Effect of low temperature storage on survival and reproduction of Indianmeal moth (Lepidoptera: Pyralidae)

ELSFVIER

Effect of low temperature storage on survival and reproduction of Indianmeal moth (Lepidoptera: Pyralidae) J.A. Johnson*, K.A. Valero and M.M. Hannel Horticultural Crops Research Laboratory, 2027 South Peach Avenue, USA

Fresno,

CA 93727,

Low temperature storage is one component of an integrated control program suggested as an alternative to methyl bromide fumigation for Indianmeal moth, P/o&a interpunctellu (Hiibner), in postharvest dried fruits and nuts. Long-term exposure to 10°C lengthened the life of adults; 5% mortality was reached after 49 days of exposure, almost four times the average adult lifespan of 13 days at 27°C. Adult mortality reached 90% after 70 days of exposure. Exposure to 10°C for greater than 25 days reduced egg production by more than half and reduced the number of viable eggs by nearly 90%. Eggs were most susceptible to 10°C; the exposure time estimated to obtain YS% egg mortality was 11.h days. From these data this it was estimated that clean product that has been under storage at 10°C and undisturbed for at least 4weeks should be relatively free of Indianmeal moth. Published by Elsevier Science Ltd Keywords: Plocfia interpunctelle; postharvest;

Introduction The Indianmeal moth, Plodiu intqvunctella (Hiibner), is a cosmopolitan pest whose larvae can infest a variety of grains, nuts, pulses, meals, dried fruits, and processed foods (Simmons and Nelson, 1975). In California, it is a major problem during processing and storage of dried fruit and nuts. Larvae reduce product quality by their presence and the production of frass and webbing, and also cause direct damage by feeding. Currently, control practices rely on scheduled fumigation with methyl bromide or hydrogen phosphide. Methyl bromide has recently been classified as an ozone depleter (USEPA, 1993), which may result in its use being severely restricted or eliminated. Insect resistance to hydrogen phosphide has been documented in other commodities (Zettler et al., 1990). Consequently, concern over the possible restriction of these fumigants has generated interest in developing alternative treatment methods. Although several nonchemical methods have been demonstrated to provide either immediate control or sustained protection, no single method provides an economical alternative to fumigation throughout the postharvest system. One possible solution is to integrate short-term disinfestation methods with longterm protective techniques to overcome the limitations of individual nonchemical methods. Low temperature storage is a well-known insect management technique for postharvest grains (Arthur and Johnson, 1995; Maier, 1994). Disinfestation of fresh fruits with cold treatments to meet quarantine *TO whom

correspondence

should

he addressed

low temperature

standards has also been suggested and several such treatments have been approved (Gould, 1994). Although disinfestation of dried fruits and nuts with low temperatures is largely impractical because of the excessive exposure times needed, storage at 10°C or less will prevent reinfestation of dried fruits and nuts that have been disinfested with more expedient methods. One of the proposed nonchemical alternatives for dried fruits and nuts integrates an initial short-term disinfestation of incoming product with long-term protection against #reinfestation by low temperature storage (Johnson et al., 1995a). Because low temperature storage is an inefficient disinfestation method, the success of the integrated method depends on the product entering storage as free of insect populations as possible. Assuming effective initial disinfestation, the most serious threat to the product would occur during transfer to low temperature storage, when the product is vulnerable to flying moths. For this reason, it is important to understand the effect of low temperature storage on both adults and eggs of the Indianmeal moth. Our study shows the effect of long-term exposure to 10°C on Indianmeal moth adult survival, reproduction and egg hatch.

