 &-5.1
% Surviva1 47 70 60 83 85 77 87 90 82 85
TABLE ~-AVERAGE WEIGHTOFTHJZADULTS(~~~~~ ~~~),A~~GED~~TIoN OFTHELARVAL STAGE,
AND
PERCENTAGESURVIVALTOTHE ADULT STAGE.EXPERIMENTIII Adult males obtained
Adult females obtained
No.
Mean wt. (mg)
Mean duration larval stage (days)
39.9 + 5.0 37.3 f 3.5 38.4 f 3.9
29 33 25
229.8 + 40.9 242.4 f 36.5 238.1 k 45.7
33.9 + 3.0 33.3 + 2.9 32.5 + 2.5
87 88 83
183.6 f 32.3
36.6 + 4.6
28
244.5 f 34.5
33.9 f 2.9
82
15
169.2 f 34.9
39.7 + 6.0
33
226.1 + 49.8
33.4 f 2.4
80
19 10 26 21 20
180.3 179.3 185.0 184.1 180.6
38.3 f 42.1 + 38.0 + 38.8 + 37.2 +
34 33 28 30 33
225.9 234.0 242.6 238.7 239.9
33.2 36.2 33.8 33.1 33.3
88 72 90 85 88
Diet
No.
Mean wt. (mg)
1% Myristic acid 0.1% Myristic acid 0.01% Myristic acid 1% Methyl palmitate 0.1% Methyl palmitate O-01% Methyl palmitate 1% Laurie acid 0.1% Laurie acid 0.01% Laurie acid Control
23 20 25
165.1 f 30.4 187.5 + 31.4 189.0 f 32.8
21
f f k f f
20.5 36,4 33.6 31.1 27.4
Mean duration larval stage (days)
3.7 5.3* 5.0 3.3 3.0
+ 33.9 f 37.5 + 53.8 + 40.7 f 37.8
f + + + f
2.7 2,6* 3.6 3.3 3.1
% Surviva1
INHIBITION
OF THE GROWTH OF AN INSECT BY FATTY
ACIDS
1251
Experintent III In this experiment, different levels of three hitherto effective chemicals were The adult results are presented in Table 3. Laurie acid at the 1% level gave prolonged male and female developmental periods. Laurie acid at the 1o/o level also inhibited growth significantly when weights were taken at 11 and 21 days. It was the only effective chemical. Repetition of this experiment gave similar results, but lauric acid at the 1% level was less effective than in Experiment III.
fed to the insects.
Experiment IV In a preliminary experiment, using a strain selected for the winged form of this polymorphic species, methyl palmitate not only prolonged the male and female larval periods significantly, but also reduced the incidence of wing development. Methyl palmitate gave 9 per cent winged forms (survivors = 46/60), whereas the control gave 36 per cent winged forms (survivors = 55/60). DISCUSSION
AND CONCLUSIONS
It seems likely that the effective chemicals entered the body through the cuticle of the tarsi as the larvae walked over the filter paper. This is concluded because lauric acid was the only effective chemical when presented in the diet (and at a high level), and because the filter paper with adsorbed chemicals was not eaten in any experiment. As the effective chemicals very likely entered the haemolymph, the gut must play a role in their detoxification when they are presented in the diet. With methyl palmitate, of course, hydrolysis in the gut would probably be sufficient to detoxify it. The effect seems to have been mainly on the young larvae, except in the case of stearic and behenic acids, where inhibition did not show up until after 21 days of growth. Either the chemicals were only effective on young larvae, or the young larvae absorbed relatively more than older ones. These possibilities could be investigated by determining just how much of the chemicals actually enters the body, and by testing the effect of injecting known amounts into the body. Speculation on other results is probably not useful at the present time. All the effective free fatty acids were saturated. Wing development was inhibited, as well as growth, by methyl palmitate. Both males and females were affected by methyl palmitate, and by lauric acid when fed. Otherwise only the males were affected by free fatty acids, and only the females by the esters except for methyl stearate. Adult weight was usually not affected. All these results suggest that fatty acids and/or their esters might play a role in the control of growth and development, and not be merely a passive reserve of energy. Any or all of the fatty acids might be involved. This is in addition to the polyunsaturated fatty acids-especially linoleic and linolenic-which have been shown to be absolutely required by a number of insects (FAST, 1964). While none of the polyunsaturated acids is required in the diet of Gryllodes sigillatus,
1252
J. E. MCFARLANEANDG. 0. HENNEBERRY
they probably are essential in a metabolic sense. Changes in the fatty acid composition of insect fat during growth and development have been shown to occur in a number of insects (FAST, 1964), but no meaning has as yet been attributed to them. The changes in the depot fat may not, however, be as meaningful as changes in the circulating fat. The work of SLAMA (1962) on the effect of fatty acids when topically applied to the wing pads of larval Pyrrhocoris apterus is interesting in this connexion. SlQma found that certain fatty acids inhibited wing growth whereas others stimulated it. His primary interest was in demonstrating the juvenile hormonelike activity of these compounds. However, the effects he obtained were restricted to the area of application, rather than being systemic as in our work. Further, we make no claim as to the specific endocrinological relationships of the fatty acids. We intend to follow carefully the changes in depot fat and circulating fat The amount of effective chemical which peneduring growth and development. trates into the insect and its fate will also be determined. The effects of injecting fatty acids and their esters will be investigated. REFERENCES FAST P. G. (1964) Insect lipids: a review. Mem. mt. Sot. Canada No. 37, 50 pp. MCFARLANEJ. E. (1964) Interaction between two species of crickets reared together. Canad. J. Zool. 42, 689-691.
SLAMAK. (1962) The juvenile hormone-like effect of fatty acids, fatty alcohols and some Acta Sot. mt. &chosIov. 59, 323-340. other compounds in insect metamorphosis.