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The effect of relative humidity on egg-laying, hatching, and survival in various spider mites
J. ins. Physiol.,
1958, Vol. 2, pp. 65 to 72.
Pergamon Press Ltd., London.
THE EFFECT OF RELATIVE HUMIDITY ON EGG-LAYING, HATCHING, AND SURVIVAL IN VARIOUS SPIDER MITES* H. BRUCE Louisiana
State University, (Received
BOUDREAUX Baton
Rouge,
6 September
Louisiana,
U.S.A.
1957)
Abstract-Mites ovipositing in a dry atmosphere lay more eggs at a higher rate and live longer than do mites ovipositing in a near saturated atmosphere. Hatching of eggs is apparently not greatly affected by extremes of humidity. Newly hatched mites survive poorly in a moist atmosphere. The results are explained on the basis of the ability to ingest larger amounts of, and to utilize more, food in a dry atmosphere through the elimination of body moisture by evaporation from the cuticle. It is suggested that spider mites may be more easily controlled in greenhouses if a high relative humidity is maintained.
IT has long been known that outbreaks of spider mite infestations are favoured by hot, dry weather, and that highly humid conditions tend to suppress major damage to plants by spider mites, presumably as a result of reduction of the population. LINKE (1953) has shown that rainfall, beating the active stages off plant leaves, cannot explain entirely the decrease in populations of Tetranychus telurius (L.) (= T. akhaeae v. Hanst.) on hops in Germany. He indicated that constant high relative humidity has the effect of causing the death of spider mites during the “chrysalis” stages (the inactive stages immediately preceding moults). The exact cause of this lethal effect is unknown. HUECK et al. (1952) surmised that egg production in Metatetranychus ulmi (Koch) was suppressed by high humidity. RODRIGUEZ (l954) h as shown that the uptake of radioactive phosphorus from plants by laying females of T. telarius is quite high, and is correlated with the process of egg-laying. Much of the ingested phosphorus becomes deposited in the eggs. This indicates that a great deal of feeding occurs during oviposition. Observations during culture of several species of Tetranychus in our laboratory tend to confirm this observation of a high rate of feeding during the oviposition period. For instance, a young cotton cotyledon, measuring about 14 in. in diameter, can support approximately 50 mites during nymphal growth with very little deterioration or loss of colou:r over a period of 8-12 days at summer temperatures, but as soon as females on such a culture mature and begin egg-laying, the leaf deteriorates rapidly, losing most of its food value to the mites in three days. In addition, I have maintained large numbers (up to 40) of sterile interspecific hybrids on similar cotton cotyledon cultures for one week without their causing any noticeable deterioration of the plant. Similarly, when only males are produced in cultures * Acknowledgement is made for the support of this research by the Agricultural Station of Louisiana State University and the Freeport Sulphur Company. 65
Experiment
66
H. BRUCEBOUDREAUX
from unfertilized mothers, there is no resulting deterioration of the host plant such as occurs when ovipositing females are present. In 1953, Dr. G. WHARTON, of the University of Maryland, suggested in a conversation that high relative humidity decreases the rate of feeding in mites, while low relative humidity has the effect of increasing the feeding rate. The basis for this is the fact that small objects have a high surface area to volume ratio. Spider mites are rarely over one millimetre in diameter. In Dr. WHARTON’Sview, the loss of water from the body by evaporation through the cuticle, results in tissue dehydration, which is compensated for by more intensive feeding if the atmospheric relative humidity is low. Spider mites take their food in liquid form. On the other hand, a high relative humidity would interfere with water loss (and concentration of nutrients) and result in lessened feeding activity. Thus in hot, dry weather, the reproductive potential would be elevated by the combination of higher physiological activity at high temperature (LINKE, 1953) and increased food intake as a result of water loss. Since the egg-laying rate is apparently correlated with the feeding rate, experiments were designed to measure the effects of relative humidity upon feeding as expressed by egg-laying, and at the same time observations were made upon hatching and survival of nymphs under extremes of relative humidity.
MATERIALS
AND METHODS
Six species of spider mites were employed in this study : Tetranychus cinnabarinus (Bois.), T. telarius (L.), T. Zobosus Boudreaux, T. tumidus Banks, T. desertorum Banks and T. gloveri Banks. * Stock cultures of each species were maintained on the cotyledons of cotton seedlings whose roots rested in tap-water. The experiments were performed at uncontrolled summer laboratory temperatures, fluctuating between 75°F and 95”F, during the months of June, July, August, and September, and different species were tested at different times. However, the tests at high and at low relative humidities for each species were carried on simultaneously. In one test, diapausing females of T. cinnabarinus were used. These had been induced to undergo diapause by rearing them from eggs in a constant temperature chamber held at 58”F, in which a controlled photoperiod of 8 hr per day was established. Diapause was terminated by bringing these females (which had not laid eggs) into the laboratory after it was apparent that the mites were in diapause as evidenced by inactivity, lack of oviposition and the appearance of an orange colour of the body, and allowing them to feed upon cotton plants as described below. The reason for using these also in the test was to determine if there was a possible correlation between the fact that diapausing females lack cuticular lobes on the skin folds such as are present on summer females (PRITCHARD and BAKER, 1952 ; BOUDREAUX,1956) and the assumption that these lobes may * Z’etranychus gloveri Banks has been recognized recently in our area. Information in favour of resurrecting this species from the synonymy of T. tumidus Banks (BAKER and PRITCHARD,1953) is in press.
