RESPONSE OF ADULTS OF THE GRAIN MITE, ACARUS sma L. TO MODIFIED ATMOSPHERES'

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1 RESPONSE OF ADULTS OF THE GRAIN MITE, ACARUS sma L. TO MODIFIED ATMOSPHERES' S. NaV81'ro,2 O. Lider,2 and U. Gerson J Ab~lract: Mortality of newly emerged adults of Acarus siro L. was assayed by exposing them to 2, 4. 6,10.21% O2 in Nz. to 10, % CO2 in 21% O2 and with the balance of the atmosphere made up of Nz. Test.s were conducted at 75% relative humidity and 15 or 26"C in a specially designed apparatus. Mean duration of mite survival in air was 1Land 7 d at 15 and 26 C. respectively. Exposure to 2% O 2 was required to obtain 100% mortality with 72 h at l5"e; the same result was obtclined within 120 h at 26"C by exposure to 10% 02' At 15 C, 30% CO 2 was required for 96 h to achieve 100% mortality; at 26"C. 20% CO 2 or above produced the same result. within 72 h. Key Words: Acarus siro, grain mite, modified atmospheres. J. Agric. EntomoL 2(1): (January 1985) The modified atmosphere (MA) storage method involves alteration of the concentrations of the normal atmospheric gases, nitrogen (N 2 ), oxygen (0 2 ), and carbon dioxide (C0 2 ), to an atmosphere lethal to stored-product pests. Mortality of stored-product insects exposed to MAs has been extensively investigated by numerous workers (Bailey and Banks 1980). Field trials demonstrating the feasibility of using MA as an efficient control method to prevent insect damage were reported by Banks (1979), Jay and Pearman (1973) and Navarro et al. (1979). Alt.hough insects are considered to be the major pests of stored-products, under certain climatic and storage conditions mites also cause serious losses to stored grains and food (Hughes 1976). Mites are often associat.ed with grain and grain products stored in the temperate belt. of the northern hemisphere, where humid climatic conditions are favorable for mite development (Sinha 1979). A mite infestation renders the grain and its products unacceptable or unattractive to humans. The most economically important group of mites infesting food belong to the family Acaridae (Acaridida). The grain (or the nour) mite, AcaruB siro L., a member of this family, is a serious and widespread pest of grain products (Griffiths 1964; Sinha 1979; Solomon 1962). Acarus siro may complete its development. at temperatures of from 5 to 25 C at a relative humidity of 70% (Cunnington 1965). Little has been published on the effect of MAs on stored product mites (Bailey and Banks 1980). Hughes (1943) reported that complete mortality was obtained when A. siro was exposed for four d at 20 C to either 100% N 2 or 100% CO 2 Stepien (1974) studied the effect of 99.5% CO 2 on all developmental stages of the mould (or the copra) mite Tyrophagus putrescentiae (Schr.) (Acaridida). Complete mortality of all developmental stages (except eggs) was obtained during a 24-h exposure. Mitsura et a1. (1973) exposed T. pulrescentiae to four different gases at I Contrihution from the Agricultural H"seafch OrgonilPtion. The VolclOni Center, Bet Dagan, Jaf.-.el No. 402 E, 1982 ~erier;. Received for publication 8 February 198 1; llcccillml 7 JflllUllry DepllrluHHlI of Stored Products, Agricultural Research Organization, Het Dagon, Israel :1 Di'isioll of EnlOmolDg}, The Hehrew UniveTsity of Jeru811lem, faculty of AgricullUre. Rehovul, lonici. 61

2 62 J. Agrie. Entomol. Vol. 2. No_ 1 (1985) high pressures. With a CO 2 aunosphere at 16 kg/cm 2 pressure complete mortality was obtained in 10 min. The present study provides further basic data required to evaluate the feasibility of using atmospheres with low O 2 or moderately high CO 2 levels to control A. siro adults. MATERIALS AND METHODS Test Mites Acarus siro was obtained from infested peanuts stored in commercial silos in IsraeL Mites were reared in 130-ml jars containing 35g of wheat germ Oakes in the manner described by Griffiths (1966). The jars were tightly sealed with a metal lid which had the center removed and replaced by 325-mesh wire screen. The cultures were maintained at 15 (Ie in desiccators over a 29.6% H 2 S0 4 solution to maintain the relative humidity (r.h.) at about 75% (Solomon 1951). For each replicate of each experiment, ten newly emerged adults (five males and five females) were transferred into each exposure cell (Fig. I), to which two or three wheat germ flakes were added. Burkholder et al. (1966) indicated that a good criterion for the determination of newly emerged adults is the attachment of the male to the female. Using this criterion, pairs of adult mites were removed daily from the cultures using a fine paint brush. The cells were covered with a glass slide so that the mites could be observed without disturbing them. (a ) (b) ~glassslide metal clips I Perspex p~... 1 ~~;Q[ mesh screen Fig. 1. A diagram of (a) the parts and (b) the assembled cell used to expose Acarus siro adults to modified lltmospheres (perspex = plexiglass). Equipment used to obtain desired modified atmospheres Atmospheric gases were supplied from pressurized gas cylinders (Fig. 2). A specially designed apparatus was used to regulate and mix 02. N 2 and CO 2 in the

