Conotrachelus dimidiatus

Torres, Betzabeth Cecilia Pérez, Skuhrovec, Jiří, Marín-Cevada, Vianey & Elizalde-González, María P., 2018, Conotrachelus dimidiatus Champion, 1904 (Coleoptera: Curculionidae: Molytinae): morphological re-description of the immature stages, keys, tribal comparisons and biology, Zootaxa 4433 (1), pp. 127-140: 137

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Conotrachelus dimidiatus


Biology of Conotrachelus dimidiatus 

Field studies. The adults of Conotrachelus dimidiatus  live on trees, mainly on the bottom side of leaves, on hard, green immature fruits (length of 2‒3 cm), in their peduncles or, preferably, in the calyx. The adults avoid direct sunshine and remain on the side with shade and fly short distances from one branch to another. During daytime, they walk, eat and rest, and are almost undetectable, becoming more active with the sunset. In the darkness of the night, flight and copulation intensify, the latter lasting an average of 10 minutes. Females are mounted while eating. Adults feed on any part of the smooth surface of the fruit by “drilling” first with their rostrum, immersing it completely, and sucking up fluids for a long period. The “guava sawdust” produced during drilling is expelled and remains on the fruit surface as residue.

Ovipositioning holes of 1‒2 mm diameter remain open and function as the point of deterioration on the fruit. Pregnant females select one undamaged fruit, drill a hole as long and wide as their rostrum, turn around and deposit one single egg, turn around again, push the egg with the rostrum into the depth of the fruit and seal the perforation and its opening with a soft mixture of sawdust and secretions from their mandibles. The creamy seal turns into a dark, hard cap, signaling that oviposition has occurred, and the selected immature guava remains untouched by other females.

After 8‒10 days, a single larva emerges, falls and burrows into the soil. This occurs in September‒October when the larvae penetrate the soil to a depth of 15‒20 cm to form a cocoon formed by soil and secretions. Adults start emerging from the soil in July, and this event is triggered by the start of the rainy season in Calvillo.

Breeding in the laboratory. The orchard subjected to investigation in this study consisted of 80 Psidium guajava trees, of which 80‒90% exhibited a severe infestation level of Conotrachelus dimidiatus  . Approximately 96% of guava fruits collected in Calvillo (n = 100) from different trees contained a solitary larva or a single egg. In the laboratory, 30 immature fruits marked with oviposition were placed inside a plastic container until the larvae were born in the laboratory. After an average of five days, the larvae (n = 10) emerged and were transferred individually to smaller plastic containers (3 x 3.5 cm) on a 10 cm bed of Calvillo soil mixed with peat-moss (1:1). The soil was preliminarily sieved, sterilized and humidified. The substrate consisted of grated and sieved immature guava (36%) and a mixture of agar, formaldehyde, sorbic acid, ascorbic acid, wheat and yeast. The incubation period can be estimated as longer than five days, since the observed fruits arrived in the laboratory already containing an egg. In a parallel trial, 20 dead larvae and 40 live larvae were extracted from 60 mature fruits in the laboratory. They were kept as previously described. This observation indicated a high mortality rate for the larvae inside the guava after cutting, and when compared to the survivors in the laboratory, the larvae born in the laboratory lived longer than the larvae that arrived inside guava fruits. The fact that few larvae from cut fruits emerged may be due to diminishing water and nutrients in the fruit setting after cutting. The observation of lower mortality of newborn larvae in comparison with those that arrived developed can be explained by the greater time of adaptation to new conditions and by the greater sensitivity of the larvae in the stage prior to pupation. Fruits and larvae were kept at controlled conditions of 26 ± 1°C, 70 ± 10% humidity and a photoperiod of 12 h.