Effects of gamma irradiation on different stages of mealybug Dysmicoccus neobrevipes (Hemiptera: Pseudococcidae)
Highlights
► Gamma Co-60 irradiation as a potential phytosanitary for quarantine treatments. ► Reproduction ability of D. neobrevipes has been efficiently inhibited at low dose. ► Pattern of tolerance to irradiation was 1st<2nd<3rd instars<adults. ► Doses from 200 to 250 Gy could be efficient to prevent the reproduction of mealybug.
Introduction
Dysmicoccus neobrevipes (Beardsley), Planococcus lilacinus (Cockerell) and Planococcus minor (Maskell) (Hemiptera: Pseudococcidae) are pests found on red dragon fruits (Hylocereus spp.) in Vietnam (APHIS, 2008b). The gray pineapple mealybug Dysmicoccus neobrevipes was described from specimens collected in Hawaii and determined that this is one of the most economically important mealybug pests in Hawaii (Beardsley, 1959). This pest attacks a wide range of plants, including agricultural, horticultural and forest species (Ben-Dov et al., 2006). D. neobrevipes goes through three nymphal stages before becoming an adult. The life span (first instar to death as an adult) varies from 59 to 117 days. This species does not lay eggs. Instead they are ovoviviparous, meaning the eggs hatch within the female. Thus, births live young (nymphs). First instars, called “crawlers” are the primary dispersal stage in all mealybug species. Female nymphs molt three times before reaching adult maturity (Kessing and Mau, 1992). Feeding on young growth causes severe stunting and distortion of leaves, thickening of stems, and a bunchy - top appearance of shoots; in severe cases the leaves may fall prematurely. Also, honeydew deposited on leaves and fruit by mealybugs serves as a medium for growth of black sooty molds, which interfere with photosynthesis and reduce the crop's market value (Jahn, 1993). Since 2008, Vietnam dragon fruit has been exported to the United States using the generic radiation dose of 400 Gy because that dose is currently required if mealybugs are ([APHIS], Animal and Plant Health Inspection Service, 2008b, Hallman et al., 2010). The generic radiation doses can be lowered for specific pests and commodities if this is practical. If lowering of the dose for the commodity of interest, cost of treatment will be reduced and the capacity of the treatment facility may be increased owing to shorter treatment time. For example, Follett (2009) reported that a reduction in dose for Hawaiian sweet potato from 400 to 150 Gy resulted in a 60% reduction in cost of treatment.
Research data of quarantine irradiation on mealybugs are limited. Dohino and Masaki (1995) found that 200 Gy did not completely prevent hatch of eggs of Pseudococcus comstocki (Kuwana) laid by irradiated adults. Some of the hatchlings developed to the adult stage, but did not reproduce. Studies on the radiation tolerance of pink hibiscus mealybug, Maconellicoccus hirsutus (Green) found that at 100 Gy 1.2% of eggs laid by irradiated adults hatched, while non hatched at the next highest does, 250 Gy (Jacobsen and Hara, 2003). Another study showed that while 100 Gy to adult Planococcus minor resulted in 49% egg hatch, none hatched at >150 Gy (Ravuiwasa et al., 2009).
In order to reduce the treatment dose of 400 Gy and to provide more information on a generic dose, the effect of radiation on different stages and the most tolerant stage of above mentioned mealybugs infesting on dragon fruits is needed to be researched. In this work, we study effects of radiation on different stages and reproduction of adult D. neobrevipes to determine the optimum dose lower than of 400 Gy for quarantine treatment.
Section snippets
Insect rearing
Dysmicoccus neobrevipes was collected from the field on dragon fruit in Binh Thuan province (Vietnam) and reared in the laboratory on the surface of pumpkin (Cucurbita moschata) at 28±2 °C, RH 70±5%. The pumpkin was used as a host for D. neobrevipes because the shelf life of dragon fruit is rather short (approximately 4–5 days). Development stages and the life cycle of D. neobrevipes were determined. Under laboratory conditions, the total life cycle of this mealybug is 30.5±0.3 days. Development
Effect of gamma irradiation on mortality of nymphs and adults
The mortality of D. neobrevipes was determined by day 21 after irradiation treatment at each stage of life cycle. The obtained results in Table 1 indicated that mortality of all stages and irradiation dose had significant interaction (F=4.86, P=0.0002) after treatment. The natural mortality in non-irradiated treatment fluctuated from 0.67 to 5.0% depending on their development stages. It could be concluded that all of treatment doses affected on the survival ability at different development
Conclusions
All irradiation doses delayed the development of immature stages of D. neobrevipes and caused eventual death in comparison with the control.
Irradiation dosages from 100 to 250 Gy with 50 Gy increment affected on the survival ability of development stages D. neobrevipes. Percentage of the mortality increased with the increase of dose. Immature stages are more sensitive to irradiation than the adult one. The pattern of tolerance to irradiation in D. neobrevipes was 1st instars<2nd instars<3rd
Acknowledgments
We are grateful to Dr. G.J. Hallman (USDA, Agricultural Research Service, Weslaco, TX) for helpful discussions. This research is partly supported by the International Atomic Energy Agency (IAEA) under Research Contract no. 15635.
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