MARINE PROGRAMS
Strategic Priorities

Biological Control of Weeds Projects

What is Biological Control?

Safety in Biological Control

Biological Control of Spartina

Biological Control of Knotweeds

References

Biocontrol Staff

"Invasive Spartina in Willapa Bay" website

"Olympic Region Harmful Algal Blooms" website

Biological Control of Spartina spp.

Overview

The planthopper, Prokelisia marginata, was introduced into Willapa Bay, WA beginning in 2000 for the biological control of Spartina alterniflora.  The introduction was made only after extensive host specificity testing by scientists from the University of California Davis and a review by the Technical Advisory Group on Biological Control of Weeds determined that the risks to non-target plants were minute.  The planthopper is native to the Atlantic and Gulf Coasts and also occurs in San Francisco Bay, CA.  P. marginata is a Spartina specialist.  It weakens and kills the plant by sucking sap from the leaves. 

Since its introduction, P. marginata has demonstrated potential to become an effective biocontrol agent.  Reproductive rates of P. marginata in Willapa Bay are among the highest recorded for the species and localized impacts on the target plant have also been measured (see Grevstad et al. 2003).  Our studies have identified habitat characteristics that correlate with improved performance of the planthopper, including low spider abundance, high leaf nitrogen, and the presence of intact thatch through the winter (Grevstad et al. 2004).  In addition, introductions of east coast biotypes of P. marginata are also contributing to improved performance.  Significant impacts on seedset have been measured in areas of moderate to high densities of P. marginata.  The insect populations are still expanding and the full impacts will require more time.

Biocontrol agent information

Prokelisia marginata is native to the Gulf and Atlantic coasts of North America.  It also occurs along the central California coast (San Francisco Bay and nearby estuaries), where it is believed to be introduced.  The known host range of P. marginata includes three very closely related species: S. alterniflora (invasive in Willapa Bay),  S. anglica (invasive in Puget Sound), and S. foliosa (native to California and the Baja Peninsula).  It has never been reported in association with any other plant and host range testing confirmed its its specificity (Grevstad et al. 2003).

In Willapa Bay, Washington, P. marginata has 2 generations per year.  Adult females insert eggs through the upper leaf surface into the vascular bundle using a piercing ovipositor.  The nymphs pass through 5 instars before molting into the adult stage.  Both nymphs and adults ingest the sap of S. alterniflora by means of a stylet inserted through the top of the leaf into the phloem tissue.  Excess sugars and liquids are excreted as honeydew onto the plant surface.  P. marginata overwinters primarily in the nymph stage.  The nymphs remain in the rolled leaves of senesced culms until early spring, when they leave wintering sites to find new green shoots to feed on.

Like many planthoppers, P. marginata adults are polymorphic for wing length.  Brachypterous individuals have full-length forewings but greatly reduced hind wings.  Macropterous individuals have 2 pairs of full-length wings and are able to fly longer distances.  In both east and west coast populations, the frequency of macroptery ranges from 20 to 91% depending on location and environmental conditions (Denno, 1978).  The frequency of macroptery increases when nymphs develop under crowded conditions (Denno, 1976; Strong and Stiling, 1983). 

Impacts on Spartina

We have measured a 90% reduction in seed viability on culms on which a moderate to high density of planthoppers have been feeding (see figure). 

“Hopper burn”, or the browning of plants from feeding damage, is readily visible at some release sites toward the end of the season. 

In a controlled field cage experiment, we measured a 50% reduction in above-ground biomass of Spartina after just one summer of exposure to P. marginata (Grevstad et al. 2003). 

These initial results are encouraging signs that P. marginata could have a very substantial impact on Spartina in the future.  However, the full extent of the impacts at larger temporal and spatial scale will only be known with more time. 

Vulnerability of Willapa Bay Spartina to herbivory

The Spartina biocontrol project was encouraged by earlier work by Daehler and Strong (1997) demonstrating a high degree of vulnerability in Willapa Bay S. alterniflora to P. marginata.  In greenhouse experiments, all S. alterniflora clones collected from Willapa Bay were severely stunted or killed after two growing seasons with P. marginata.  In contrast, S. alterniflora from Florida, Maryland, and San Francisco Bay were tolerant of even high densities of the planthopper.  The possibility that this vulnerability was the result of a plant disease such as phytoplasma or Xylella bacteria vectored by planthoppers was ruled out by Davis et al. (2002).  Another explanation is that the Willapa Bay population, having grown in the absence of insect natural enemies for over 100 years, has evolved to lose its resistance to the planthopper. 

Potential future agents from Spartina’s native range

The use of multiple agents can enhance biocontrol effectiveness by contributing to control in different habitats, providing additive stress on the plants, and by increasing the odds that at least one very effective agent will be established. 

We have completed field surveys for additional natural enemies of Spartina residing on the Atlantic Coast of North America. These investigations revealed 22 insect species that appear to specialize on S. alterniflora.  Among the more promising are three stem-boring flies Chaetopsis aenea, Chaetopsis apicalis (Ottitidae) and Thripticus violaceous (Dolichopodidae).  The larvae of these flies develop inside young S. alterniflora shoots, feeding on meristem tissue and developing leaves.  The result is death of the shoot tip and no flower production in ~ 100 % of the stems that are infested.  All three species occur from Florida to Maine.  The potency of these flies lies in the fact that a single larva can kill a shoot.  For comparison, it takes approximately 200 Prokelisia planthoppers to kill a shoot (Daehler and Strong 1997).  In our surveys, we found several sites where the rate of shoot death due to this insect was greater than 50%.  Another candidate agent is the katydid Orchelimum fulicinium, which chews on leaf tissue and flower heads causing substantial damage in some areas.  The mirid bug Trigonotylus uhleri, and the scale insect Haliaspis spartina are of also of special interest because they already occur in California.   Thus, introduction of these two species into Washington State would not pose risk to the California native Spartina foliosa, if the insects ever dispersed there.