THE EFFECT OF RELATIVE
HUMIDITY
ON VARIOUS
67
SPIDER MITES
be a factor in increasing even more the evaporative surface on a female mite, thus affecting feeding activity as outlined in the introduction. For maintaining different degrees of relative humidity over the test animals, rearing cells were devised in which silica gel was used to maintain a fluctuating relative humidity measured to range downwards from 35 per cent. The silica gel was changed. every 2 days, and while it was fresh the relative humidity approached zero, but it rose to nearly 35 per cent during the ensuing 2 days. Similar cells containing water provided a constant relative humidity measured as ranging from 95 to 100 per cent. TABLE
I-SUMMARY
OF EFFECTS SURVIVAL
OF RELATIVE HUMIDITY UPON EGG-LAYING. IN VARIOUS SPECIES OF ~&UfZJ&Zus
HATCHING
AND
-
I Species
Relative
No. of
1tumidity
mites
--
@asI mite
Average
1Laying days/ mite
Qas/ dayi 16.811 7.7 8.8
mite
Per cent eggs hatched
?er cent dead nymphs
--
Cinnabarims* (summer
99)
Cinnabarimst (diapause
Telariust,
99)
1
Dry Wet
123.511 49.8
10.71/
Dry Wet
70.411 19.0 92.91: 39.8 116.611 57.5 26.3 23.3 66.811 52.4 40.611 21.2
F,,
Dry Wet
Lobosust
Dry Wet
Tumidus*
Dry Wet
Tumidzrst
Dry Wet
Desertortm t
Dry Wet
Gloverit
Dry Wet
1;.:,, I:.;,, 9.5 4.9 4.6 10.1/l
z::,, 4.9 7.811 6.1 5.4 5.0 2:;
--84.13 74.1 83.0 81.4 0.0 3kZl, 68.0 70.2 70.6 tes diet
C8,, 5.111 4.4 4.7 Nne laid eggs. All test I laying 18-One female laid 18 eggs, none of hatched. Others died before laying
-3.211 22.4 3.511 11.7 --100.0 ---
which
-
* This series was tested using small cotyledons of DPL-15 cotton as host plants. t This series was tested using large cotyledons of Empire cotton as host plants. $ The 5 species from hereon were represented by summer females. $ Significant at 5 per cent. 11Significant at 1 per cent.
Test cells were prepared by boring a 4 in. diameter hole in the centre of a Petri dish by means of a steel tube chucked into an electric drill, working in a lump of automotive valve-grinding compound. A 1 in. length of rubber tubing was slipped halfway into the hole, and formed a seal with the dish. The dish could thus hold either silica gel or water. A young cotton plant in the cotyledon stage was removed from soil in germination boxes and inserted through the rubber tubing so the roots could be immersed in water held in a bottle below a supporting table.
H. BRUCE BOUDREAUX
68
This supporting table consisted of a sheet of & in. plywood, bored to pass the stems of the plants and supported by short legs. Each dish was covered by a crystallizing dish which replaced the Petri dish cover. Thus there was some air leakage into the chamber through the edge of the dish resting on the table, and through the hole in the rubber tubing. This leakage seemed to be negligible for the purposes of the experiment (Fig. 1).
A. B. C. D. E. F. G. H.
FIG. 1. Crystallizing dish used for cover. Tanglefoot barriers on expanded cotton cotyledon. Moisture-control substance (water or silica gel). Bored Petri dish. Portion of supporting plywood table. Rubber tubing. Cotton seedling, cotyledon stage. Tap-water in bottle below table.
