Synergistic effects of fertilizer and a biological control agent on the growth attributes of a perennial target weed in an outdoor environment

The efficacy of biological control agents in combating invasive weeds can be influenced by biotic (predation, intraguild competition) and abiotic (climate, edaphic) factors. We compared the impacts of fertilizer applications on the growth attributes of Brazilian peppertree saplings before and after releasing Pseudophilothrips ichini (Thysanoptera: Phlaeothripidae), a biological control agent that targets flushing stem tips and new leaves. A garden plot containing 96-103 saplings was established at two research facilities in southeastern Florida. Saplings were 6 mo-old and roughly 70 cm-tall at the time of planting in September 2017. Each plant was randomly assigned a permanent fertilizer (24N-8P-16K) treatment administered bimonthly: no fertilizer n=69 (0g/L), medium fertilizer n=66 (0.8g/L), or high fertilizer n=66 (3.6g/L). Thrips were released after 22 mo and releases continued every 1-3 mo. Growth attributes were collected approximately monthly from October 2017 to April 2021. Crown volume and height steadily increased, with greater rates for higher fertilizer levels. The number of stem tips per cm of canopy diameter increased dramatically during the thrips release period and was greatest for the high fertilizer level. The percentage of dead tips quadrupled during the second year of thrips releases, but tip mortality did not differ between fertilizer levels. Fertilized plants appear to regenerate more unseasonal lateral buds after thrips attack, providing new resources for thrips population increase. Plants growing in fertile soils at field sites may help sustain thrips populations when neighboring plants lack flushing tips. Repeated thrips-induced tip mortality is expected to reduce seed production and canopy expansion.