Hot off the Press: Cool Temperature Performance of a Biological Control Agent of the Invasive Water Hyacinth

Wow.. I can’t believe May was my last post.. ugh! Ive been swamped with starting my new postdoc, moving into a new place, and writing an NSF-OCE grant! Anyhow, I am back and will try to be more regular again!

I am excited to announce a new article that is hot off the press! I coauthored this article with Dr. Angelica Reddy (first author) and Dr. Paul Pratt at the USDA, along with researchers from Argentina and Uruguay. You can read it here with free access for 50 days: Article in Biological Control.

This study was in conjunction with some of the work I did as a Delta Science Postdoctoral Fellow to investigate the mechanisms limiting the current biological control of invasive water hyacinth (Eichhornia crassipes) in the Sacramento-San Joaquin River Delta in northern California, USA (hereafter ‘Delta’). Classical biological control uses natural enemies (predators/herbivores, parasitoids and parasites) to control invasive populations of weeds, pests and disease vectors in the introduced range. 
Successful biological control agents can reduce pest populations below threshold levels that cause problems for humans and native species. Once established, biological control can provide a sustainable, long-lasting management option.

In a previous study, my coauthors and I conducted a one-year field survey 34 years after the initial releases of several biological control agents of water hyacinth in the Delta (Hopper et al. 2017). We found that two biological control agents, the herbivorous weevils, N. bruchi and N. eichhorniae (Coleoptera), were still present in the Delta and the associated tributaries. Although N. bruchi was broadly distributed throughout the Delta, N. eichhorniae was only found in the southernmost tributaries. Densities of N. bruchi during the warmer months in the Delta are comparable to densities in other regions with successful control of water hyacinth, but were not high enough year-round to reduce water hyacinth biomass and cover. Thus one idea to improve control is to re-introduce the more rare weevil, N. eichhorniae, in order to increase its abundance and distribution in the Delta and compliment the impacts of the existing weevil populations.

One theory for the difference in the current abundance and distribution between these two weevil species is that the present biotype of N. eichhorniae in the Delta is less cold-tolerant than N. bruchi. Thus, the researchers from the USDA and myself were interested in determining whether a cold-temperature biotype of N. eichhorniae is present. If a cold-tolerant biotype exists, then the goal will be to screen it in the quarantine (host range tests and pathogen screening), access the necessary permits, and then release it into the Delta to improve the performance of the current population of N. eichhorniae and ultimately enhance the control of the invasive water hyacinth in the Delta. 

To achieve these goals, we (Reddy et al. 2019) examined the cool temperature performance and cold tolerance of four populations of the biological control agent, N. eichhorniae. These populations consisted of N. eichhorniae from: the Delta (California: USA), a population within the native range (Uruguay), and two temperate populations (Kubusi River, Stutterheim, South Africa and Jilliby, Australia). The geo-locations of these populations are noted as red markers in the green-highlighted regions on the map.

In this study, we measured life history parameters of these weevil populations under temperatures occurring in the Delta during the cooler seasons (Fall and Winter). These life-history parameters included: Egg survivorship and development, juvenile (larval and pupal) survivorship and development, adult fecundity and adult longevity. I then used these parameters to construct stage-structured matrix models and calculate the intrinsic growth rates, doubling times, generation times and reproductive potential of each of these populations (as I have detailed in a previous blog and linked here).

In summary, Reddy et al. (2018) found that the population from Jilliby, Australia had the highest intrinsic rate of increase under conditions simulating Fall temperatures in the Delta due to the fact this population had the highest fecundity compared to all of the other populations (including the existing population residing in the Delta). Permission is thus being sought to release the Australian population into the Delta to improve the biological control of the invasive water hyacinth.

Please read the published paper for more details! 

And I will be back to update you with much more soon, especially on the results from my study on the population genetics of both of these weevils (N. bruchi and N. eichhorniae), using these same populations pictured above and many others! I finally submitted this manuscript for review.. so stay tuned!