Bioecology of the guava weevil Conotrachelus psidii Marshall (Coleoptera: Curculionidae)

Abstract

The guava weevil, Conotrachelus psidii Marshall (Coleoptera: Curculionidae), is a key pest in guava and information concerning the bioecology of this insect is scarce. This work aimed to verify the bioecology of certain aspects of this insect's lifecycle by observations of the period during which the larvae abandon the fruits in order to enter the soil to pupate, the effect of different soil types on penetration times and the subsequent emergence of adults from the soil. Adult population levels were monitored over a two year period and the possible relationship between population density and climatic factors investigated. The abandonment of guava fruits by the larvae was evaluated during two time periods (day-time and night-time). The time taken for 4th instar larvae to penetrate 3 different soil types was observed. The time these insects remained underground and subsequent adult emergence periodicity was evaluated following the release of 4th instar larvae into trays filled with soil. These trays were maintained in the field and observations carried out three times a day (morning, noon, and afternoon). Path analysis was used to investigate the relationship between the climate variables and adult population density. Soil penetration by 4th instar larvae was fastest in clay soils and slowest in sand. There was no difference (p = 0.075) between the number of larvae that abandoned the fruits during the day (51%) and during the night (49%). A majority of the adults emerged from the soil during daytime (86.84%), which was significantly different (p < 0.001) to the number of adults emerging during the night (13.16%). When comparing adult emergence during different periods of the day, the majority emerged in the afternoon (93.94%), with only 6% emerging in the morning (p < 0.001). The average time period between penetration of the soil by 4th instar larvae and adult emergence was approximately nine months (272 days). Adult population density fluctuations were similar during both years studied here, with the lowest population seen from May to August, a peak in November, and then a decline in population density until March. Path analysis showed that rainfall positively influenced C. psidii population density. The results of this work will be important for the development of sustainable IPM programs aiming to control this serious pest of guava orchards.