Materials and methods Adult mortality Test insects were obtained

from laboratory colonies maintained continuously on a wheat bran diet (Tebbets et al., 1978). The laboratory isolate was obtained from a walnut packinghouse in Modesto,

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CA, in November 1967. Normal rearing conditions were 28°C 60% RH, and a photoperiod of 14 h:lO h (L:D). Strips of corrugated cardboard were placed along the inside of the rearing jars to serve as pupation sites. Pupae were removed from the cardboard strips and placed individually in glass culture tubes (12 mm x 75 mm). A strip of paper toweling was placed in each tube to collect excess moisture and the tube was closed with plastic tube caps. A single hole was made through the caps with fine-tipped forceps for air exchange. Pupae were held under rearing conditions and examined each day for adult emergence. When a sufficient number of moths had emerged (more than 120 of each sex), they were moved to a refrigerated room held at 10 k l.O”C. Adults were placed at 10°C between 6 and 72 h after adult emergence. Adults’ mortality was determined weekly until all moths were dead. The test was repeated three times. Mortality data for male and female moths were compared using the GLM procedure for repeated measures (SAS Institute, Inc., 1989). Adult reproduction Adults of known age and sex were obtained as described above. Mated females were obtained by releasing equal numbers of males and females into a glove box and allowing them to mate. At least 100 pairs were carefully placed back in individual culture tubes while still in copulo. The pairs were held at rearing conditions for approximately 3 h and examined. Moths that had failed to uncouple were discarded. Tubes containing the mated pairs and an additional 100 unmated females still in culture tubes were placed at 10°C. Every 5 days, 10 mated pairs and 10 unmated females were removed from 10°C and placed in small cages (approximately 20 mm x 29 mm diameter) made by cutting the bottoms from plastic vials and heat-sealing 100 mesh brass screen to the cut edge. The screen was of sufficiently fine a mesh to retain unhatched eggs but allowed neonate larvae to pass through. The cages were closed with snap-on lids with 60 mesh brass screen centers. A single untreated male (6-24 h old) was added to each cage containing an unmated female. Ten mated pairs of recently emerged, untreated moths were placed in similar cages as controls. The cages were placed on wheat bran diet in plastic food containers (250 ml) and held under rearing conditions. This design allowed larvae hatching from eggs laid within the cages to move into the diet through the 100 mesh screen, while retaining egg chorions. After 10 days the cages were placed in a freezer and held there until hatched and unhatched eggs could be counted. The test was continued for 30 days, and replicated four times. The number of eggs laid per female was analyzed using the GLM procedure with the effect of exposure interval nested within the main treatment effects of untreated moths, moths mated before exposure, and moths mated after exposure (SAS Institute, Inc., 1989). Within each exposure, means were separated

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using Tukey’s Studentized range test. A similar analysis was done on the percentage of viable eggs after an arcsine transformation. Using the GLM procedure, the exposure time required for 95% egg mortality was estimated. Egg mortality

Eggs were collected from 1-Zday-old adults. Oviposition jars containing loo-150 adults of both sexes were set up from 16:OO to 17:OOh (Pacific Standard Time) and placed in environmental chambers kept at normal rearing conditions. Eggs were collected 16 h later. Scales were removed from eggs by gently shaking the collection container under a fume hood. The eggs were then passed through a 32 mesh brass screen to remove clumps of eggs and moth body parts. Samples of approximately 100 eggs (2.1 mg) were transferred to dishes made from hollow plastic test-tube caps (9 mm tall, 13 mm diameter). The bottoms of the caps were cut off, and brass screen (100 mesh) was heat-sealed to the cut edges. Three egg dishes were placed in each of 30 Petri dishes containing about 50 g of bran diet. Dishes were held at 292 1°C until placement at 10°C. Ten Petri dishes each were placed at 10°C when the eggs were 9 +8, 30+8, and 54&8 h old. An additional three egg dishes were held under rearing conditions as untreated controls. For each age, a single Petri dish was removed from 10°C after 4 days of exposure, and then every 2 days until 22 days. After removal from 10°C the dishes were held at rearing conditions for 10 days, after which they were placed in a freezer and held until hatched and unhatched eggs could be counted. The test was replicated four times for the two youngest ages, and three times for the oldest age. We used the GLM procedure (SAS Institute, Inc., 1989) to compare the effect of age and exposure time on the percentage of egg mortality after an arcsine transformation. We also did a logit regression analysis of egg mortality against exposure time using the probit procedure (SAS Institute, Inc., 1989) on untransformed data for 30 f g-h-old eggs. Results Adult mortality