3 NA VARRO et a1.: Grain Mite Response to Modified Atmospheres 63 GAS FLOW DIRECTION 26 c ~ I--<,.". 15 C CHAMBEo'R~_l;:::==.---n rr1~c~h:am:b=e=r~~~... GAS WASHING BOTTLE FILTER r F.~~f-t-RH MEASURING CHAMBER TO AIR FlOWMET!::RS 0, \ / (,t.s CYLINDERS TO AOOlTlQNAl!::XF'ERIMENTAL FLASKS experimental FLASK Fig. 2. Equipment used to obtain modified atmospheres for exposure of Acarus siro. proportions necessary to obtain the required compositions of MAs. After passing through the flowmeters and the reservoir, the gas mixture was humidified by being passed through gas washing bottles containing solutions of 29.6% and 30.0% of H 2 S0 4 for 15 and 26 C, respectively. The gas mixture was maintained at a flow rate of 10 mvmin/experimental nask (250 ml). Cas samples of 100 J.ll were withdrawn from the test flasks twice daily for analysis by gas chromatography (Navarro and Donahaye 1972). The gas concentrations used were 2, 4, 6, to and 21 % 02 in an atmosphere of N 2, and concentrations of 0, to, 20, 30 and 40% CO 2 in an atmosphere containing 21% O 2 with the balance of the atmosphere made up of N Z " Test procedure Initial tests were carried out to obtain basic data on the adult life span of A. siro. The cells containing the mites were placed in the chambers at 15 or 26 C and exposed to a normal air atmosphere containing ca. 21% 0z, at a mean r.h. of 75%. Mortality was recorded daily and dead and live mites were counted immediately after the removal of the cells from the exposure chambers. Moribund mites were considered dead during these counts. The response of the mites to the MAs was tested by placing the cells containing the mites in the 250-ml exposure nasks and exposing them for different lengths of time. Another group of mites, which served as controls, were exposed to normal air in desiccators under identical temperature, relative humidity and exposure conditions. Exposure periods for the various experiments were 24, 48, 72,96 and 120 h at temperatures of 15 and 26 C. The r.h. was held at a mean of 75% throughout the tests. Each experiment was replicated four times. Following each exposure, the treated and control mites were transferred to desiccators held at 15 and 75% r.h.

4 64 J. Agric. Enlomol. Vol. 2, No. 1 {1985} and a normal atmospheric gas composition. After 24 h. dead and live mites were counted. Mortalities were corrected by the use of Abbott's formula (Abbott 1925). Results were analyzed by computing the S.E. for each set of four replicates. RESULTS Survival of Acarus siro in air Acarus siro survived longer at 15 than at 26 C (Fig. 3). At 15, 50% mortality was obtained after approximately 11 d, but at 26 C, after only 7 d. For 95% mortality 22 and 14 d was required at 15 and 26 C, respectively '1,( 2 ; 21%02;75%RH 80 ;;<:.-J <:( > 60 > n:: 1,0 :::> <J) 0 20 a 2 I, II, 16 lb , 26 TIME (days) Fig. 3. Survival of newly emerged Acarus siro adults in air at 75% r.h. and at two temperatures. Effect of low O 2 concentrations The effect of different O 2 concentrations at 15 and 26 C on A. siro adult mortality is shown in Fig. 4. After l20 h mortality of mites exposed to 21 % 02 was 20% at 26 C but only 5% at ISoC. This figure indicates that 100% mortality was obtained after 120 h exposure when O 2 concentrations were 6% or less at ISoC, whereas at 26 C complete mortality was obtained after a 72 h exposure. At 26 C an exposure for 120 h to 10% O 2 produced 100% mortality. The lowest O 2 concentration tested (2%) gave similar results at 26 C and 15 C as 100% mortality was obtained after 48 hand 72 h exposures, respectively. Effect of elevated CO 2 concentrations All CO 2 concentrations were tested at 21% O 2 to eliminate the influence of 02 in these experiments. At 26 C an exposure to 20% CO 2 for 72 h caused complete mortality (Fig. 5). However, exposure to 30% or 40% CO 2 did not decrease the time required to produce this level of mortality. The tolerance of A. siro adults to

