1. bookVolume 31 (2022): Issue 2 (July 2022)
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2719-9509
First Published
01 Jan 1992
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Phosphine Susceptibility of Adult Megaselia scalaris (Loew) (Diptera: Phoridae)

Published Online: 15 Aug 2022
Volume & Issue: Volume 31 (2022) - Issue 2 (July 2022)
Page range: 101 - 105
Received: 03 Oct 2021
Accepted: 14 Apr 2022
Journal Details
License
Format
Journal
eISSN
2719-9509
First Published
01 Jan 1992
Publication timeframe
4 times per year
Languages
English
INTRODUCTION

Megaselia scalaris (Loew) (Diptera: Phoridae) is a cosmopolitan and synanthropic fly mainly found near areas inhabited by humans in warm climate regions (1). It is essentially a detritivore species feeding on a variety of foods of both animal and plant origins, regardless of whether they are fresh or decaying (2). These diverse feeding habits make this species a facultative predator, parasite, and parasitoid of invertebrates and vertebrates (2, 3) and it is classified as a quarantine pest in the Eurasian Economic Union (4). Effective disinfestation methods of this insect have not been developed because it is not regarded as an agricultural/veterinary pest in the other countries. Recently, Rosselkhoznadzor (Federal Service for Veterinary and Phytosanitary Surveillance of Russia) announced that this quarantine species was found in imported containers with raw tobacco from various countries in Africa, South and North America, Europe, and Asia, and the phytosanitary regulation of tobacco has been enhanced (5, 6). The larvae of this species depend on moist decaying and relatively fluid plant or animal matter suitable for their sponging mouthparts, and females lay eggs on decomposing material that has become a fluid, allowing larvae to feed as soon as they hatch (7). Therefore, cured tobacco is not infested by this species, and the adults intercepted in Russian ports must have been hitchhikers during marine transportation.

In the tobacco industry, phosphine has been the most widely used insecticide to control of common insect pests associated with cured tobaccos such as the cigarette beetle, Lasioderma serricorne (Coleoptera: Ptinidae) and the tobacco moth Ephestia elutella (Lepidoptera: Pyralidae) (8). There are no data on the efficacy of phosphine against M. scalaris. At first, we exposed eggs, larvae, pupae and adults to 200 ppm phosphine to determine relative susceptibility to phosphine in developmental stages. A phytosanitary treatment should be effective against the most tolerant stage that may be present in a traded commodity (9). Because life stages other than adults are not associated with cured tobacco, the following study was conducted to examine the effects of phosphine on adult flies and to estimate the dose required for a phytosanitary treatment for this species.

MATERIALS AND METHODS
Test insects

Megaselia scalaris was originally purchased from Sumika Technoservice Corp. (Hyogo, Japan) and maintained on a 1:1:2 mixture of rodent diet (Oriental Yeast Co. Ltd., Tokyo, Japan), wheat bran, and water (10), under 24–28 °C. Twenty unsexed adults (2–3 d after emergence) were released on 4 g of feed in 29 mm ∅ × 95 mm polystyrene vials, and the vials were capped with a polyurethane foam plug. After 24–48 h, the adults were removed, and another 48–72 h later, ca. 1 g of wheat bran was used in the diet for pupation. The pupae were collected from the bran and maintained under rearing conditions. The adults were collected within 24 h of emergence and fed a 10% glucose solution.

Fumigation procedure

The third instar larvae and pupae for the evaluation of phosphine susceptibility were collected from rearing vials. Adults within 24 h after emergence were collected after being kept at 5 °C for 5–10 min to suppress their moving. To prepare eggs, 30 unsexed adults of 48–72 h after emergence were enclosed in 25 mm ∅ × 50 mm vials and covered with #80 nylon mesh. A piece of absorbent cotton soaked in 1% solution of beef extract (Nacalai Tesque, Kyoto, Japan) was put on the mesh cover, and the vial was kept at 27 °C for 12–14 h under darkness. The deposited eggs on the mesh and cotton were picked up with a fine brush. Fifty eggs, 30 larvae, 30 pupae, or 30 adults were placed as a group into polystyrene vials measuring 29 mm in diameter and 95 mm long with 1 g feed. The vials were then placed in gas-tight acrylic resin container measuring 160 mm × 295 mm × 100 mm. Three vials for each developmental stage were used in the experiment. Phosphine fumigation was performed using the protocols described by HORI and KASAISHI (11). Phosphine was generated from aluminum phosphide (Tyvek®, Degesch Japan Co. Ltd., Saitama, Japan) and introduced into the containers after adjusting the concentrations using a phosphine gas analyser (Komyo Rikagaku Kogyo K.K., Kanagawa, Japan).

To assess the susceptibility of developmental stages, the containers with 200 ppm phosphine were maintained at 25 °C for either of 2, 4, 8, or 24 h in an MLR-352 plant growth chamber (PHC Corporation, Tokyo, Japan). For untreated control, the container prepared in the same manner without phosphine were kept at 25 °C for 24 h. For dose-response assay, the containers with/without phosphine were maintained at either of 15, 20, or 25 °C for 6 h. After exposure, the insects in vials were held at 27 °C until their survival was checked. Life or death of adults was determined 1–2 h after treatments and those of the other stages were determined 14 d after treatments.