The cumulative mortality of adult Indianmeal moths at 10°C is shown in Table I. Repeated measures analysis of variance did not show significant difference (P = 0.1776) between male and female mortality. An average mortality of 50% was reached after 49 days of exposure, and 90% mortality was obtained after 70 days of exposure. Earlier work (Johnson, unpublished data) showed that the average lifespan of unmated moths kept at 27°C was 12.6 days (9-15) for females and 14.3 days (11-16) for males. Adult reproduction

Long-term exposure to 10°C reduced the number of eggs laid per female (Table 2). Analysis of variance across exposure intervals (GLM procedure, SAS Institute, Inc., 1989) showed that although moths

Low temperature Table 1. Cumulative percentage adults exposed to 10°C

of

Indianmeal

Females

Exposure (days) 7 14 21 28 3s 42 49 56 63 70 77 x4 91 98 Mean

mortality

of three

replicates

of

l.iY-221

5.‘) * 1.30 7.7 & I .87 11.8+2.35 16.7*3.81 23.Y k4.54 42.1 &S.Sl 65.7+5.62 79.2 k 2.55 X8.1 k2.21 Y2.h+ 1.86 96.1 kO.93 ‘)7.7+0.9x 98.7 f 0.55 Y9.8 f 0.23 moths (total II

fcmalcs

was

521. total II males was 5l)Y).

exposed to 10°C produced significantly fewer eggs than untreated moths (F = 143.57, d.f. = 1, 53, P = 0.0001). there was no difference between moths and those mated after mated before exposure exposure (P = 0.14). The number of eggs by moths exposed to 10°C for 15 days or more was reduced by over 50% when compared with untreated moths. The percentage of viable eggs was also reduced after adult exposure to 10°C (Table 2). Analysis of variance across exposure intervals showed that the percentage of viable eggs from untreated moths was significantly higher than that from moths exposed to 10°C (F= 258.55, d.f. = 1, 53, P= 0.0001). Moths exposed to 10°C before mating (unmated) produced slightly more viable eggs than did moths that had mated before exposure (mated) (F = 4.33, d.f. = 1, 53, P = 0.0422). Because the difference was so slight, we pooled the values for both treatment groups (mated and unmated) in further analyses. Regression analysis of the percentage of viable eggs against adult exposure time (GLM procedure, SAS Institute, Inc., 1989) yielded an R2 of 0.784 (Figure I). The estimated exposure time needed for a 95% reduction of viable eggs, calculated from the resulting regression equation, was 27 days. Adult female mortality at a comparable exposure time (28 days) was only 10% (Table I).

We previously estimated the developmental threshold for Indianmeal moth to be between 14 and 18”C, depending upon life stage and larval diet (Johnson et al., 1992, 1995b). These estimates agree with those recently reported (15.3 k 0.76”C) by Subramanyam and Hagstrum (1993). Because 10°C is within the range of temperatures suggested to prevent development of storage insects (Maier, 1994) and is well below the threshold for Indianmeal moth, we selected it as the target temperature ‘for larger-scale pest management tests. Tests completed with walnuts have shown that 10°C provides adequate control of Indianmeal moth while maintaining product quality (Johnson et al., 1995a). Actual and suggested use of low temperatures to control insects is limited primarily to aeration or chilling of grain (Arthur and Johnson, 1995; Maier, treatments for perishable 1994) or quarantine products (Gould, 1994). Consequently, much work

Eggs were relatively sensitive to exposure to 10°C (7X& 3). For all ages, 95% mortality was exceeded

Total Exposure (days) 5

IO IS 20 35 30 Values Tukcy‘s

J.A. Johnson et al.

