Spatial and temporal variation of biological control agents associated with Eichhornia crassipes in the Sacramento-San Joaquin River Delta, California

Authors: HOPPER, JULIE - University Of California; Pratt, Paul; McCue, Kent; PITCAIRN, MICHAEL - California Department Of Food And Agriculture; Moran, Patrick; Madsen, John

Submitted to: Biological Control
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/11/2017
Publication Date: 5/12/2017
Citation: Hopper, J.V., Pratt, P.D., McCue, K.F., Pitcairn, M.J., Moran, P.J., Madsen, J.D. 2017. Spatial and temporal variation of biological control agents associated with Eichhornia crassipes in the Sacramento-San Joaquin River Delta, California. Biological Control. 111:13-22. doi:10.1016/j.biocontrol.2017.05.005.

Interpretive Summary: Water hyacinth (Eichhornia crassipes) is a floating aquatic weed that invaded the Sacramento-San Joaquin River Delta in 1904. Water hyacinth severely limits the recreation, irrigation, and water services provided by the Delta. Strategies for controlling water hyacinth in the Delta include: herbicides, mechanical control and classical biological control. In classical biological control, a pest's natural enemies (for example, insect herbivores that eat water hyacinth) are collected from the geographic origin of water hyacinth (South America) and released into the invaded region (the Delta). Successful biological control agents can reduce pest populations below threshold levels that cause problems for humans and native species. Once established, biological control agents can provide a sustainable, long-lasting management option as they are self-reproducing and self-distributing. Although biological control agents will not eradicate every target pest individual, this strategy prevents population crashes of the agent and the pest. As part of the biological control program in the Delta, two beetles known as Neochetina bruchi and N. eichhorniae, and a moth, Niphograpta albiguttalis, were released into the Delta the early 1980s, and another biological control agent, the planthopper: Megamelus scutellaris was released from 2011-13. In this research study, we conducted monthly surveys at 16 sites throughout the Delta to determine the resulting establishment, abundance and distribution of these biological control agents (the introduced herbivores). In addition to using physical characteristics of the insect herbivores for identification, we also looked at the DNA (the genetic makeup) of the two beetle herbivores and sequenced a fragment of a conserved gene (COI) in the beetle’s mitochondria. We determined that both species are still present in the Delta region, with 83% similarity at the DNA level between these two beetle species. The more abundant beetle species, N. bruchi, was present in all of the study sites and the more rare beetle, N. eichhorniae, was recovered from two sites in the southern extreme of the Delta tributaries. Beetle densities varied in space and time with the highest number of beetles per plant in August (with an average of 5 beetles per plant and a maximum of 39 beetles per plant), and the lowest densities of beetles occurring in June (average of 0.54 beetles per plant). The proportion of damaged leaf area on water hyacinth from feeding by the beetles with increasing densities of beetles. The planthopper, M. scutellaris, remained established at its original release site from 2011-2013, but has not travelled into the other surveyed areas from this study. We suggest explanations for the current patterns in the abundance and distribution of these biological control agents, with potential mechanisms for improved future biological control.

Technical Abstract: Invasive aquatic weeds, such as water hyacinth (Eichhornia crassipes), severely limit the ecosystem services provided by the Sacramento-San Joaquin River Delta. As part of the biological control program in the Delta, two weevils, Neochetina bruchi and N. eichhorniae (Coleoptera: Curculionidae) and a moth, Niphograpta albiguttalis (Warren) (Lepidoptera: Pyralidae), were released in the early 1980s, and another agent, Megamelus scutellaris (Hemiptera: Delphacidae) was released in 2011. We conducted monthly surveys at 16 sites throughout 1,667 km2 of the Delta to determine the resulting establishment, abundance and distribution of these introduced herbivores. Morphological identifications, and sequencing of 616 bp from the mitochondrial cytochrome oxidase subunit 1 (COI) gene (94 SNPs between weevil species) determined that both species are still present in the Delta region. From all of the study sites, 96.6% of the examined weevils were identified as N. bruchi, and all of the 3.4% N. eichhorniae individuals were recovered from two sites in the southern extreme of the Delta tributaries. Weevil densities (larvae and adult weevils per destructively sampled plant) varied spatially and temporally. Peak mean densities (averaged across August-November) decreased with increasing distance (km) from the original biological control release sites. Peak mean densities of 6.31 weevils were found at one site and 0.31 weevils at another site just 12 km away, Densities averaged across sites were the lowest in June 2015 (0.54 weevils), increasing in August to 5.35 weevils, and peaking in November at 6.22 weevils, with a maximum density of 39 weevils per plant in August. The proportion of damaged leaf area from weevil feeding increased concomitantly with weevil densities. M. scutellaris remained established at its original release site but has not dispersed into the other surveyed regions. We propose hypotheses to explain patterns in species establishment and distribution, with potential mechanisms for improved future biological control.