Data analysis

Probit analysis was performed using the PriProbit (ver. 1.63) computer program developed by SAKUMA (12) which was downloaded from http://www.ars.usda.gov/Services/docs.htm?docid=11284. The phosphine concentrations needed to achieve probit 9 mortality (≈LC99.9968) were read from the data table for linear graphic display.

RESULTS AND DISCUSSION
Relative susceptibility to phosphine in developmental stages

Table 1 shows the lethality of phosphine to each developmental stage. All adults exposed to 200 ppm phosphine at 25 °C died within 4 h, and the most pupae survived the same treatment for 24 h exposure. These results show that the pupa is the most tolerant and that the adult is the most susceptible stage to phosphine. Fortunately, life stages other than adults are not associated with cured tobacco, and therefore a phytosanitary treatment should be effective against adult, the least tolerant stage.

Phosphine susceptibility of developmental stages of Megaselia scalaris (200 ppm, 25 °C).

Stage Number of insects Mortality (%)
Exposure time (h)
0 (control) 2 4 8 24
Egg 150 2.0 4.0 18.0 84.7 100
Larva 90 0.0 4.4 20.0 71.1 100
Pupa 90 4.4 1.1 7.8 13.3 15.6
Adult 90 0.0 75.6 100 100 100
Probit 9 mortality of adult

No adult flies died that were not treated with phosphine. The mortality data obtained in this test were well fitted with the probit model (Figure 1). The phosphine concentrations required to achieve probit 9 mortalities (≈LC99.9968) in the adult were calculated to be 636.2 ppm at 15 °C, 565.9 ppm at 20 °C, and 280.1 ppm at 25 °C (Table 2). The concentration × time products (Ct, ppm·d) at the respective temperatures were calculated to be 159.1 ppm·d at 15 °C, 141.5 ppm·d at 20 °C, and 70.0 ppm·d at 25 °C. These values are lower than those recommended for the control of insect pests of cured tobacco leaves by CORESTA, which are 1800 ppm·d (300 ppm × 6 d) at 16–20 °C and 800 ppm·d (200 ppm × 4 d) at temperatures higher than 20 °C (8). Phosphine is much more effective at lower concentrations over longer exposure periods than at higher concentrations for a shorter period; the relationship can be expressed by the modified form Cnt = k (constant), where n is less than 1 (13). Such a relationship has not been confirmed in this species. However, it appears that the CORESTA recommendations for cured tobacco fumigation will also control the adults of M. scalaris.

Figure 1

Probit mortality of adult Megaselia scalaris in relation to phosphine concentration with 6 h exposure at three different temperatures. Filled circles, cross marks, and open circles represent mortality at 15 °C, 20 °C, and 25 °C, respectively.

Phosphine concentrations (ppm) required to achieve 99% (LC99) and 99.9968% (LC99.9968 Probit 9) mortalities of adult Megaselia scalaris with 6 h exposure.

Temperature Number of insects Slope ± SE Intercept ± SE LC99 (95% Fiducial limits), ppm LC99.9968 (95% Fiducial limits), ppm
15 °C 1350 4.394 ± 0.334 −8.319 ± 0.658 264.6 (223.9 – 332.0) 636.2 (478.5 – 942.9)
20 °C 1260 4.081 ± 0.383 −7.234 ± 0.703 22.0 (176.4 – 305.2) 565.9 (388.5 – 996.6)
25 °C 1440 4.208 ± 0.269 −6.298 ± 0.430 112.0 (96.9 – 135.0) 280.1 (218.3 – 387.3)

Large-scale confirmatory tests should be further performed to validate the efficacy for quarantine treatment. Theoretically, a minimum of 93,613 insects are required to verify the probit standard at the 95% confidence level (14). The estimated probit 9 values obtained in this study can be utilized for the development of phosphine-based quarantine and preshipment treatments for this species.

Figure 1

Probit mortality of adult Megaselia scalaris in relation to phosphine concentration with 6 h exposure at three different temperatures. Filled circles, cross marks, and open circles represent mortality at 15 °C, 20 °C, and 25 °C, respectively.
Probit mortality of adult Megaselia scalaris in relation to phosphine concentration with 6 h exposure at three different temperatures. Filled circles, cross marks, and open circles represent mortality at 15 °C, 20 °C, and 25 °C, respectively.

Phosphine concentrations (ppm) required to achieve 99% (LC99) and 99.9968% (LC99.9968 Probit 9) mortalities of adult Megaselia scalaris with 6 h exposure.

Temperature Number of insects Slope ± SE Intercept ± SE LC99 (95% Fiducial limits), ppm LC99.9968 (95% Fiducial limits), ppm
15 °C 1350 4.394 ± 0.334 −8.319 ± 0.658 264.6 (223.9 – 332.0) 636.2 (478.5 – 942.9)
20 °C 1260 4.081 ± 0.383 −7.234 ± 0.703 22.0 (176.4 – 305.2) 565.9 (388.5 – 996.6)
25 °C 1440 4.208 ± 0.269 −6.298 ± 0.430 112.0 (96.9 – 135.0) 280.1 (218.3 – 387.3)

Phosphine susceptibility of developmental stages of Megaselia scalaris (200 ppm, 25 °C).

Stage Number of insects Mortality (%)
Exposure time (h)
0 (control) 2 4 8 24
Egg 150 2.0 4.0 18.0 84.7 100
Larva 90 0.0 4.4 20.0 71.1 100
Pupa 90 4.4 1.1 7.8 13.3 15.6
Adult 90 0.0 75.6 100 100 100

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