Each cotyledon was ringed with Tanglefoot, and divided in half by a line of Tanglefoot, so that four confined areas were available per plant. On each such area was placed a female mite in the deutonymph stage. When these moulted into the imago, they were observed daily, during which eggs were counted and observations made upon egg-hatching and survival of the offspring. Some recentlyemerged females were lost in the Tanglefoot before laying began, and were immediately replaced with deutonymphs. Others which were entangled later were not used in computing egg-laying data, but the eggs and young were kept under observation. In experiments with T. cinnabarinus eggs were removed daily as they were counted. In other experiments the larvae were allowed to develop until they
THE
EFFECT
OF RELATIVE
HUMIDITY
ON
VARIOUS
SPIDER
MITES
69
matured. Some were lost by becoming entangled in the barrier, and were not used in computing “per cent dead nymphs” as shown in Table 1. A second type of experiment was performed to test the concept that the cuticle of mites is easily permeable to water. Fifteen teneral summer females of T. telurius were confined in a small shell vial loosely plugged with cotton and the preparation was placed in a covered dish containing water. Another group of 15 teneral females was confined in a similar vial and the preparation was placed in a covered dish containing activated silica gel. These 2 preparations simulated the oviposition test conditions. The mites were examined daily until all were dead. In a third type of experiment an attempt was made to determine if mites living in a dry atmosphere accumulated more solids in their bodies than would mites living in a moist atmosphere. Six cotton seedlings were prepared in such a way that mites could move freely from one cotyledon to the other, or from the upper surface to the lower surface. Each plant was wrapped with cotton at the point where it passed through the rubber tube (Fig. 1). Just above this the stem was ringed with Tanglefoot. Forty teneral summer females of T. telarius were placed on each plant. Three plants (120 female mites) were installed in dry chambers as previously explained, and the other 3 (120 female mites) were installed in moist The plants in the moist chambers were left undisturbed for 4 days. chambers. Mites on the plants in dry chambers were transferred to new plants after 2 days, and fresh silica gel was used to replace the original. On the fourth day, all the mites remaining in each series were weighed in a group in a weighing bottle on a laboratory balance sensitive to 10 pg. Both groups were dried in an oven at 6O”C, and weighed again. The loss in weight was converted to per cent moisture. The eggs laid by each group were counted at the end of the experiment. RESULTS
The data summarized in Table 1 indicate that conditions of constant high relative humidity allow for the production of fewer eggs at a lower rate, and for a shorter life-span of egg-laying females than a constant low relative humidity in T. cinnabarinus, T. telarius, T. lobosus, and T. desertorum. The exceptions, T. tumidus and T, gloveri, are unexplained. The optimum relative humidity for each species has not been established, although uncontrolled laboratory atmospheric humidity permits successful stock culturing of all species. Where a small cotyledon was used for testing T. tumidus (one of the first of the series of experiments), the wet and dry atmospheres had the same effect, but on the large cotyledons of the Empire variety, which are about twice as large as those of Deltapine 1.5 cotton, the results were similar to the other species. In T. gloveri, all the deutonymphs except 1 failed to transform into adults under the test conditions, and that 1 laid 18 eggs, none of which hatched. In these experiments in which hatching rate of the eggs was noted, the hatchability of the eggs did not seem to be affected differently by either moisture condition, except for T. tumidus tested on Empire cotton. An important effect of high relative humidity is indicated in the survival of Significantly larger numbers of young mites died under wet hatched young.
H. BRUCE BOUDREAUX
70
conditions than did under dry conditions. With few exceptions death occurred during the first “resting” stage (protochrysalis) following the larval stage. The 15 teneral females confined without food in a dry chamber were all dead and shrivelled in 2 days. The 15 females in the moist chamber were all dead in 6 days, but their bodies did not shrink appreciably after death. In the third type of experiment, after 4 days only 41 live females remained on the plants in moist chambers. A total of 339 eggs were counted on these 3 plants. The 41 live females weighed 600 pg. After drying, these weighed 130 pg. The water-loss upon drying amounted to 470 pg or 78.3 per cent. At the same time, 80 live females remained on the plants in the dry chambers. A total of 2215 eggs were counted on these 3 plants. The 80 live females weighed 1460 pg. After drying these weighed 310 pg. Th e water-loss upon drying amounted to 1150 pg or 78.8 per cent (Table 2). TABLE
~--SUMMARY
OF EXPERIMENT TO DETERMINE BODY MOISTURE CONTENT OF MITES FED UNDER EXTREMES OF RELATIVE HUMIDITY FOR 4 DAYS
I Condition of test
Dry Wet
I Original no. of mites 120 120
I
I
I 118
Final no. alive after 4 days
Eggs laid
Live weight
Dry weight
Per cent body moisture
80 41
, DISCUSSION