5 NAVARRO el. 01.: Grain Mite Response to Modified Atmospheres '. 1 _._._._ ,, Is, C.0'J.C01 ~j?l~. ~ :. 50 ~ ::; :: "0 ~ o ~ 20 '. < o 2' 72 r e...,,,, o " EXPOSURE TIME [lid.. e.t... ret,...!...,..! Fig. 4. Mortality of newly emerged A. siro adults exposed to different O 2 concentrations in N 2 at 75% r.h. and at two temperatures. /0 CO2 0-'-'-'-0 ''0 ~ ;g f S.E SoC:21'l. 01.~~~' o ~y;:'::' r.' 9, J T...~... t... 0 I.k~/..., o i o " "r 1-'" [ Ilrl o ;i" t j... ~... A ~ 'I do Fig. 5. Mortality of newly emerged A. siro adults exposed to CO 2 concentrations with the O 2 maintained at 21 % and the balance of the MA N 2 at 75% r.h. and at two temperatures.

6 66 J. Agric. Entomol. Vol. 2, No.1 (1985) CO 2 increased when mites were exposed to similar concentrations at 15 C. At this temperature, complete mortality was obtained after 72 and 96 h at CO 2 coneen trations of 40% and 30%, respectively. DISCUSSION Effect of exposure to low O 2 concentrations Very little has been published on the effect of MAs on stored product mites. Hughes (1943) demonstrated that A. siro failed to survive a 3~d exposure to 100% N 2 at 20 C; however, he did not indicate the specific developmental stage of A. siro tested. Although the present study has shown that 100% adult mortality was obtained at 2% O 2 and 26 C after an exposure of 48 h. other developmental stages may respond differently and further investigations are required to explore this area, A low O 2 concentration in the environment of this mite may cause a progressive adaption of metabolic activity from aerobic to anaerobic respiration. At 20 C and 0.8% O 2 these mites are immobilized after 48 h, and eventually die (Hughes 1943). Total mortality of A. siro adults was observed in the present study at O 2 concentrations of 6% at 15 C, and 10% O 2 at 26 C. At 26 C, the mortality levels obtained were much higher than at 15 C, indicating that temperature plays an important role in enhancing the lethal effect of low O 2 concentrations. The fact that at 26 C even a 10% O 2 atmosphere causes complete mortality in a 120 h exposure indicates the high sensitivity of treated mites to low 02 concentrations, and the possibility of using MAs to control A. siro adults. Effects of exposure to CO 2 concentrations Hughes (1943) found that exposure of A. sira populations to 20% CO 2 and 16% O 2 for 96 h did not affect mite activity; however, at 27.5% CO 2 and 14.5% 02' the mites became immobilized after exposures of 24 h or longer. From these observations, Hughes (1943) concluded that a CO, level of about 30% had a limiting effect on mite activity, although the decrease of O 2 concentrations to 14.7% was not considered. He also observed that A. siro were killed when exposed to 100% CO, at 100% Lh. for 72 h. Although the effects of very high CO, concentrations were not assayed in the present study, it is evident that relatively low CO 2 levels (i.e., 20%) could be utilized at 26 and 75% r.h. to obtain 100% adult mortality. To control most stored product insects a CO 2 concentration of 60% in air was recommended by Jay (1971). At this level of CO 2 the approximate O 2 concentration is 8%. An atmosphere containing 6% O 2 at 26 C caused 100% mortality in 72 h, but for an atmosphere containing 10% O 2 it took 120 h to produce this effect (Fig. 4). In the present work the combined effect of high CO 2 and low O 2 was not studied. This effect would have probably resulted in shorter exposure times for complete mortality of A. siro. However, it appears that a MA containing 60% CO 2 in air would mask the mortality caused by the low (ca. 8%) 02 concentration acting alone. Therefore, the recommended atmosphere containing 60% in air at 26 C would probably produce 100% mortality of A. siro adults in less than 72 h. The detailed studies conducted on insects in atmospheres containing high CO 2 concentrations suggest that r.h. has an important effect on insect mortality (Jay et al. 1971; Navarro and Calderon 1973). Acarus siro and other acarid mites have no respiratory mechanisms such as those found in insects. Since r.h. is important in