Discussion

Egg mortality

Table 2. Number and viability of eggs laid by Indianmeal

moths:

after just 12 days of exposure. Very low numbers of 54 f&h-old eggs were found to have hatched after 16 days or more of exposure to 10°C. Mean developmental period for Indianmeal moth eggs at temperatures of 28-30°C is 79-69 h, respectively (Johnson et al., 1995b). These eggs had probably hatched either just before or during exposure. Analysis of arcsine transformed data across exposure intervals showed that egg age had a significant effect on response to 10°C (F = 18.21, d.f. = 2, 47, P = 0.0001). When the different ages were contrasted directly, 30 *8-h-old eggs were significantly more tolerant than the younger (F = 16.01, d.f. = 1, 47, P = 0.0002) or older eggs (F = 30.55, d.f. = 1, 47, P = 0.0001). The youngest and oldest eggs were not found to be significantly different in their response (P = 0.0725). Because the 30+&h-old eggs were found to be the most tolerant of exposure to lO”C, we limited further analyses to this age. Logit regression analysis (Probit procedure with logistic model, SAS Institute, Inc., 1989) gave the best fit. Although the Pearson x’ (8.07) is high, a plot of actual and expected values (Figure 2) shows no systematic departure from the model. The analysis predicted an exposure of 11.6 days to produce 95% mortality, with lower and upper 95% fiducial limits of 11.3 days and 12.0 days, respectively.

Males

4.Y& 1.71 5.9 * 1.77 7.2i_ 1.47 10.2+2.33 l.5()*33.21 27.OiS.01 47.3 + 8.29 67.‘) * Y.73 s4.3*5.71 94.9 * 2.52 07.7+1.13 00.1 k0.S I YY.7+0.27 100.0

valucs+SE

moth

effects on lndianmeal

moths exposed to 10°C

eggs per female

Percentage

viable eggs

Mated

Untreated 432.8 369.9 308.7 451.1 325.1 369.8 are mcanaiSE Studentized

+ 48.8 f 18.0 f 30.5, +21.5 iS4.4 + 24.6

a a a a a a

of four test).

before

exposure

273.9 i 27.6 238.4k2l.S 171.4117.8 146.3 i 14.2 147.5 f 37.1 152.8 f 34.6 replicates.

Analysis

Among

performed

Mated

329.3 + 18.0 ba 236.4 + 59.4 a 252.Sf6.1 b 187.0+2Y.l b 149.6 + 28.6 b 140.4 f 22.2 b

b a h b b b

treatments

on arcsinc

after exposure

for each variahlc

transformed

and

90.7 f 2.5 85.3 50.9 89.3k3.4 92.7 + 2.6 87.0+2.1 85.3 * 7.5

exposure, means

values for percentage

Mated

Untreated

viable

a a a a a a

followed

before

exposure

Mated

74.3 * 7.3 a 56.0 f 4.7 a 41.3* 10.5 b 12.7i3.2 b lO.Oi9.0 b 0 hy different

Icttcn

arc significantly

after exposure

85.7& 1.7 a 50.5 * IS.8 a 52.3iS.6 b 28.3iS.3 b Y.3 + 4.6 b 5.3 & 3.3 b different

(P~w)~,

eggs.

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70 l

Mated

0

Unmated

60

--

y = 73.53 -2.28.X R’=0.78

J 0

5

10

15

20

25

30

Exposure (days) Figure 1. Linear regression of percentage of viable eggs (after arcsine transformation) from Indianmeal moths exposed to 10°C before (unmated) and after (mated) mating. Data were combined for analysis, dotted lines are 95% confidence limits

has been done on the effects of suboptimal and lethal low temperatures for various stored product and quarantine insects. Most of the work done on stored product moths has focused on developmental stages and has neglected the effect on the non-feeding adults (Fields, 1992). Within a range of optimal temperatures, adult Indianmeal moths are short-lived, and their longevity Table 3. Mortality exposed to 10°C

of different

ages

of

Indianmeal

moth

eggs

Age of eggs Exposure (days)

9kXh

4 6 8 10 12 14 Ih

34.4 f 13.7 44.3 f 12.5 59.4 k 12.8 87.2 k 6.8 98.1 + 1.1 100.0 100.0

Mean valucs~SE of four for eggsof 54 h age.

