Icerya purchasi Maskell (Hemiptera: Monophlebidae) (Australian bug) is a polyphagous and cosmopolitan pest of plants that includes citrus. Dense populations of Australian bug cause plant stress, leading to defoliation, fruit drop and a decrease in plant vitality. Australian bug excretes honeydew, which supports the growth of sooty mould, disfiguring the plant. Novius cardinalis (Mulsant) and Novius iceryae (Jenson) (Coleoptera: Coccinellidae) (Vedalia beetles) are natural enemies of the Australian bug that are generally successful in controlling their populations. Vedalia beetles are, however, susceptible to a number of insecticides used to control other insect pests in citrus orchards. High incidences of Australian bug were recently recorded in mandarin orchards in some citrus growing regions in South Africa. In order to better understand the causative factors leading to an increase in incidence of Australian bug, populations of Australian bug and Vedalia beetles were monitored on a monthly basis over two years (May 2021 – May 2023) in four mandarin orchards on four different farms in Mpumalanga Province, South Africa.

Chemical application programmes for the selected orchards were obtained and correlated with Australian bug and Vedalia beetle populations. High populations of Australian bug (>60% trees infested) were recorded between May and July 2021. After July 2021, there was a rapid decline in both Australian bug and Vedalia beetle populations. This decline was attributed to the use of chemical insecticides targeting spring pests. Populations of Australian bug returned the following year (2022) but remained low. Vedalia beetles were only recorded on two of the farms in 2022 as compared to all four farms in the first year of the study. In the second part of the study N. iceryae were released into four selected orchards with little to no chemical application histories. Novius iceryae was able to control I. purchasi populations in two of the orchards where there were no chemical interventions. It was evident from the study that insecticidal applications disrupted the biological control of Australian bug in mandarin orchards.

Click here to read the full article.

Source:

Project 1314 (April 2021 March 2023) by Leani Serfontein, Aruna Manrakhan and Evans Mauda
(CRI)