7 NA VARRO et a!.: Grain Mile Response to Modified Atmospheres 67 the development of astigmatic mites (Cunnington 1965), its influence on the mortality of A. siro exposed to various CO 2 concentrations merits further attention. Effect of temperature A higher mortality was observed at 26 than at 15 C during exposure to the same MAs. Although the lower temperatme is considered more favorable for the survival and reproduction of A. siro than 26 C (Hughes 1976), the data in Figs. 3 to 5 indicate that appropriate atmospheric gas compositions could be a feasible control method for A. siro adults even at this low temperature. This is clearly different from the extended exposure periods required to control stored-product insect pests at 15 C (Jay 1980). CONCLUSIONS There is increasing interest in the control of stored-product pests by methods which do not leave harmful residues in the food. Mites constitute a part, and often an economically important part, of the fauna encountered in stored products, and can present a problem because of a lack of effective means of control. The results obtained in the present study show that even atmospheres containing 10% O 2 in N 2 would kill A. siro adults efficiently at 26 C. However, at the same temperature, atmospheres containing 20% CO:.! would cause the same effect. Further studies are needed on immature stages to show that MAs can be used to control mites in stored products. ACKNOWLEDGMENTS The authors thank Mrs. 1'\'1. Rindncr and Mr. A. Azrieli for their technical assistance. This wol'k was partially supported by a grant from t.he Israel National Council for Research and Development. REFERENCES CITED Abbott, W. S A method for computing the effectiveness of an insecticide. J. Ecan. Entomal. 18: Bailey. S. W., and H. J. Banks A review of recent studies of l.he effects of controlled atmospheres on stored pests. pp I" J. Shejbal led.l, Controlled Atmosphere Storage of Grains. Elsevier Scient Pub!. Co., Amsterdam. Bonks, H. J Recent advances in the use of modified atmospheres for stored product pest control. Proc. 2nd lnt. Wkng Conf. on Stored Product Entomology (Ibndun, Nigeria, 1978) pp Burkholder, W. E., E. W. Tilton, and R. R. Cogburn Effect.s of gamma radiation on the grain mite Acarus siro. J. Econ. Entomol. 59: Cunnington, A. M Physical limits for complete development of the grain mite, Acarus siro (Acarina, Acaridae), in relation to its world distribution_ J. Appl. Ecol. 2: Griffiths, D. A A revision of the genus Acorns (Acaridae. Acarina). Bull. Brit. Mus. (Nat. Hist) Zool. 11: Griffiths, D. A Nutrition as a factor influencing hypopus formation in the Acarus siro species compie (Acarina, Acaridae). J. Stored Prod. Res. I:

8 68 J. Agric. Entomo!' Vol. 2, No.1 (1985) Hughes, A. M The Mites of Stored Food and Houses. Tech. Bull. No.9. H. M. Stationery Office, London. 400 pp. Hughes, T. E The respiration of Tyroglyphus (arirwc. J. Exp. BioI. 20: 1-5. Jay, E. G Suggested conditions and procedures for using carbon dioxide to control insects in grain storage facilities. U. S. Dept. Agric., ARS pp. Jay, E. G Low temperatures: Effects on control of Sitophilus oryzae (L.) with modified atmospheres. pp /11 J. Shejbal led.], Controlled Atmosphere Storage of Grains. Elsevier Scientific Pub!. Co., Amsterdam. Jay, E. G., R. T. Arbogast, and G. C. Pearman, Jr Rellltive humidity: its importance in the control of stored-product insects with modified atmospheric gas concentrations. J. Stored Prod. Res. 6: Jay, E. G., and G. C. Pearman, Jr Carbon dioxide for control of an insect infestation in stored corn (maize). J. Stored Prod. Res. 9: Mitsurn, A., R. Amana, and H. Tanabe The acaricidal effects of compressed gas treatments on the grain mile, Tyrophagus flutrescentiae. J. Food Hyg. Soc. Japan, 14(6): Navarro, S., and M. Calderon Carbon dioxide and relative humidity; interrelated,factors affecting the loss of water and mortality of Ephestia cautella (Walker) (Lepidoptera, Phycitidae). Israel,I. Entomo!. 8: Navarro, S., and E. Donahaye An apparatus for studying the effect of controlled low pressures and compositions of atmospheric gases on insects. J. Stored Prod. Res. 8: Navarro, S., M. Gonen, and A. Schwartz Large scale trials on the use of controlled atmospheres for the control of stored grain insects. Proc. 2nd Int. Wkng Conf. on Stored Product Entomology. (Ibadan, Nigeria, 1978), pp Sinha, R. N Role of Acarina in the stored grain ecosystem. pp In J. G. Rodriguez led.j, Recent Advances in Acarology. Vol. 1. Academic Press, New York, NY. 631 pp. Solomon, M. E Control of humidity with potassium hydroxide, sulphuric acid, or other solutions. Bull. Entomol. Hes. 42: Solomon, M. E Ecology of the nour mite, Acarus siro L. (= 7yroglyphus (arinae De G.). Ann. Appl. BioI. 50: Stepien, Z Effect of carbon dioxide all 7'yrophagus pulrescellliae (Schrank) (Acarina; ACllridae). Proc. 4t.h Ent. Congr. on Acarology (Budapest). pp