The results of these experiments tend to confirm the hypothesis that high relative humidity interferes with feeding of ovipositing mites by preventing loss of moisture from the body by evaporation. The cuticular lobes on the folds covering the body apparently increase the rate of evaporation of body moisture through the increased evaporative surface provided, as can be inferred from the behaviour of the female mites lacking such lobes (emerged diapause females) compared to active summer females possessing the lobes. One defect of this experiment is that different varieties of cotton were used as host plants for comparing diapause with non-diapause females. When T. tumidus was tested on both varieties of cotton, different results were obtained in each case. It may be that the two varieties of cotton (differing mostly in the cotyledon stage by size differences) have different effects upon relative humidity in the test cells, and that neither extreme of humidity represents an optimum condition, particularly for T. tumidus and T. gloreri. These 2 species, incidentally, differ from the other species studied in having cuticular lobes which are larger and cover more of the body ventrally (all over the entire body surface except the ventral side of the gnathosoma) than the other species. Roth species are larger than the others except T. desertorum, whose cuticular lobes are less extensive ventrally, as in T. Zobosuswhich is much smaller in body size. The remaining species, T. telarius and T. cinnabarinus, completely lack any ventral
THE EFFECT OF RELATIVE HUMIDITY
ON VARIOUS SPIDER MITES
71
cuticular lobes. However, the lobeless diapausing females showed a much greater suppression of oviposition by a wet atmosphere than did the active summer females bearing cuticular lobes. The lack of lobes on the cuticle of diapausing mites may be an important factor in survival by minimizing desiccation during the inactive non,-feeding period. Another indication of the high rate of feeding at low humidity was shown in the appearance of the faeces of the ovipositing mite. Those at low humidity voided black faecal .pellets, which shrank very little upon drying. The mites under high humidity voided many liquid colourless faecal pellets which shrank slowly to white tiny specks upon drying, or the pellets contained only small amounts of the black faecal material. Also, females of the dry series which died between observation periods were found shrivelled and dry at the next observation period, while dead females in the wet series remained plump after death for several days, indicating of cuticular evaporation that cuticular evaporation does occur. Another indication is apparent in the results of the second type of experiment, in which unfed mites died and dried up more quickly in the dry chamber than in the moist chamber. The high mortality of larvae during the protochrysalis stage may be the result of the inabilzty to store adequate food reserves needed for moulting when reared The results of these experiments seem to in an atmosphere high in moisture. suggest that water-loss by evaporation from the cuticle is necessary for the most favourable utilization of the liquid cell contents ingested by mites. This would provide for the ingestion of large quantities of plant liquids and the concentration of the nutrients, avoiding a rapid passage through the alimentary canal. This is an odd counterpart of the role of the production of honeydew by homopterous insects through filtering action of the midgut, although there is some doubt that honeydew production helps nutrient concentration (EWART and METCALF, 1956). It is not intended to imply that the only effect of water-loss by cuticular evaporation is to allow more food to be ingested and used. There are probably other unknown ph;fsiological effects involved. The ovipositing adult mites probably do not store much food reserves, as is indicated by 1:he apparent similarity in the dry matter/moisture ratio under different humidity conditions. The rapid utilization of food in a dry atmosphere is reflected in its conversion into eggs at a much higher rate than happens under moist conditions. It is noticed from Table 2 that mites from the dry chamber were larger than those from the moist chamber: Average = 1X.25 tJ_gvs. average = 14.63 pg. The “dry” females were visibly larger when transferred for weighing. It has frequently bt,en observed that healthy females from stock may have l-2 eggs at a time fully developed in the oviduct. The larger size of the “dry” mites is probably the result of their having a few eggs of nearly full size developing together in the oviducts. The results of the experiments also suggest that it may be possible to minimize spider-mite damage, to greenhouse plants which can withstand water-saturated atmospheres, particularly in such ranges where spider mites have become resistant to acaricides, by maintaining extremely high relative humidities in the greenhouse.
H. BRUCE BOUDREAUX
72
MUNGER’S (1955,1956) findings rooms enhanced the production result
of more
change,
through
favourable
that fresh air or purified air circulated in rearing of the citrus red mite on lemons, may partly be the
conditions
the same effect
for food
postulated
utilization
in the present
brought
about
by air
paper.
REFERENCES BAKER E. W. and PRITCHARDA. E. (1953) A guide to the spider mites of cotton. Hilgardiu 22, 203-234. BOUDREAUX H. B. (1956) A revision of the two-spotted spider mite (Tetranychus telurius L.) complex (Acarina, Tetranychidae). Ann. ent. Sot. Amer. 49, 43-R EWART W. H. and METCALF R. L. (1956) Preliminary studies of sugars and amino acids in the honeydews of 5 species of coccids feeding on citrus in California. Ann. ent. Sot. Amer. 49, 441447. HUECK H. J., KUENEN D. J., DEN BOER P. J., and JAEGER E. (1952) The increase of egg production of the fruit tree red spider mite (Metatetranychzls zdmi Koch) under influence of DDT. Physiol. camp. 2, 371-377. LINKE W. (1953) Investigation of the biology and epidemiology of the common spider mite, Tetranychus althaeae v. Hanst. with particular consideration to hops as the host. HofchenBriefe Bayer PJEanz. Nuchr. 6, 181-232. MUNGER F. (1955) Rearing citrus red mites in the laboratory. r. econ. Ent. 48, 72-74. MUNCER F. (1956) Activated-carbon filter for purification of air for rearing citrus red mite. r. econ. Ent. 49, 138. PRITCHARDA. E. and BAKER E. W. (1952) A guide to the spider mites of deciduous fruit trees. Hilgurdiu 21, 253-287. RODRIGUEZJ. G. (1954) Radiophosphorus in metabolism studies in the two-spotted spider mite. r. econ. Ent. 47, 514-517.
