Damage to citrus and vegetables by Thaumatotibia leucotreta (Lepidoptera: Tortricidae) and prospects for control with entomopathogenic fungi

Abstract

Globally, orange is one of the major fruit crops contributing to nutrition and monetary income. False codling moth (FCM), Thaumatotibia leucotreta, is one of the major constraints of orange production. Before this study, little was known regarding the bio-ecology of FCM in orange and vegetables in Kenya and Tanzania and the potential use of dry conidia of entomopathogenic fungi for control of T. leucotreta moths has not been tested. There is also no IPM strategy available for FCM in East Africa. This PhD study therefore aimed at generating information on these aspects. Field surveys on damage inflicted by the pest on orange and vegetables were conducted in Kenya and Tanzania. The spatial-temporal population dynamics and genetic diversity of FCM were evaluated in citrus orchards in these two countries. The highest FCM larval incidence (46%) was recorded on orange produced at high altitudes in Kenya while the lowest (33%) was recorded at low altitudes in Tanzania. The highest FCM infestation amongst the vegetables was recorded on African eggplant (12%) while the lowest was on okra (3%). A similar spatio-temporal pattern of FCM was observed in both countries, with the highest catches being recorded in August, during the 2017 and 2018 orange fruiting seasons in these regions. Microbial control of the pest was tested by screening dry conidia of entomopathogenic fungi isolates of Metarhizium anisopliae and Beauveria bassiana species. Dry conidia of these EPF isolates were found to be pathogenic to the moths, the ICIPE 69 isolate caused the highest mortality of 94.2%. Fecundity was reduced by 33.6 and 25.9% for the donor (fungal contaminated moths) and recipient (fungus-free moths allowed to mate with fungal contaminated moths) FCM females, respectively after horizontal transmission. Compatibility of the potent entomopathogenic fungal isolate, ICIPE 69 and the FCM sex pheromone was tested in an auto-inoculation device. The fungus remained viable and was therefore compatible with the pheromone. The fungus in the autoinoculation device was integrated with other control tactics and evaluated in citrus orchards in Machakos and Makueni counties in Kenya. In this trial, a lower percentage of infested fruit (4.67% and 6.67%) was recorded in orchards where the treatment combination “ICIPE 69 campaign + dry conidia of ICIPE 69 applied in the autoinoculation device + Last call FCM” was applied, compared to the untreated orchards (48.67% and 54.33%) at Machakos and Makueni respectively. The effect of FCM infestation was also reflected on marketable yield, with the highest yield (10,880.68 and 11,192.26 kg orange fruit/ha) recorded in orchards where this treatment combination was applied, while the lowest yield was recorded in untreated orchards (5,944.28 and 5,458.63 kg orange fruit/ha) at Machakos and Makueni, respectively. The findings from this study indicated that T. leucotreta is present and causes significant losses in Kenya and Tanzania thus control tactics need to be implemented. The compatibility of ICIPE 69 and FCM sex pheromone in the auto-inoculation device provide for its use in integrated management strategies for the pest. A significant reduction in the T. leucotreta population and fruit infestation as well as an increase in marketable orange fruit yield were obtained with the combined use of entomopathogenic fungi and Last Call FCM. This combination can therefore be used as an integrated management strategy for FCM. Low genetic diversity of FCM specimens from Kenya, Tanzania, Uganda, Sudan and the Republic of South Africa, as well as from different hosts was determined. Similar management strategies for control of T. leucotreta can therefore be used across Africa.