Host utilization by the Mediterranean fruit fly, Ceratitis capitata (Diptera: Tephritidae)

DIEGO F. SEGURA; MARÍA T. VERA; JORGE L. CLADERA

Stellenbosch, South Africa

Simposio; 6th International Symposium on Fruit Flies of Economic Importance; 2002

Host utilization by fruit flies has been traditionally studied using data on infestation levels found in different hosts. This analysis hinders the comparison among hosts, because the values of use are affected by the carrying capacity (i.e. maximum number of pupae/kilogram of fruit) of each host. It also prevents the observations of different hosts species from being pooled to obtain a global measure of the degree of exploitation of available resources. In this study, host utilization was inferred calculating a new variable, percent of exploited resource (PER), which relates the extracted pupae in a host with the maximum quantity of pupae extractable in the same host. The objectives of this work were: 1) to analyze the dynamics of host utilization by C. capitata 2) to compare PER values with host availability along the fruit producing season and 3) to compare PER values among different host species.  The study was conducted at San Pedro (33º 41´ S, 59º 41´ W), in Argentina. Two types of sampling were performed: 12 fortnight samplings to describe temporary variations in PER and in hosts availability during 1999-2000 and 2 larger samplings to compare PER among hosts species in 2001. The fruits sampled were weighed and conditioned until adult emergence. Correlation analysis were carried out between PER and biotic and abiotic variables. The results showed that the percent of exploited resource (PER) reached by any host species was never above 60%, and the highest PER value obtained by pooling all species together in any fortnight sampling was below 35%, indicating that host availability is not a limiting resource in this ecosystem. However, PER was not constant along the time, probably indicating periods in which the competition become more intense. These fluctuations are associated with biotic and abiotic factors, which in turn are related to fluctuations in the activity and abundance of adults. No association was found between global PER and global availability of hosts, probably due to the lack of synchronicity between C. capitata population levels and host availability. For each species, PER usually showed a gradual growth that ended sharply when the host disappeared, while the availability grew reaching a maximum and then felt gradually. Apparently during this gradual reduction in availability the flies concentrate its egg-loadings on the few remaining fruits of the same species, instead of using other hosts species with higher availability at the time. Finally, significant differences were found comparing PER among hosts species. In most cases, the hosts species showing higher PER correspond to that with higher carrying capacity, indicating that C. capitata is exploiting preferentially the hosts with better yield. Probably this behavior tends to improve its fitness.