Abstract
Bactrocera minax, one of the most devastating citrus pests in Asia, has two developmental stages (mature larva and pupa) that complete their life cycle in the soil. Currently, southern China has a climate with abundant autumn rains, and soil moisture can be a major factor affecting the survival of larvae and pupae of B. minax. In the present study, we evaluated the effects of water immersion and high soil moisture content on the development of mature larvae and pupae of B. minax. When immersed in water for 1 d, 100% of mature larvae of B. minax were knocked out. When larvae were immersed for less than 6 d, however, more than 92% of knocked-out larvae recovered within 24 h. The days of water immersion with 50% and 90% recovery ratios (indicated as RD50 and RD90) were 10.3 d and 6.4 d, respectively. When larvae were immersed less than 6 d, the mortality ratios of larvae were not significantly different from those that were not immersed at all. The days of immersion causing 50% and 90% mortality of larvae (MD50 and MD90, respectively) were 7.6 d and 11.1 d, respectively. The pupation ratios of larvae were also observed to be not significantly different compared to non-immersion, and the days of immersion causing 50% and 90% pupation (PD50 and PD90, respectively) were 6.6 d and 0.8 d, respectively. Larval respiration rates were reduced after water immersion as a strategy for larval survival. High water content was not detrimental to pupae of B. minax. Adult emergence did not significantly decrease in soil with high water content, even though pupae were under those conditions for 161–175 d. The respiration rates of pupae were lower in soil with different moisture levels and were not significantly different, which ensured the survival of pupae in high water content. Reduced respiration rate is a strategy for survival of larvae and pupae, and remarkable tolerance to high moisture conditions could explain the high rate of spread and geographical distribution of B. minax. The results of this study provide a reference for the occurrence and control of B. minax.
Highlights
The respiratory organs of insects consist of tracheal tubes with external spiracular valves that control gas exchange [1]
We suggest that the lower respiration rates are one survival strategy for B. minax larvae forced into water immersion
We suggest that B. minax pupae survived in the high water content soil by maintaining lower respiration rates, which decreased with the oxygen requirement for the entire pupa stage
Summary
The respiratory organs of insects consist of tracheal tubes with external spiracular valves that control gas exchange [1]. Many insects have discontinuous patterns of gas exchange, and the respiratory system of insects consists of a highly branched system of cuticle-lined tubes extending throughout the body [2], which may have initially evolved in underground insects to deal with hypoxic conditions [3]. Soil oxygen concentrations are affected by moisture content [4]. Eskafi and Fernandez [5] identified the lack of oxygen as an important factor for increased mortality in pupa of Ceratitis capitata, when moisture content reached a saturation point. Hetz and Bradley [1] reported decreased respiration of underground insects. Behavioral and physiological adaptations for surviving moist environments are important life traits for terrestrial invertebrates. Tolerance of insects to water immersion is different among species
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