S. Navarro and E. Donahaye Departmenr of Stored Products, ARO, The Volcani Center, Bet Dagan, Israel

Transcription

1 SENSITIZATION OF INSECTS TO FUMIGATION TECHNIQUES, INCLUDING REDUCED PRESSURE* S. Navarro and E. Donahaye Departmenr of Stored Products, ARO, The Volcani Center, Bet Dagan, Israel Abstract The unique properties and inherent advantages offered by fumigation for controlling storage insects are under careful reconsideration because of the occurrence of insect resistance to fumigants, and restrictions related to the undesirable effects of fumigant residues. Mixtures of fumigants have been investigated with the aim of potentiating the toxic action against insects. However, since the number of approved fumigants has been reduced over recent years, increasing attention has been devoted to methods that employ physical means for sensitizing the insects to fumigant action. The effect of carbon dioxide in enhancing the toxicity of fumigants has been investigated on several stored-product insects. The use of carbon dioxide - fumigant mixtures is desirable to reduce both the length of the exposure period and the pesticide residue levels in the treated commodities. In studies carried out using methyl bromide (MB)-carbon dioxide mixtures the susceptibilities of test insects have been found to increase by a factor of about two. For the fumigation of dried fruit in fumigation chambers turn-over time and toxic residue levels are both critical factors that require optimization. Currently, "vacuum" fumigation is widely employed to achieve these objectives. Laboratory fumigations for up to 18 hours of larvae, pupae and adults of two species of Carpophilus (Coleoptera: Nitidulidae) which are major pests of dates in the field and in storage in Israel, revealed differences in sensitization between ME fumigation in a 0% CO atmosphere and MB fumigation at 100 mm Hg. Sensitization to ME by CO was in the order of x to x3 for 18-hours exposure of C. hemipterus, while maximum sensitization by 100 mm Hg was xl.5 at 18 hours' exposure. For C. mutilatus maximum sensitization to MB by CO was x.1 (for adults). At hours' exposure differences in sensitization were marginal, while for more prolonged fumigations C0 was generally more effective than 100 m Hg. h e significance of these findings is discussed. Introduction The role of fumigants in stored product protection is at present under revision and reappraisal. This is due to a number of constraints * Contribution from the Agricultural Research Organization. No E, 1986 series. Proc. 4th Int. Work. Con$ Stored-Product Protection, Tel Aviv, Israel, Sept [Eds. E. Donahaye and S. Navarro], pp

2 that have arisen over the years and have seriously compromised fumigation as an overall answer to infestation problems. Among these may be mentioned: a) Reduction in the list of widely used fumigants to two, namely, methyl bromide (MB) and phosphine. b) Limitations to dosage levels and restrictions on repeat fumigations because of the dangers of toxic residues and establishment of tolerance levels (Bond, 1983). C) The need to increase the efficacy of treatments so as to minimize the development of resistance by stored productinsects. Measures aimed at side-stepping these and other limitations are: a) Integration of non-chemical methods to reduce the number of repeat fumigations and counteract the development of resistance. b) Improvement of fumigation techniques in order to reduce excessive dosages and at the same time achieve 100% kill. These include improved methods of distribution, maintenance of concentration, and monitoring procedures. C) Sensitization of fumigation techniques in order to reduce dosages and exposure periods, and still obtain total kill. The main input in this field has been the use of released or sustained "vacuum" fumigation techniques, although fumigation mixtures have also been employed for this purpose, including the admixture of fumigants with CO - Vacuum fumigation was investigated many years ago, its primary objective being to hasten and improve penetration of the fumigant into the treated material (Brown and Heuser, 1953; Bond, 1984). The two basic vacuum fumigation techniques are: a] Sustained vacuum fumigation, and b) Vacuum fumigation with restored atmospheric pressure. This method is based on the creation of an initial low pressure followed by the restoration of atmospheric pressure by one of the following ways: immediate restoration of atmospheric pressure; gradual restoration of atmospheric pressure; delayed restoration of atmospheric pressure; or simultaneous introduction of air and fumigant. Work on the effect of vacuum fumigation on a variety of commodities indicates that penetration of the fumigant was most rapid with the sustained vacuum method. In most cases insects were more susceptible at reduced pressure than at atmospheric pressure,

