Improving the Efficiency and Safety of Sentinel Stink Bug Eggs Using X-rays

Abstract

Sentinel eggs used to monitor field parasitism of stink bug pests (Hemiptera: Pentatomidae) can only be deployed for a few days to avoid releasing the pest in the monitored area. Using sterile eggs removes the risk of accidental pest introduction and extends deployment time. Freezing the eggs before deployment is one common method of sterilizing sentinel eggs. However, some egg parasitoid species have low or no parasitism on frozen eggs. In this study, X-ray irradiation was used to sterilize Bagrada hilaris sentinel eggs intended for monitoring parasitism by Gryon aetherium (Hymenoptera: Scelionidae), the most promising biological control candidate. In this case, freezing sentinel eggs is not recommended because G. aetherium has low levels of parasitism on frozen eggs. Doses as low as 10 Gy induced 100% sterility. Irradiated eggs successfully sustained the development of G. aetherium and Ooencyrtus californicus (Hymenoptera: Encyrtidae), another egg parasitoid attacking B. hilaris, and parasitism levels were comparable to that of fresh eggs up to seven days old. In addition, G. aetherium showed no preference for fresh non-irradiated eggs over seven-day-old irradiated eggs. Our results indicate that X-ray irradiation is a suitable alternative to produce safe and reliable sentinel eggs to monitor the egg parasitism of B. hilaris and possibly other species.

Keywords: biological control; egg parasitoids; egg sterilization; field monitoring; irradiation; pentatomids.

Figure 1

Interior of the X-ray irradiator showing samples in Ziploc bags rotating adjacent to the radiation source. Real-time dose measurements from the attached ion probe were read from an external display, and X-rays were discontinued when the desired dose was reached.

Figure 2

Influence of X-ray irradiation (0–40 Gy) on the percentage of sand-free (“no sand”) and sand-covered (“sand”) B. hilaris eggs showing no or some embryo development or eclosed 12 days post irradiation.

Figure 3

Percentage of 1, 2, 3, and 7 d old (refrigerated) eggs showing no or some embryo development, or eclosed 9 days post irradiation (40 Gy).

Figure 4

Percentage of parasitism and sex ratio (percentage female) by G. aetherium on (a) fresh (<24 h old), (b) 7-, and (c) 12-day-old irradiated (40 Gy) B. hilaris eggs. Controls were fresh B. hilaris eggs, with the addition of one control consisting of 7-day-old non-irradiated eggs when testing 7-day-old irradiated eggs. Bars with different letters are significantly different (GLM or Tukey’s HSD, p < 0.05).

Figure 5

Percentage of parasitism and sex ratio (percentage female) by O. californicus on 7-day-old irradiated (40 Gy) B. hilaris eggs. Controls were fresh B. hilaris eggs. Bars with different letters are significantly different (GLMM, p < 0.05).

Figure 6

Percentage parasitism and female production when G. aetherium was given a choice between 7-day-old irradiated eggs (40 Gy) and fresh (<24 h old) B. hilaris eggs. Bars with different letters are significantly different (GLMM, p < 0.05).