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1958, Vol. 2, pp. 65 to 72.
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THE EFFECT OF RELATIVE HUMIDITY ON EGG-LAYING, HATCHING, AND SURVIVAL IN VARIOUS SPIDER MITES* H. BRUCE Louisiana
State University, (Received
BOUDREAUX Baton
Rouge,
6 September
Louisiana,
U.S.A.
1957)
Abstract-Mites ovipositing in a dry atmosphere lay more eggs at a higher rate and live longer than do mites ovipositing in a near saturated atmosphere. Hatching of eggs is apparently not greatly affected by extremes of humidity. Newly hatched mites survive poorly in a moist atmosphere. The results are explained on the basis of the ability to ingest larger amounts of, and to utilize more, food in a dry atmosphere through the elimination of body moisture by evaporation from the cuticle. It is suggested that spider mites may be more easily controlled in greenhouses if a high relative humidity is maintained.
IT has long been known that outbreaks of spider mite infestations are favoured by hot, dry weather, and that highly humid conditions tend to suppress major damage to plants by spider mites, presumably as a result of reduction of the population. LINKE (1953) has shown that rainfall, beating the active stages off plant leaves, cannot explain entirely the decrease in populations of Tetranychus telurius (L.) (= T. akhaeae v. Hanst.) on hops in Germany. He indicated that constant high relative humidity has the effect of causing the death of spider mites during the “chrysalis” stages (the inactive stages immediately preceding moults). The exact cause of this lethal effect is unknown. HUECK et al. (1952) surmised that egg production in Metatetranychus ulmi (Koch) was suppressed by high humidity. RODRIGUEZ (l954) h as shown that the uptake of radioactive phosphorus from plants by laying females of T. telarius is quite high, and is correlated with the process of egg-laying. Much of the ingested phosphorus becomes deposited in the eggs. This indicates that a great deal of feeding occurs during oviposition. Observations during culture of several species of Tetranychus in our laboratory tend to confirm this observation of a high rate of feeding during the oviposition period. For instance, a young cotton cotyledon, measuring about 14 in. in diameter, can support approximately 50 mites during nymphal growth with very little deterioration or loss of colou:r over a period of 8-12 days at summer temperatures, but as soon as females on such a culture mature and begin egg-laying, the leaf deteriorates rapidly, losing most of its food value to the mites in three days. In addition, I have maintained large numbers (up to 40) of sterile interspecific hybrids on similar cotton cotyledon cultures for one week without their causing any noticeable deterioration of the plant. Similarly, when only males are produced in cultures * Acknowledgement is made for the support of this research by the Agricultural Station of Louisiana State University and the Freeport Sulphur Company. 65
Experiment
66
H. BRUCEBOUDREAUX
from unfertilized mothers, there is no resulting deterioration of the host plant such as occurs when ovipositing females are present. In 1953, Dr. G. WHARTON, of the University of Maryland, suggested in a conversation that high relative humidity decreases the rate of feeding in mites, while low relative humidity has the effect of increasing the feeding rate. The basis for this is the fact that small objects have a high surface area to volume ratio. Spider mites are rarely over one millimetre in diameter. In Dr. WHARTON’Sview, the loss of water from the body by evaporation through the cuticle, results in tissue dehydration, which is compensated for by more intensive feeding if the atmospheric relative humidity is low. Spider mites take their food in liquid form. On the other hand, a high relative humidity would interfere with water loss (and concentration of nutrients) and result in lessened feeding activity. Thus in hot, dry weather, the reproductive potential would be elevated by the combination of higher physiological activity at high temperature (LINKE, 1953) and increased food intake as a result of water loss. Since the egg-laying rate is apparently correlated with the feeding rate, experiments were designed to measure the effects of relative humidity upon feeding as expressed by egg-laying, and at the same time observations were made upon hatching and survival of nymphs under extremes of relative humidity.
MATERIALS
AND METHODS
Six species of spider mites were employed in this study : Tetranychus cinnabarinus (Bois.), T. telarius (L.), T. Zobosus Boudreaux, T. tumidus Banks, T. desertorum Banks and T. gloveri Banks. * Stock cultures of each species were maintained on the cotyledons of cotton seedlings whose roots rested in tap-water. The experiments were performed at uncontrolled summer laboratory temperatures, fluctuating between 75°F and 95”F, during the months of June, July, August, and September, and different species were tested at different times. However, the tests at high and at low relative humidities for each species were carried on simultaneously. In one test, diapausing females of T. cinnabarinus were used. These had been induced to undergo diapause by rearing them from eggs in a constant temperature chamber held at 58”F, in which a controlled photoperiod of 8 hr per day was established. Diapause was terminated by bringing these females (which had not laid eggs) into the laboratory after it was apparent that the mites were in diapause as evidenced by inactivity, lack of oviposition and the appearance of an orange colour of the body, and allowing them to feed upon cotton plants as described below. The reason for using these also in the test was to determine if there was a possible correlation between the fact that diapausing females lack cuticular lobes on the skin folds such as are present on summer females (PRITCHARD and BAKER, 1952 ; BOUDREAUX,1956) and the assumption that these lobes may * Z’etranychus gloveri Banks has been recognized recently in our area. Information in favour of resurrecting this species from the synonymy of T. tumidus Banks (BAKER and PRITCHARD,1953) is in press.