3 I sometimes by a factor of between and 3. These findings were documented in the reports of detailed studies by Nahal (1953). Monro (1959) and, recently, Calderon and Leesch (1983). The gradual development of insect resistance to fumigants and the undesirable effects related to fumigant residues in food have led to the idea of employing mixtures containing C0. The use of CO in the mixture is based on the concept that it is a natural cons?ituent of the atmosphere and its presence in the food environment will not lead to the production of harmful residues. The addition of C0 to a fumigant may be considered from the viewpoints of: increasing the toxicity of the fumigant+c0 mixtures, improving the distribution pattern, and limiting the production of harmful residues in the treated commodity. The toxicities of fumigant+co mixtures were investigated in the late 190's (Cotton and Young, 1993 and subsequently by Jones (1938). More recent studies were carried out by AliNiazee and Lindgren (1969) with hydrocyanic acid (HCN) and MB, and by Calderon and Leesch (1983) and Williams (1985) with MB. In these studies the susceptibility of test insects to the MB+CO mixture was found to increase by a factor of two over MB alone. The subject considered here is the sensitization of insects to a fumigation process required to solve a specific problem arising in the date cultivation and marketing industry in Israel, with similar situations arising in the dried fruit industry in general. Dates in the grove are harvested and transported to packing houses while still containing a diverse population of insect pests, and in particular Nitidulid species that are both field and storage pests. On arrival at the packing houses fumigations are undertaken in chambers to control these pests. Fumigations pose a bottle-neck in the handling process and a rapid turnover of fumigations is essential. In the past ethylene dibromide (EDB) was used to treat dates in Israel. Since the US Environmental Protection Agency banned its use, it has been replaced by MB. The mode of action of both these products induces hyperactivity, in contrast to the narcotic action of other fumigants, thereby inducing the insects to abandon the fruit before they die. The action of both ECB and MB may therefore be considered one of "disinfestation". Even at moderate levels of infestation it is extremely impressive to observe the numerous insects that have left the dates and line the floor of the fumigation chamber upon completion of fumigation. In this study we made a laboratory comparison of two possible sensitization methods for MB fumigations. This was done first because ELB is also under scrutiny for possible carcinogenic effects and therefore it is essential to reduce dosages and residues to a minimum,

4 and second because of the requirement to reduce the fumigation chamber turnover time to a minimum. Since recent studies have shown that the addition of C0 to MB results in an increase in the susceptibility of some stored-product insects, this effect will have to be compared with that produced by vacuum fumigation on date pests before it will be possible to decide which method to recommend for use. Suffice it to say that the construction of fumigation chambers that can hold 100 mm Hg, and in addition their upkeep, are far more expensive than for chambers at atmospheric pressure with provisions for CO flushing. Experimental Test insects: Carpophilus hemipterus L. and Carpophilus mutilatus Er. were collected from infested dates. The two species were reared at 6O~ and 70% R.H. on an artificial diet. Treatments: Exposures of insects to MB alone or in combination with 0% C0 or at 100 mm Hg were carried out in glass containers. For each test 30 insects at each development stage were confined in small plastic exposure cages, placed in the chambers. In all experiments the relative humidity in the exposure chambers was maintained at 75%. and the tests were run at 6O~. Exposure periods for MB alone, for MB + 0% CD, and for MB at 100 mm Hg were, 6, and 18 h. Mortality counts for adults were made 4-7 days after exposure and for larvae they were based on those insects that failed to pupate within approximately weeks after exposure. Similarly, pupal mortality was considered as those pupae that had failed to produce adults about weeks after exposure. Probit analyses were carried out for each exposure time by using different concentrations of MB, while for combinations of the treatment with CO and at 100 mm Hg, the C0 level and the low pressure were maintained constant. Results and Discussion To determine the relative effect of sensitization in the presence of a 0% CO atmosphere or at 100 mm Hg, an index of sensitization was established. This index was based on the ratio of LD obtained for MB 95 alone, to MB in 0% CO or MB at 100 mm Hg. In Figures I and the sensitization factors?sf) for the tested two species are shown.

