Cold winter temperatures affect biological control agents of Pontederia crassipes in South Africa
Pontederia crassipes (Pontederiaceae – listed as Eichhornia crassipes, EPPO A2 List) is one of the world’s most invasive aquatic plants. Native to South America, P. crassipes has been introduced to countries throughout the world where it causes significant negative impacts including blocking water channels, degrading biological diversity and providing breeding grounds for mosquitoes. Biological control against P. crassipes has been implemented in many countries using a number of biocontrol agents with the most common being the weevils Neochetina eichhorniae and N. bruchi (Coleoptera: Erirhinidae). In South Africa, a further 7 biocontrol agents have been released with limited success compared to other countries. Reasons for this can be due to the high nutrient status of rivers and dams in South Africa which enhance the growth rate of P. crassipes, the improper or overuse of non-specific chemical herbicides, and cold winter temperatures. In the case of the latter, new biocontrol agents have been sought which have shorter generation times and can build up the population rapidly post-winter. Megamelus scutellaris (Hemiptera: Delphacidae) is a host specific natural enemy of Pontederia crassipes with short and overlapping generations and was approved for release in South Africa in 2013. Over a 15-month period (May 2017-August 2018) the population dynamics of M. scutellaris was studied along the Kubusi River. Upon the onset of the winter, M. scutellaris incurred a severe population decline when Pontederia crassipes became frost damaged. This decline coupled with the low minimum winter temperatures (6.1°C) caused a post-winter lag in the density increase of the biological agent population. M. scutellaris population density only recovered to its maximum level at the end of the following summer growing season which allowed the P. crassipes population to recover. Supplementary releases of M. scutellaris from mass-reared cultures at the beginning of the growing season (spring) could act as a potential method of reducing the lag-period in field populations in colder areas.
Sources
Miller BE, Coetzee JA, Hill MP (2020) Mind the gap: the delayed recovery of a population of the biological control agent Megamelus scutellaris Berg. (Hemiptera: Delphacidae) on water hyacinth after winter. Bulletin of Entomological Research 111(1), 1–9. https://doi.org/10.1017/S0007485320000516