Climate Mismatch between Introduced Biological Control Agents and Their Invasive Host Plants: Improving Biological Control of Tropical Weeds in Temperate Regions.

Many weed biological control programs suffer from large-scale spatial variation in success due to restricted distributions or abundances of agents in temperate climates. For some of the world's worst aquatic weeds, agents are established but overwintering conditions limit their survival in higher latitudes or elevations. The resulting need is for new or improved site- or region-specific biological control tools. Here, we review this challenge with a focus on low-temperature limitations of agents and propose a roadmap for improving success. Investigations across spatial scales, from global (e.g., foreign exploration), to local (selective breeding), to individual organisms (molecular modification), are discussed. A combination of traditional (foreign) and non-traditional (introduced range) exploration may lead to the discovery and development of better-adapted agent genotypes. A multivariate approach using ecologically relevant metrics to quantify and compare cold tolerance among agent populations is likely required. These data can be used to inform environmental niche modeling combined with mechanistic modeling of species' fundamental climate niches and life histories to predict where, when, and at what abundance agents will occur. Finally, synthetic and systems biology approaches in conjunction with advanced modern genomics, gene silencing and gene editing technologies may be used to identify and alter the expression of genes enhancing cold tolerance, but this technology in the context of weed biological control has not been fully explored.

Biological and Host Range Characteristics of Lysathia flavipes (Coleoptera: Chrysomelidae), a Candidate Biological Control Agent of Invasive Ludwigia spp. (Onagraceae) in the USA.

Exotic water primroses (Ludwigia spp.) are aggressive invaders in aquatic ecosystems worldwide. To date, management of exotic Ludwigia spp. has been limited to physical and chemical control methods. Biological control provides an alternative approach for the management of invasive Ludwigia spp. but little is known regarding the natural enemies of these exotic plants. Herein the biology and host range of Lysathia flavipes (Boheman), a herbivorous beetle associated with Ludwigia spp. in Argentina and Uruguay, was studied to determine its suitability as a biocontrol agent for multiple closely related target weeds in the USA. The beetle matures from egg to adult in 19.9 ± 1.4 days at 25 °C; females lived 86.3 ± 35.6 days and laid 1510.6 ± 543.4 eggs over their lifespans. No-choice development and oviposition tests were conducted using four Ludwigia species and seven native plant species. Lysathia flavipes showed little discrimination between plant species: larvae aggressively fed and completed development, and the resulting females (F1 generation) oviposited viable eggs on most plant species regardless of origin. These results indicate that L. flavipes is not sufficiently host-specific for further consideration as a biocontrol agent of exotic Ludwigia spp. in the USA and further testing is not warranted.

Biology and Host Range of Digitivalva delaireae (Lepidoptera: Glyphipterigidae), a Candidate Agent for Biological Control of Cape-ivy (Delairea odorata) in California and Oregon.

Cape-ivy (Delairea odorata Lemaire) is an ornamental vine native to South Africa that has escaped into natural areas in coastal California and Oregon, displacing native vegetation. Surveys in South Africa led to the discovery of the leaf- and stem-mining moth Digitivalva delaireae Gaedike and Kruger (Lepidoptera: Glyphipterigidae: Acrolepiinae) as one of several common and damaging native herbivores on Cape-ivy. In greenhouse studies, adult female life span averaged 16 d (46 d maximum). Most (72%) mated females began laying eggs within 72 h of emergence. Females had an average lifetime fecundity of 52 eggs, with >70% laid on leaf laminae, and 89% of eggs were laid by the 15th day postemergence. Lifetime fertility (adult production) averaged three to four offspring per female. At 25 °C, egg hatch required 10 d, pupal formation 26 d, and adult emergence 41 d, while under variable greenhouse and laboratory conditions development to adult required 54-60 d. In four-way choice tests, involving 100 plant species other than Cape-ivy, including 11 genera and 37 species in the Asteraceae, subtribe Senecioninae from both native and invaded ranges, D. delaireae inflicted damage and produced pupae only on Cape-ivy. Leaf mining damage occurred on 30% of leaves of native Senecio hydrophilus in no-choice tests and on 2% of leaves in dual-choice tests, but no pupation occurred. If approved for field release in the continental United States, the moth D. delaireae is expected to produce multiple generations per year on Cape-ivy, and to pose little risk of damage to native plants.

Field testing Diorhabda elongata (Coleoptera: Chrysomelidae) from Crete, Greece, to assess potential impact on nontarget native California plants in the genus Frankenia.

When laboratory host specificity tests on weed biological control agents produce ambiguous results or are suspected of producing false-positive findings, field cage or open field tests can be used to help determine the true ecological host range of the agent. The leaf beetle Diorhabda elongata (Brullé) from Crete, imported to the United States for the control of saltcedar (Tamarix spp., Tamaricaceae), showed a low but variable ovipositional response to nontarget Frankenia spp. (Frankeniaceae) in previous laboratory tests conducted in small cages, where up to 11.4% of eggs were laid on these native plants. Results from field tests presented in this article show that no eggs were laid on Frankenia palmeri S. Watson and significantly more eggs were always laid on Tamarix ramosissima Ledebour than Frankenia salina (Molina) I. M. Johnston. Furthermore, the ovipositional response to F. salina was substantially lower than that recorded in laboratory tests. The percent of eggs laid on F. salina in field tests was 3.7 in a paired choice cage test, 4.3 in a multiple choice cage test, and 2.5 in a multiple choice open field test, suggesting that the true acceptance rate of the nontarget by D. elongata in the field will be lower than laboratory tests predicted. However, some damage was caused to F. salina by adult and larval feeding in the field, although this occurred only at the very end of the open field test, when D. elongata densities were extremely high, and all of the surrounding saltcedar had been totally defoliated. Scientific representatives from various stakeholder organizations (state, county, university, and environmental groups) viewed the open field test when in progress and reviewed the final results before advising State regulatory agencies on beetle redistribution. These test results, and the open review process, led regulators to conclude that redistribution of D. elongata in California was warranted owing to its significant ability to defoliate saltcedar, and its low rate of feeding on nontarget Frankenia spp. The introduction of D. elongata provides an interesting case study for risk assessment of a potentially efficacious weed biocontrol agent that may also be capable of using nontarget native plants.