5 *. Figure 1: Sensitization factors (SF=Dose (LD ) of MB/Dose (LD ) of MB+ 0% CO or MB at 100 mm Hg) for 9?arpophilus hemi~5erus at 6O~ an 70% r.h. - MB* O%C0 e----- MB AT 100mm Hg LARVAE PUPAE "I ; ; 6 ;a 1; I; ib EXPOSURE TIME [hl For C. hemipterus the presence of CO in the atmosphere markedly increased the SF, especially for longer 'exposure times (Fig. 1). However, at 100 mm Hg the 1. SF recorded at the short exposure time increased to only 1.5 for adults even at 18 h exposure, while for pupae it decreased progressively with the extension of exposure times. The different development stages of C. hemipterus were found to respond very similarly to these combined treatments. Additional tests carried out by exposing these insects either to 0% CO without MB or to 100 nun Hg alone indicated that C. mutilatus adults were the most sensitive to low pressure treatment, complete mortality being obtained with 16 h exposure, whereas C0 was not effective. The SF levels were lower for C. mutilatus (Fig. ) than for C. hemipterus (Fig. 1). Whereas for MB in a 0% CO atmosphere pupae and larvae showed slightly increased SF with extened exposure times, the adults exhibited a marked effect of increased sensitivity with

6 Figure : Sensitization factors (SF=Dose (LD ) of MB/Dose (LD ) of MB+ 95 8% CO or MB at 100 mm Hg) for Carpophilus mut??atus at 6 C an3 70% r.h. SF M MB+ZO%C * ME AT 100mm~$ RPAE o LARVAE -'%.;-==; PUPAE LARVAE 1 I, I 1 6 EXPOSURE TIME (h) L 18 time. The response of c. mutilatus larvae in terms of SF exposed to MB at 100 mm Hg was in decreasing order in relation to time. However, the response of the adults was different and showed a steady SF at 1.4 (Fig. ). These results indicate clearly that for E. hemipterus sensitization in the order of x to x3 was possible when insects were exposed to an atmosphere of MB and 0% COZ. However, sustained vacuum fumigation did not prove effective in increasing the sensitivity of this species. An important aspect of vacuum fumigation is in the application of the treatment to materials that are particularly difficult to fumigate at atmospheric pressure. Typical materials in this category are leaf tobacco in bales or hogsheads, boxes of compressed dates, and multiple packages of manufactured articles. The results of tests made on a variety of commodities confirm that penetration of a fumigant is most rapid with the sustained-vacuum method (Brown and Heuser, 1953). However, penetration of MB into coarse granular products, such as wheat and peanuts, was so rapid at atmospheric pressure that it was concluded (Brown and Heuser, 1953; Nahal, 1954) that there could be

7 little advantage in using vacuum methods unless there were some important biological effects associated with the use of reduced pressures. In the case of the two species of Carpophilus examined here, it appears that sustained-vacuum fumigation was less advantageous than treatment with 0% CO ' An additional aspect that requires investigation is the effect of these treatments on the "disinfestation" of dates by inciting the insects to abandon the dates. This will serve as our objective for future studies. Acknowledgments The authors thank Mrs. Miriam Rindner, Miss Daliah Salah, and Mr. A. Azrieli for their technical assistance during the experiments. References AliNiazee M.T. and Lindgren D.L. (1969) Effect of carbon dioxide on toxicity of hydrocyanic acid and methyl bromide to adults of confused flour beetle and granary weevil at two different temperatures. J. econ. Ent., 6, Bond E.J. (1983) Resistance of stored product insects to fumigants. pp In Proc. Third Int. Wkng Conf. Stored Product Entomoloqy. (Manhattan, KS), pp. 76. Bond E. J. (1984) Manual of fumigation for insect control. FA0 Plant Production and Protection Paper No. 54, pp. 43. Brown W.B. and Heuser S.G. (1953) Behaviour of fumigants during vacuum fumigation. I. Penetration of methyl bromide into boxes of dates. J. Sci. Fd Agric.,4, Calderon M. and Leesch J.G. (1983) Effect of reduced pressure and CO on the toxicity of methyl bromide to two species of store3 product insects. J. econ. Ent.., 76, Cotton R.T. and Young H.D. (199) The use of carbon dioxide to increase the insecticidal efficacy of fumigants. Proc. ent. Soc. Wash. 31, Jones R.M. (1938) Toxicity of fumigant C0 mixtures to the red flour beetle. J. econ. Ent., 31, Monro H.A.U. (1959) The response of Tenebroides mauritanicus (L.) and Tenebrio molitor L. to methyl bromide at reduced pressures. J, Sci. Fd Agric., 7, Nahal A.K.M. El (1953) Responses of pests to fumigation. IV. The responses of Calandra spp. to reduced pressures. Bull. ent. Res., 44, Williams P. (1985) Toxicity of methyl bromide in carbon dioxide enriched atmospheres to beetles attacking stored grain. & appl. Ent., 17, 17-4.