Impacts of the psyllid Arytinnis hakani on invasive French broom in relation to plant size and psyllid density

Authors: Hogg, Brian; Moran, Patrick; Smith, Lincoln

Submitted to: Environmental Entomology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/18/2017
Publication Date: 4/10/2017
Citation: Hogg, B.N., Moran, P.J., Smith, L. 2017. Impacts of the psyllid Arytinnis hakani on invasive French broom in relation to plant size and psyllid density. Environmental Entomology. 46(3):552-558. doi: 10.1093/ee/nvx074.

Interpretive Summary: The effects of herbivores on plants may change as plants grow. In general, mature plants are more impervious to attack than younger plants. Older plants may harbor higher levels of toxins in their tissue, or they may be able to sustain damage more easily because of their size. The reduced vulnerability of older plants may have implications for insect agents that are released for weed control. Nonetheless, the effectiveness of biological control agents is rarely compared between plants of different ages. Here, we tested whether the survival and impacts of the psyllid Arytinnis hakani changed with the age of its host plant, the invasive shrub French broom, Genista monspessulana. The psyllid pierces plant tissue and feeds on sap. We also examined whether high numbers of the psyllid could consistently kill plants. Psyllids survived to adulthood at similar rates on small and large plants, indicating that the toxicity of plant tissue did not change with plant age, or that the psyllid was unaffected by any changes in toxin levels. However, survival of adult psyllids was lower on small plants, suggesting that adult and immature psyllids responded differently to age-related changes in the chemistry or food quality of plants. Psyllids affected the growth of small and large plants similarly. In a separate experiment, plants consistently died at high psyllid densities. Thus, results suggest that the psyllid would be equally damaging to immature and mature plants, particularly at high densities, and show the potential of the psyllid to help control French broom in California.

Technical Abstract: The performance and impacts of weed biological control agents may vary with plant ontogeny. As plants grow, profound structural and chemical changes can alter plant resistance, which may reduce herbivory via chemical or structural defenses, and plant tolerance, which may enable plants to maintain fitness despite attack. Resistance and tolerance generally increase as plants grow. Nonetheless, pre-release tests of agent efficacy often overlook plant ontogeny. Here, we assess the performance and impacts of a candidate biocontrol agent, the psyllid Arytinnis hakani, in relation to the age of its host plant, the invasive perennial shrub French broom, Genista monspessulana. We also examined whether the psyllid can consistently kill plants when its densities are sufficiently high. Survival of psyllids to adulthood and the timing of adult emergence did not differ between plant sizes, indicating that performance of psyllid nymphs was not influenced by plant size. However, adult psyllid survival was reduced on small plants, suggesting that psyllid nymphs and adults responded differently to ontogenetic changes in plant quality. Psyllids affected the growth of small and large plants similarly; all measured plant growth parameters were lower in the presence of psyllids regardless of plant size. In a separate experiment, effects on plant survival were dependent on psyllid density, as higher densities of 4-6 psyllids per cm stem length were necessary to consistently kill plants. Thus, results suggest that the psyllid would be equally effective on a range of plant of sizes, particularly at high densities, and show the potential of the psyllid to help control French broom in California.