EFFICACY O"FTEMPERATURE TREATMENTS FOR INSECTDISINFESTATION OF DRIED FRUITS AND NUTS

J. A. Johnson, H. R. Bolin, G. Fuller,and J. F. Thompson

ABSTRACT

In order to determine the most resistant stage to various high temperaturetreatments for disinfesting dried fruits and nuts of postharvest insect pests, effectof different Indianmeal moth (lMM) and navel orangeworm (NOW) egg ages onresponse to heat was estimated. Of the ages treated, 9 :i: 8 hr old IMM eggs, and42 :i: 6 hr old NOW eggs proved to be the most resistant stages to hightemperatures. Theseages were usedto determineLT96 for temperatures ofbetween 40 and 47°C for IMM eggs. When compared to LT96sfor IMM pupae,eggs were found to be far more resistant at temperatures of less than 47°C.Resistance of pupae was equal to or greater than eggs at higher temperatures.NOW eggs were found to be much more resistant to high temperature~ than IMMeggs. The LT5o, LT95and LT99for NOW eggs exposed to 40°C were estimated tobe about 27, 34, and 38 hours, respectively. Lower developmental thresholds forIMM from laboratory and w!ld-type cultures were found to be similar. For bothIMM and NOW, pupae just prior to adult emergence were the most resistant toheat. The lower thresholds for IMM eggs and pupae were 14.8 and 13.9°C,respectively. Exposure of pupae to 46°C for 10 minutes resulted in a decrease inthe average number of progeny produced, depending upon pupal age. Fertility ofmales was more effected by high temperatures than females. These results areencouraging because they indicate that individual insects that survive treatmentswill be sterile, which should reduce possible reinfestations and the development ofresistance. Initial results from field studies using high temperature treatmentscombined with night-air recirculation are encouraging. Even though targettemperatures were not reached, IMM was completely controlled. Other studiesshowed that higher temperatures could be obtained using higher air speeds duringthe treatment.

OBJECTIVES

The purpose of the study is the development of temperature treatments asalternatives to fumigation for insect disinfestation of postharvest dried fruits andnuts. In accomplishing this goal the following objectives have been set.

(a) Determine the temperature extremes and associated exposure times required tokill various stages of the Indianmeal moth (lMM) and the navel orangeworm (NOW).

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(b) Compare control efficacy of different heat application methods against theabove insects in infested raisins, prunes and walnuts.

(c) Evaluate the effect of the above treatments on product quality.

(d) Estimate and compare the costs and feasibility of the above treatments tocurrently used fumigants and other alternatives.

PROCEDURES

To determine the effect of egg age on response to high temperatures, IMM eggswere exposed to 45°C for 10 or 20 minutes in a water bath. IMM eggs weretreated every 6 hours. NOW eggs were exposed to 47°C for 30 minutes every 8hours. After treatment, eggs were held at 27°C for 7 to 10 days and then egghatch was determined. The ages found to be the most resistant were used todetermine the response of eggs to a range of high temperatures. The effect of ageon response of IMM and NOW pupae to high temperatures was also examined.

The effect of high temperatures applied to IMM pupae on subsequent reproductionwas determined by exposing pupae of different ages to 46°C for 10 m.inutes. Theadults emerging from these pupae were then paired with untreated adults in smallplastic cages, and allowed to mate and lay eggs for 7-10 days. Hatched andunhatched eggs were then ~ounted to determine fertility.

Developmental thresholds for IMM were determined by incubating eggs and pupaeat constant temperatures of 19, 22, 25, 28, 31, and 34°C. Both the laboratorycolony and a recently isolated wild-type culture were studied.

A field test using a heat treatment combined with night-air recirculation to controlIMM in bins of prunes was begun. Bins of prunes were heated with forced hot airin a modified cargo container (20 x 8 x 8ft). Temperature profiles within the bins.were monitored with thermocouples and recorded on data loggers throughout theheating process. Control efficacy was determine.d by placing cages containingIMM eggs, larvae and pupae in the center of the bins. After heating, the pruneswere cooled through night-air recirculation. Temperatures will be monitoredthroughout the cooling period. Adult IMM in cages will be placed in the containerperiodically throughout the cooling period to determine efficacy. Prune quality willbe assessed by the processor.

RESULTS

IMM and NOW egg response to high temperatures was strongly modified by eggage (Figs. 1 and 2). The mO$t heat resistant IMM eggs were those 9 or 45 ~ 8hrs old. NOW eggs were most resistant when 42 or 66 ~ 6 hrs old. In earlierstudies, 9 ~ 6 hrs old IMM eggs were the least resistant to sub-freezing

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temperatures. Response of NOW eggto sub-freezing temperatures was verysimilar to NOW egg response to heat.In subsequent studies, 9 :i: 8 hrs oldand sometimes 45. :i: 8 hrs old IMMeggs were used to determine LT9SSforhigh temperatures (Table 1). Similarly,42 :i: 6 hrs old NOWeggs were usedto determine lethal exposures to 40°C.NOW eggs were much more resistantto heat than IMMeggs,. with LTso, LT9sand LT99for 40°C being 26.85, 33.78and 37.67 hrs, respectively.

