Submitted to: Biological Control
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/8/2004
Publication Date: 7/7/2005
Citation: Dudley, T.L., Kazmer, D.J. 2005. Field assessment of the risk posed by diorhabda elongata, a biocontrol agent for control of saltcedar (Tamarix spp.), to a non-target plant, frankenia salina. Biological Control. 35:265-275.
Interpretive Summary: Classical biological control of weeds is based on the importation and release of non-native herbivores and pathogens that debilitate or kill the target weed. Because this type of biological control involves the release of non-native organisms, great emphasis is placed on identifying organisms that will have significant impacts on the target weed yet have no or negligible impacts on non-target organisms. Host specificity testing is a key step in screening candidate biological control organisms for these attributes. However, the results of host specificity tests are often difficult to interpret because they are done under artificial laboratory conditions as opposed to field conditions. Under laboratory conditions, where the candidate organisms are typically confined in small cages, herbivorous insects may feed, lay eggs, and complete development on non-target plants that they would not attack under field conditions. The saltcedar leaf beetle, Diorhabda elongata, is the first biological control agent to be introduced against saltcedar (a complex of several Tamarix species and their hybrids) in North America. Earlier laboratory host specificity testing of this agent indicated that that some feeding and oviposition may occur on plants in the genus Frankenia, especially the species F. salina. This was cause for concern because native North American Frankenia occur in some of the same habitats as saltcedar and, until recently, one Frankenia species was a federally endangered species. To determine if Frankenia would be attacked under field conditions, we transplanted F. salina and F. jamesii into experimental field release sites in Nevada and Wyoming where “free-range” D. elongata were highly abundant and causing extensive damage to the saltcedar. We then monitored the Frankenia plants for insect activity and feeding damage. Despite the high density of D. elongata on saltcedars adjacent to or over the Frankenia plants, and a dwindling supply of the preferred food (saltcedar foliage) of the beetle, only minor feeding damage was observed on F. salina at the Nevada site, only a handful of individual D. elongata were observed on the Frankenia plants over several weeks, and the Frankenia plants continued to grow vigorously. We conclude that D. elongata poses an extremely low risk to native Frankenia populations and that the benefits of D. elongata for biological control of saltcedar greatly compensate for this risk.
Technical Abstract: The biological control program for saltcedar (Tamarix spp.) has led to open releases of a specialist beetle (Chrysomelidae: Diorhabda elongata) in several research locations, but the controversy over potential impacts to native, non-target plants of the genus Frankenia remains unresolved. To assess the potential for non-target impacts under field conditions, we installed cultivated Frankenia spp. (primarily two forms of F. salina but also including F. jamesii) at locations in Nevada and Wyoming where D. elongata densities were expected to be very high, saltcedar defoliation near complete, and insects would be near starvation with high probability of attacking non-targets if these were suitable under such conditions. D. elongata populations were high, as expected, and minor, but significant, impact was observed (approximately 6% foliar damage) on both forms of F. salina under these ‘worst case’ conditions; there was no impact to F. jamesii. However, no oviposition nor larval development were observed on any plants, there was no dieback of damaged F. salina stems, and plants continued growing vigorously once insect populations subsided (but were never absent). These results indicated that, unlike in caged host-range tests, in which feeding, development and minor oviposition occurred, under ‘natural’ field conditions non-target hosts received only trivial damage, with no mortality and rapid recovery. Other ecological factors, such as distance from natural Frankenia spp. populations, inability for agents to survive at those sites, and intrinsic insect behavior that makes colonization and/or genetic adaptation extremely unlikely, ensure that non-target impacts following program implementation will be insignificant, and probably non-existent. Host range testing of new agents, while necessary to ensure safety, must put greater attention on assessing the ecological context in which establishment would occur, and on balancing speculated risks against potential benefits of biological control.