THE EFFECT OF RELATIVE
HUMIDITY
ON VARIOUS
67
SPIDER MITES
be a factor in increasing even more the evaporative surface on a female mite, thus affecting feeding activity as outlined in the introduction. For maintaining different degrees of relative humidity over the test animals, rearing cells were devised in which silica gel was used to maintain a fluctuating relative humidity measured to range downwards from 35 per cent. The silica gel was changed. every 2 days, and while it was fresh the relative humidity approached zero, but it rose to nearly 35 per cent during the ensuing 2 days. Similar cells containing water provided a constant relative humidity measured as ranging from 95 to 100 per cent. TABLE
I-SUMMARY
OF EFFECTS SURVIVAL
OF RELATIVE HUMIDITY UPON EGG-LAYING. IN VARIOUS SPECIES OF ~&UfZJ&Zus
HATCHING
AND
-
I Species
Relative
No. of
1tumidity
mites
--
@asI mite
Average
1Laying days/ mite
Qas/ dayi 16.811 7.7 8.8
mite
Per cent eggs hatched
?er cent dead nymphs
--
Cinnabarims* (summer
99)
Cinnabarimst (diapause
Telariust,
99)
1
Dry Wet
123.511 49.8
10.71/
Dry Wet
70.411 19.0 92.91: 39.8 116.611 57.5 26.3 23.3 66.811 52.4 40.611 21.2
F,,
Dry Wet
Lobosust
Dry Wet
Tumidus*
Dry Wet
Tumidzrst
Dry Wet
Desertortm t
Dry Wet
Gloverit
Dry Wet
1;.:,, I:.;,, 9.5 4.9 4.6 10.1/l
z::,, 4.9 7.811 6.1 5.4 5.0 2:;
--84.13 74.1 83.0 81.4 0.0 3kZl, 68.0 70.2 70.6 tes diet
C8,, 5.111 4.4 4.7 Nne laid eggs. All test I laying 18-One female laid 18 eggs, none of hatched. Others died before laying
-3.211 22.4 3.511 11.7 --100.0 ---
which
-
* This series was tested using small cotyledons of DPL-15 cotton as host plants. t This series was tested using large cotyledons of Empire cotton as host plants. $ The 5 species from hereon were represented by summer females. $ Significant at 5 per cent. 11Significant at 1 per cent.
Test cells were prepared by boring a 4 in. diameter hole in the centre of a Petri dish by means of a steel tube chucked into an electric drill, working in a lump of automotive valve-grinding compound. A 1 in. length of rubber tubing was slipped halfway into the hole, and formed a seal with the dish. The dish could thus hold either silica gel or water. A young cotton plant in the cotyledon stage was removed from soil in germination boxes and inserted through the rubber tubing so the roots could be immersed in water held in a bottle below a supporting table.
H. BRUCE BOUDREAUX
68
This supporting table consisted of a sheet of & in. plywood, bored to pass the stems of the plants and supported by short legs. Each dish was covered by a crystallizing dish which replaced the Petri dish cover. Thus there was some air leakage into the chamber through the edge of the dish resting on the table, and through the hole in the rubber tubing. This leakage seemed to be negligible for the purposes of the experiment (Fig. 1).
A. B. C. D. E. F. G. H.
FIG. 1. Crystallizing dish used for cover. Tanglefoot barriers on expanded cotton cotyledon. Moisture-control substance (water or silica gel). Bored Petri dish. Portion of supporting plywood table. Rubber tubing. Cotton seedling, cotyledon stage. Tap-water in bottle below table.
