Sulfuryl Fluoride: A Disinfestation Treatment for Walnuts and Almonds

J. Larry Zettler and James G. Leesch, Horticultural Crops Research Laboratory, USDA, ARS, 2021 South Peach Avenue, Fresno, CA 93727

In California, the codling moth, Cydia pomonella (Linnaeus), is the major insect pest of walnuts while the navel orangeworm, Amyelois transitella (Walker), is the major insect pest of almonds. The navel orangeworm is also a major pest of walnuts, particularly if populations are allowed to increase in orchards where nuts have already been damaged by the codling moth. Although these field pests are carried into the storage environment with harvested commodities, they do not thrive in storage and subsequently perish. However, their presence in the post harvest environment, albeit short lived, presents a problem for some export markets because both insects are considered quarantine pests by some countries. In addition, the European Union requires imported walnuts to be fumigated with methyl bromide to eliminate the codling moth and navel orangeworm to meet phytosanitary requirements.

Sulfuryl fluoride (=Vikane®) has been identified as a potential replacement for several methyl bromide uses. It has been registered and labeled in many parts of the world as a structural fumigant for 40 years but has no food tolerance. Its insect efficacy is comparable to that of methyl bromide against the active life stages of many insects. The objective of this study was to evaluate the toxicity of sulfuryl fluoride to codling moth and navel orangeworm to determine if it could be used to replace methyl bromide as a treatment for nuts infested with codling moth and navel orangeworm.

Naked life stages of the codling moth and the navel orangeworm contained in glass dishes or vials were exposed to sulfuryl fluoride inside 28.3-liter Labconco® vacuum desiccators. For codling moth, last instars (2 wk in diapause) were tested as well as aged (1, 2, or 3 days old) eggs. For navel orangeworm, last instars and similarly aged eggs were used. The insects were taken from laboratory cultures reared at 27°C, 60% RH, and a 12:12 (L:D) photoperiod. Diapausing codling moth larvae were reared under diapause-inducing conditions at 18 ± 0.5°C, 60% RH, and an 8:16 (L:D) photoperiod. Tests were conducted at normal atmospheric pressure (NAP) and at 100 mm Hg pressure (VAC). Fumigant concentrations were determined by gas chromatography as described by Zettler et al. (in press). Exposure times were 2 and 4 hours at 20 and 25°C. All mortality data were analyzed by probit analysis. Based on the LC₉₉, the CxT was calculated for each life stage tested.

Walnuts artificially infested with diapausing larvae of the codling moth were exposed to sulfuryl fluoride. Larvae were individually placed into walnuts through a hole previously drilled through the shell. After inserting the larva, the hole was sealed. Infested nuts along with non-infested walnuts were allowed to equilibrate at 15.6°C overnight. On the day of the fumigation, fifty infested nuts were placed in a loose-mesh bag, admixed with 6.8 Kg of non-infested nuts and placed in each chamber to produce a chamber load of 33%. Chambers were equipped with an internal fan for recirculating the chamber atmosphere during fumigation. Each dose was replicated at least three times, each on a different day. Exposure times were 24 hours at normal atmospheric (NAP) followed by forced aeration for 2 hours at a rate of 3.4 to 3.6 liters per minute and then 22 hours of passive aeration with the chamber doors open. Following aeration, infested nuts were stored at 15.6°C for 7 days at which time insect counts were made.

Sulfuryl fluoride was toxic to diapausing codling moth larvae at relatively low dosages. The LC₉₉ CxT product was 127 mg hours/liter (63.6 mg/liter in 2 hour fumigation). VAC fumigation reduced the CxT product by about half to 68 mg hours/liter (34 mg/liter in 2 hour fumigation). Tebbets et al. (1986) also found that, when compared with NAP fumigation, VAC fumigation reduced LC values of this insect by half when fumigated with MB. On the other hand, eggs were relatively tolerant to sulfuryl fluoride fumigation. The tolerance differed with age (3-day > 2-day > 1 day). Whereas VAC fumigation decreased CxT values for larvae, it had no significant effect in reducing tolerance of eggs of any age. In fact, VAC fumigation was antagonistic to 1 and 2 day-old eggs resulting in CxT values higher than those for NAP fumigation.

Similar results are shown for navel orangeworm. Based on the CxT products, sulfuryl fluoride was about equally toxic to non-diapausing 5^th instars of navel orangeworm as to diapausing larvae of codling moth. However, VAC fumigation had more effect on increasing sulfuryl fluoride susceptibility of navel orangeworm than of codling moth. Four-hour VAC fumigation reduced the NAP dose of 35.3 mg/L (CxT=141 mg hours/liter) by more than 80% to 6.6 mg/liter (CxT=26 mg hours/liter). Navel orangeworm eggs were more tolerant to sulfuryl fluoride than were codling moth eggs. Like codling moth eggs, navel orangeworm eggs showed varying tolerance to the fumigant and the tolerance was age dependent. VAC had no effect in reducing the CxT values except for 3 day old eggs, the most tolerant age for eggs of both species.

The present fumigation schedule for disinfesting walnuts infested with diapausing codling moth involves 24 hour fumigation at NAP at a dose of 56 mg/liter methyl bromide, the minimum effective dose (MED) required for complete control. Results of confirmatory tests showed that the MED for sulfuryl fluoride under the same conditions was 8 mg/liter. Thus, sulfuryl fluoride was 7X more toxic than methyl bromide. Sorption of sulfuryl fluoride by the walnuts was only 30% compared with 80% for methyl bromide. Low sorption is indicative of rapid penetration into and aeration out of a commodity and thus reflects reduced chemical residues on the commodity. When walnuts and almonds are harvested, first to fifth instars of codling moth and navel orangeworm are present from overlapping broods in the orchards. These larvae can freely infest the nuts while on the tree. Only rarely, however, does the codling moth remain in walnuts. When it does, in late August and early September, the fifth instars may enter diapause. This is a very rare occurrence because most larvae destined for diapause leave the nut. The fumigation treatment approved by Japan for these insects in walnuts is aimed at the diapausing larvae of codling moth because this is the most tolerant life stage that could infest the nut at harvest. The eggs don't occur naturally on walnuts or almonds at the time of harvest. Thus, based on larval toxicity, low sorption by the walnuts, and the fact that only the larvae are present in exported nuts, sulfuryl fluoride appears to be a viable replacement for methyl bromide under these conditions.

Although a 24-hour fumigation is not commercially practical for disinfesting walnuts being shipped to the European Union or Japan, the increased toxicity of sulfuryl fluoride under VAC conditions indicates the exposure period could be shortened to as little as 4 hours and still produce a CxT product that would produce complete control.

Last Updated: May 4, 2000