0

3Oi8

replicates

2

4

S4*8

h

3.2 i 0.6 7.5 * 2.0 27.8 k 4.5 79.0 * 3.6 96.4 * 1.9 99.9&0.1 100.0

34.x + 59.9 + 86.3 k 96.9 * 99.3 + 99.5 * 99.8 +

for eggsof Y and 30 h apt, three

6

8

10

h 13.4 20.6 10.3 1.5 0.4 0.3 0.5

replicates

12

14

16

Indianmeal

moth

eggs

Exposure (days) Figure 2. Logit analysis exposed to 10°C

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is strongly influenced by temperature (Hamlin et al., 1931). Our study showed that exposure to 10°C more than tripled the length of adult life. However, the primary damage caused by non-feeding adults is through the production of offspring. Whereas a’bout 50% of the moths survived exposure to 10°C for 49 days, egg mortality from exposed moths was estimated at 95% after 27 days, and egg production was reduced by 50%. Tauber et al. (1993) found similar results with Chrysupa camea Stephens, where long-term exposure of diapausing adults to 5°C caused a reduction in both fecundity and fertility. More work has been done on the effect of low temperatures on pyralid eggs (Cline, 1970; Daumal et al., 1974; Mullen and Arbogast, 1979; Strati1 and Reichmuth, 1984). Eggs tend to be more susceptible to low temperatures than other life stages, and age of the egg may affect response (Fields, 1992). Strati1 and Reichmuth (1984) found that mortality of Ephestiu cuutellu (Walker) and Ephestia elutella (Hiibner) eggs after exposure to 11°C increased with age. The response of E. cuutella was similar to our results for Indianmeal moth, but E. elutellu was far more cold tolerant. Cline (1970) showed that the mortality of Indianmeal moth eggs exposed to 2.4”C decreased with age, increasing slightly just before hatch. When exposing Indianmeal moth eggs to subfreezing temperatures, Johnson and Wofford (1991) found a similar pattern. In the current study, the youngest and oldest eggs were both more susceptible to lO”C, and more variable in their response than the middle-aged eggs. The oldest eggs were consistently more susceptible than middle-aged eggs at exposure intervals of 10 days or less, but their response was similar at exposure intervals of 12 days or longer. We believe that this was due to very low numbers of eggs hatching either just before or during exposure. Fields (1992) points out that there is considerable variability in response to low temperatures by geographical isolates of stored product insects, and between field and laboratory populations. The results of our study should be confirmed with field populations before final recommendations are made. Storage of dried fruits and nuts at 10°C is a useful method of preventing insect infestation of clean product by rendering the product environment unsuitable for insect development (Johnson et al., 1995a). Walnuts are often stored at temperatures of approximately 5°C to preserve quality, and the quality of other nut crops would benefit from storage at comparable temperatures. Because the long exposure times necessary to produce adequate control of postharvest insects makes it impractical as a disinfestation technique, the product must be as free as possible of insect populations when it enters low temperature storage. Thus, low temperature storage is most useful in combination with more efficient disinfestation techniques. Assuming successful disinfestation, product is most vulnerable to reinfestation by flying moths as it is transferred to low temperature storage. Our work shows that more than half of these moths may survive for 7 weeks or longer under low temperature condi-

Low temperature

tions; however, females are nearly sterile after only 4 weeks. Preliminary laboratory observations indicate that females ready to oviposit will lay their eggs within l-2 weeks while exposed to 10°C (Johnson, unpublished data). Because eggs are relatively sensitive to lO”C, and will fail to hatch after about 2 weeks of exposure; this gives a similar period of 3-4 weeks for complete mortality of any offspring. Thus, clean product that has been under storage at 10°C and has been undisturbed should be relatively free of Indianmeal moth. The efficiency of low temperature storage may also be enhanced by strict sanitation methods and by moving product during the day, when moths are less likely to be flying. Acknowledgements We wish to thank Shirley May (Horticultural Crops Research Laboratory, USDA-ARS) for her technical assistance. We also thank Patrick V. Vail and Richard Gill (Horticultural Crops Research Laboratory, USDA-ARS), Lisa Neven (Fruit and Vegetable Insect Research, USDA-ARS), and Richard Arbogast (Postharvest and Bioregulation Research, USDAARS) for reviewing this manuscript. This paper presents the results of research only. Mention of a proprietary product does not constitute an endorsement or recommendation for its use by USDA. References Arthur, F. H. and Johnson, H. L. (1995) Development of aeration plans based on weather data: a model for management of corn stored in Georgia. Am. Entomol. 41, 241-246 Cline, L. D. (1970) Indian-meal moth egg hatch and subsequent larval survival after short exposures to low temperature. J. Econ. Entomol. 63, 1081-1083 Daumal, J.. Jourdheuil, P. and Tomassone, R. (1974) VariabilitC des effets ICtaux des basses tempkratures en fonction du stade de dCveloppement embryonnaire &ez la pyrale de la farine (Anugasra kuhniella Zell.. Leoid.. Pvralidae). Ann. Zoo/.-Ecol. Anim. 6. . > 229-243 Fields, P. G. (1992) The control of stored-product mites with extreme temperatures. J. Stored Prod. Res.