Each cotyledon was ringed with Tanglefoot, and divided in half by a line of Tanglefoot, so that four confined areas were available per plant. On each such area was placed a female mite in the deutonymph stage. When these moulted into the imago, they were observed daily, during which eggs were counted and observations made upon egg-hatching and survival of the offspring. Some recentlyemerged females were lost in the Tanglefoot before laying began, and were immediately replaced with deutonymphs. Others which were entangled later were not used in computing egg-laying data, but the eggs and young were kept under observation. In experiments with T. cinnabarinus eggs were removed daily as they were counted. In other experiments the larvae were allowed to develop until they
THE
EFFECT
OF RELATIVE
HUMIDITY
ON
VARIOUS
SPIDER
MITES
69
matured. Some were lost by becoming entangled in the barrier, and were not used in computing “per cent dead nymphs” as shown in Table 1. A second type of experiment was performed to test the concept that the cuticle of mites is easily permeable to water. Fifteen teneral summer females of T. telurius were confined in a small shell vial loosely plugged with cotton and the preparation was placed in a covered dish containing water. Another group of 15 teneral females was confined in a similar vial and the preparation was placed in a covered dish containing activated silica gel. These 2 preparations simulated the oviposition test conditions. The mites were examined daily until all were dead. In a third type of experiment an attempt was made to determine if mites living in a dry atmosphere accumulated more solids in their bodies than would mites living in a moist atmosphere. Six cotton seedlings were prepared in such a way that mites could move freely from one cotyledon to the other, or from the upper surface to the lower surface. Each plant was wrapped with cotton at the point where it passed through the rubber tube (Fig. 1). Just above this the stem was ringed with Tanglefoot. Forty teneral summer females of T. telarius were placed on each plant. Three plants (120 female mites) were installed in dry chambers as previously explained, and the other 3 (120 female mites) were installed in moist The plants in the moist chambers were left undisturbed for 4 days. chambers. Mites on the plants in dry chambers were transferred to new plants after 2 days, and fresh silica gel was used to replace the original. On the fourth day, all the mites remaining in each series were weighed in a group in a weighing bottle on a laboratory balance sensitive to 10 pg. Both groups were dried in an oven at 6O”C, and weighed again. The loss in weight was converted to per cent moisture. The eggs laid by each group were counted at the end of the experiment. RESULTS
The data summarized in Table 1 indicate that conditions of constant high relative humidity allow for the production of fewer eggs at a lower rate, and for a shorter life-span of egg-laying females than a constant low relative humidity in T. cinnabarinus, T. telarius, T. lobosus, and T. desertorum. The exceptions, T. tumidus and T, gloveri, are unexplained. The optimum relative humidity for each species has not been established, although uncontrolled laboratory atmospheric humidity permits successful stock culturing of all species. Where a small cotyledon was used for testing T. tumidus (one of the first of the series of experiments), the wet and dry atmospheres had the same effect, but on the large cotyledons of the Empire variety, which are about twice as large as those of Deltapine 1.5 cotton, the results were similar to the other species. In T. gloveri, all the deutonymphs except 1 failed to transform into adults under the test conditions, and that 1 laid 18 eggs, none of which hatched. In these experiments in which hatching rate of the eggs was noted, the hatchability of the eggs did not seem to be affected differently by either moisture condition, except for T. tumidus tested on Empire cotton. An important effect of high relative humidity is indicated in the survival of Significantly larger numbers of young mites died under wet hatched young.
H. BRUCE BOUDREAUX
70
conditions than did under dry conditions. With few exceptions death occurred during the first “resting” stage (protochrysalis) following the larval stage. The 15 teneral females confined without food in a dry chamber were all dead and shrivelled in 2 days. The 15 females in the moist chamber were all dead in 6 days, but their bodies did not shrink appreciably after death. In the third type of experiment, after 4 days only 41 live females remained on the plants in moist chambers. A total of 339 eggs were counted on these 3 plants. The 41 live females weighed 600 pg. After drying, these weighed 130 pg. The water-loss upon drying amounted to 470 pg or 78.3 per cent. At the same time, 80 live females remained on the plants in the dry chambers. A total of 2215 eggs were counted on these 3 plants. The 80 live females weighed 1460 pg. After drying these weighed 310 pg. Th e water-loss upon drying amounted to 1150 pg or 78.8 per cent (Table 2). TABLE
~--SUMMARY
OF EXPERIMENT TO DETERMINE BODY MOISTURE CONTENT OF MITES FED UNDER EXTREMES OF RELATIVE HUMIDITY FOR 4 DAYS
I Condition of test
Dry Wet
I Original no. of mites 120 120
I
I
I 118
Final no. alive after 4 days
Eggs laid
Live weight
Dry weight
Per cent body moisture
80 41
, DISCUSSION