Table 1. LTII&for IMM eggs

For most temperatures tested, IMMeggs proved to be far more resistant tohigh temperatures than IMM pupae(Fig. 3). At 48°C and above, pupaeare as resistant as eggs. These resultsindicate that for field tests of methods using lower treatment temperatures (s

. 47°C), eggs should be the stage most closely studied. When higher temperaturesare used, response of both eggs and pupae should be examined.

Survival of older pupae treated with 46°C for 10 minutes was higher for thanyounger pupae (Fig. 5). Older NOW pupae also were more resistant to hightemperatures (Fig. 4). High temperatures applied to IMM pupae caused almostcomplete sterility in all surviving males (Fig. 6). Younger females were also nearlysterile, but older females were less effected. These results are encouraging,because they show that survivors of heat treatments will probably be unable toreinfest the product. T~ese results also suggest that resistance to heat treatmentsmay be slowed.

Developmental thresholds for only IMMeggs and pupae were determined becauselarvae reared at temperatures below 20°C go into diapause, making accuratethreshold estimations difficult. The laboratory and the wild-type IMM culture hadsimilar developmental rates and thresholds (Fig. 7 and 8). Lower thresholds foreggs and pupae were 14.8 and 13.9°C, respectively. Developmental rates forboth eggs and pupae at 34°C were similar to those at 31°C, indicating that anupper developmental threshold had been reached.

Preliminary studies on the efficacy of a forced hot air treatment of a bin of prunesshowed that a minimum temperature of 48°C could be reached. None of the IMMeggs placed at the center of the bin hatched. Some IMMadults were found in thecaged prunes but these had probably emerged from pupae before the treatmenthad begun. In a larger scale test using 3 bins of prunes, minimum temperatureswere much lower (about 40°C), probably due to using a lower air speed during the

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Temperature (OC) Minutes (Hours)

40 1340 (22.3)

41 674 (11.2)

42 338 (5.6)

43 -44 87 (1.5)

45 45 (0.7)

46 19 (0.3)

47 12 (0.2)

treatment. Even so, none of the IMM larvae or pupae buried in cages in the centerof the bins were able to emerge as adults. IMMeggs buried in the bins were ableto hatch, but no newly hatched (neonate) larvae were seen, nor was any feedingdamage due to neonates observed. Cages of untreated prunes infested with IMMeggs showed considerable feeding damage from neonates. As shown by earlierstudies, IMM eggs are .highly resistant at temperatures of less than 42°C.However, because newly hatched larvae are not as resistant, even relatively lowtreatment temperatures were effective. This study is continuing and a detailedanalysis of temperature profiles is not yet available. A new test using a higher airspeed is underway.

CONCLUSIONS

Indianmeal moth eggs have been shown to be the most resistant stage to hightemperature treatments of less than 47°C. By targeting this stage, future studieswill be more efficient. The thresholds determined by our developmental studieswill also be useful in future tests by delineating lethal temperatures. The effect ofhigh temperatures on IMM reproduction showed that occasional survivors of heattreatments will be unable to reproduce. This will help prevent reinfestation of theproduct and slow the possible development of resistance to the treatment.

The initial results of field studies using high temperature treatments combined withnight-air recirculation for insect control in prunes are encouraging. Even thoughtarget temperatures were not reached in the first trial, control of IMM wascomplete. This is of interest to the walnut industry, because lower treatmenttemperatures are desirable to avoid an increase in rancidity in stored walnuts. Bycombining high temperature treatments with low temperature night-airrecirculation, chemical fumigations and space treatments may be avoided oreliminated.

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o8 9 18 21 27 33 39 48 81 87 83 88

Egg Age (hrs)

Figure 1. Effect of egg age on response of IMM to high temperatures.Eggs were exposed to 450 C for 10 or 20 minutes. .

100

o14 18 28 34 42 50 58 88 74 82 80 84

Egg Age (hrs)

Figure 2. Effect of egg age on response of NOW to high temperatures.Eggs were exposed to 470 C for 30 minutes.

-- -

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Temperatur. (OC)

Figure 3. Comparison of heat response of IMM eggs and pupae.

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Pupal Age (hrs)

Figure 4. Effect of pupal age on response of NOW to high temperatures.Pupae were exposed to 48°C for 20 minutes.

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Figure 5. Effect of age on response of IMMpupae to high temperatures.IMM pupae were exposed to 46°C for 10 minutes. .

Figure 6. Effect of high temperatures on IMMreproduction. IMMpupaewere exposed to .46°C for 10 minutes. -

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Pupa' Age (h...)

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Temperature (GC)

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Figure 7. Comparison of developmental rates for eggs of laboratory and.wild-type IMM isolates. .

Figure8. Comparison of developmental rates for pupae of laboratory andwild-type IMMisolates.

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Temperature (OC)