insects and 18

28, 89-l

Gould, W. P. (1994) Cold storage. In: Quarantine Treatments for (Ed. by J. L. Sharp and G. J. Hallman) pp. I 19- 132, Westview Press, Boulder, Colorado

Pests of Food Plunts

Hamlin, J. C., Reed, W. D. and Phillips, M. E. (1931) Biology of the Indian-meal moth on dried fruits in California. USDA Tech. Bull. 242, 26

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Johnson, J. A. and Wofford, P. L. (1991) Effects of age on response of eggs of Indianmeal moth and navel orangeworm (Lepidoptera: Pyralidae) to subfreezing temperatures. J. Econ. Entomol.

84, 202-205

Johnson, J. A., Wofford, P. L. and Whitehand, L. C. (lY92) Effect of diet and temperature on development rates, survival, and reproduction of the Indianmeal moth (Lepidoptera: Pyralidae). J. &on. Entomol. 85, 56 l-566 Johnson, J. A., Soderstrom, E. L., Curtis, C. E. and Vail, P. V. (1995a) Beyond methyl bromide: non-chemical control methods for postharvest pests of walnuts. Aust. Nutgrower 9, 19-20 Johnson, J. A., Wofford, P. L. and Gill, R. F. (lY95b) Developmental thresholds and degree-day accumulations of Indianmeal moth (Lepidoptera: Pyralidae) on dried fruits and nuts. J. Econ. Entomol.

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Maier, D. E. (1994) Chilled aeration and storage of U.S. crops a review. In: Proceedings of the 6th International Working Conference on Stored-product Protection, Canberra. Australia, 17-23 April 1994 (Ed. by E. Highley, E. J. Wright, H. J. Banks and B. R. Champ) pp. 300-31 I, CAB International, Wallingford Mullen, M. A. and Arbogast, R. T. (1979) Time-temperaturemortality relationships for various stored-product insect eggs and chilling times for selected commodities. J. Econ. Entomol. 72. 476-478

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Stratil, H. H. and Reichmuth, C. (1984) uberlebensdauer von Eiern der vorratsschidlichen Motten Ephestia cautella (Wlk.) und Ephestia elutella (Hiib.) (Lepidoptera. Pyralidac) bei Temperaturen unterhalb ihres Entwicklungsminimums. Z. Angeu: Gztomol. 97. 63-70 Subrdmanyam, B. and Hagstrum, D. W. (1903) Predicting development times of six stored-product moth species (Lepidoptera: Pyralidae) in relation to temperature, relative humidity, and diet. Eur: J. Entomol. 90. 51-64 Tauber, M. J., Tauber, C. A. and Gardescu. S. (1993) Prolonged storage of Chtysoperla carnea (Neuroptera: Chrysopidae). Environ. Entomol.

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Tebbets. J. S., Curtis, C. E. and Fries, R. D. (1978) Mortality of immature stages of the navel orangeworm stored at 3.5”C. ./. Econ. Entomol.

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USEPA (1993) Regulatory action under the Clean Air Act on methyl bromide. US Environ. Prot. Agency. Off. Radiat. Programs, Washington. DC, Update, Winter 1993 Zettler, J. L., Halliday, W. R. and Arthur, F. H. (lY90) Phosphine resistance in insects infesting stored peanuts in the southeastern United States. J. Econ. Entomol. 82. 150% IS I I

Received 24 October 19% Revised 2 April 1997 Accepted 4 April 1997

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