The results of these experiments tend to confirm the hypothesis that high relative humidity interferes with feeding of ovipositing mites by preventing loss of moisture from the body by evaporation. The cuticular lobes on the folds covering the body apparently increase the rate of evaporation of body moisture through the increased evaporative surface provided, as can be inferred from the behaviour of the female mites lacking such lobes (emerged diapause females) compared to active summer females possessing the lobes. One defect of this experiment is that different varieties of cotton were used as host plants for comparing diapause with non-diapause females. When T. tumidus was tested on both varieties of cotton, different results were obtained in each case. It may be that the two varieties of cotton (differing mostly in the cotyledon stage by size differences) have different effects upon relative humidity in the test cells, and that neither extreme of humidity represents an optimum condition, particularly for T. tumidus and T. gloreri. These 2 species, incidentally, differ from the other species studied in having cuticular lobes which are larger and cover more of the body ventrally (all over the entire body surface except the ventral side of the gnathosoma) than the other species. Roth species are larger than the others except T. desertorum, whose cuticular lobes are less extensive ventrally, as in T. Zobosuswhich is much smaller in body size. The remaining species, T. telarius and T. cinnabarinus, completely lack any ventral
THE EFFECT OF RELATIVE HUMIDITY
ON VARIOUS SPIDER MITES
71
cuticular lobes. However, the lobeless diapausing females showed a much greater suppression of oviposition by a wet atmosphere than did the active summer females bearing cuticular lobes. The lack of lobes on the cuticle of diapausing mites may be an important factor in survival by minimizing desiccation during the inactive non,-feeding period. Another indication of the high rate of feeding at low humidity was shown in the appearance of the faeces of the ovipositing mite. Those at low humidity voided black faecal .pellets, which shrank very little upon drying. The mites under high humidity voided many liquid colourless faecal pellets which shrank slowly to white tiny specks upon drying, or the pellets contained only small amounts of the black faecal material. Also, females of the dry series which died between observation periods were found shrivelled and dry at the next observation period, while dead females in the wet series remained plump after death for several days, indicating of cuticular evaporation that cuticular evaporation does occur. Another indication is apparent in the results of the second type of experiment, in which unfed mites died and dried up more quickly in the dry chamber than in the moist chamber. The high mortality of larvae during the protochrysalis stage may be the result of the inabilzty to store adequate food reserves needed for moulting when reared The results of these experiments seem to in an atmosphere high in moisture. suggest that water-loss by evaporation from the cuticle is necessary for the most favourable utilization of the liquid cell contents ingested by mites. This would provide for the ingestion of large quantities of plant liquids and the concentration of the nutrients, avoiding a rapid passage through the alimentary canal. This is an odd counterpart of the role of the production of honeydew by homopterous insects through filtering action of the midgut, although there is some doubt that honeydew production helps nutrient concentration (EWART and METCALF, 1956). It is not intended to imply that the only effect of water-loss by cuticular evaporation is to allow more food to be ingested and used. There are probably other unknown ph;fsiological effects involved. The ovipositing adult mites probably do not store much food reserves, as is indicated by 1:he apparent similarity in the dry matter/moisture ratio under different humidity conditions. The rapid utilization of food in a dry atmosphere is reflected in its conversion into eggs at a much higher rate than happens under moist conditions. It is noticed from Table 2 that mites from the dry chamber were larger than those from the moist chamber: Average = 1X.25 tJ_gvs. average = 14.63 pg. The “dry” females were visibly larger when transferred for weighing. It has frequently bt,en observed that healthy females from stock may have l-2 eggs at a time fully developed in the oviduct. The larger size of the “dry” mites is probably the result of their having a few eggs of nearly full size developing together in the oviducts. The results of the experiments also suggest that it may be possible to minimize spider-mite damage, to greenhouse plants which can withstand water-saturated atmospheres, particularly in such ranges where spider mites have become resistant to acaricides, by maintaining extremely high relative humidities in the greenhouse.
H. BRUCE BOUDREAUX
72
MUNGER’S (1955,1956) findings rooms enhanced the production result
of more
change,
through
favourable
that fresh air or purified air circulated in rearing of the citrus red mite on lemons, may partly be the
conditions
the same effect
for food
postulated
utilization
in the present
brought
about
by air
paper.
REFERENCES BAKER E. W. and PRITCHARDA. E. (1953) A guide to the spider mites of cotton. Hilgardiu 22, 203-234. BOUDREAUX H. B. (1956) A revision of the two-spotted spider mite (Tetranychus telurius L.) complex (Acarina, Tetranychidae). Ann. ent. Sot. Amer. 49, 43-R EWART W. H. and METCALF R. L. (1956) Preliminary studies of sugars and amino acids in the honeydews of 5 species of coccids feeding on citrus in California. Ann. ent. Sot. Amer. 49, 441447. HUECK H. J., KUENEN D. J., DEN BOER P. J., and JAEGER E. (1952) The increase of egg production of the fruit tree red spider mite (Metatetranychzls zdmi Koch) under influence of DDT. Physiol. camp. 2, 371-377. LINKE W. (1953) Investigation of the biology and epidemiology of the common spider mite, Tetranychus althaeae v. Hanst. with particular consideration to hops as the host. HofchenBriefe Bayer PJEanz. Nuchr. 6, 181-232. MUNGER F. (1955) Rearing citrus red mites in the laboratory. r. econ. Ent. 48, 72-74. MUNCER F. (1956) Activated-carbon filter for purification of air for rearing citrus red mite. r. econ. Ent. 49, 138. PRITCHARDA. E. and BAKER E. W. (1952) A guide to the spider mites of deciduous fruit trees. Hilgurdiu 21, 253-287. RODRIGUEZJ. G. (1954) Radiophosphorus in metabolism studies in the two-spotted spider mite. r. econ. Ent. 47, 514-517.
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