Walking velocity and estimated distance of the armored scale crawler Rhizaspidiotus donacis, a biological control agent of Arundo donax

Authors: VILLARREAL, JOSHUA - UNIVERSITY OF TEXAS RIO GRANDE VALLEY; RACELLS, ALEX - UNIVERSITY OF TEXAS RIO GRANDE VALLEY; Goolsby, John

Submitted to: Southwestern Entomologist
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/14/2021
Publication Date: 6/29/2021
Citation: Villarreal, J.A., Racells, A.E., Goolsby, J. 2021. Walking velocity and estimated distance of the armored scale crawler Rhizaspidiotus donacis, a biological control agent of Arundo donax . Southwestern Entomologist. 46. https://doi.org/10.3958/059.046.0223.
DOI: https://doi.org/10.3958/059.046.0223

Interpretive Summary: The arundo scale insect is a biological control agent of the invasive weed, Arundo donax, also known as giant reed or carrizo cane. Once the juvenile stage of this insect settles on rhizomes and side shoots it does not move. The only mobile stage is the juvenile or crawler stage. This crawler is less than 1 mm in length, lives for approximately 12 hours, and travels only a short distance. In our study, we measured the walking speed of crawlers and estimated the theoretical maximum distance walked in 12 hours. Arundo scale crawlers walked at an average velocity of 0.02 inches/second, with an estimated maximum distance of 60 ft in 12 hours. This velocity and theoretical distance is similar to measurements of black scale crawlers conducted in California in the early 1900s. In conclusion, this valuable biological control insect has a very limited ability to disperse. Therefore, additional rearing and release may be useful to achieve the full benefits of biological control along the 350 river miles of the Rio Grande River between Del Rio and Brownsville, TX where giant reed is an invasive weed.

Technical Abstract: The armored scale biological control agent, Rhizaspidiotus donacis (Leonardi) (Hemiptera; Diaspididae), also known as the arundo scale, has established on populations of the invasive weed, Arundo donax L. (Poaceae; Arundinoideae) at several locations along the Rio Grande River in Texas. The arundo scale is having significant impacts on the target weed in the areas where it is established, and no non-target plant use has been observed. Armored scale crawlers generally live for less than a day and settle within 1m of their sessile mother. This low level of dispersibility led to biological studies of the arundo scale which were conducted in quarantine prior to release to quantify aspects of its biology that influenced dispersal. In our experiments we measured walking velocity, estimated the potential distance that a crawler could travel over a 12-hour period, and variations of these attributes for crawlers that emerged over time from different geographical accessions. This information was used to prioritize populations for release at field sites or nearby rearing facility. Three accession of R. donacis crawlers from France, Italy and Greece were evaluated. Crawlers for each accession were isolated as individuals in gelatin capsules on M, W, F mornings and held for observation. Gelatin capsules with crawlers were observed using a dissecting microscope and distances walked were measured using a stage micrometer over 10 second periods. Arundo scale crawlers walked at an average velocity of 0.47 mm/s, which is similar to measurements of black scale crawlers, Saisettia olea Bern., walking on wax paper at 0.38 mm/s. The estimated distance walked by black scale crawlers in 12 hours was 9.1 m, which is similar to arundo scale which ranged from 14.1 to 20.7 m (Quayle 1911). Arundo scale crawlers from different European accessions showed significant differences in mean velocity walked and their potential estimated maximize distance over 3 and 12 hours and for days during the week that they emerged. These differences may reflect the nutrition of their plant host. All accessions were harvested from A. donax stands with high population levels of R. donacis, so rhizomes and ramets may have been significantly depleted of nutrient reserves which influenced crawler vigor. As expected, crawlers that were collected on Mondays had the least vigor since some of the crawlers may have been up to 3 days old. Arundo scale crawlers appear to have the capability to travel for up to 20 meters which is the maximum height of an A. donax stem. Although we did not estimate distance for more than 12 hours, this period of time represents the typical life span of an arundo scale crawler. Dispersal of arundo scale is limited to movement of the crawlers over short distances and displacement of infested rhizomes during flood events. Given the limited dispersal ability of R. donacis, additional rearing and release may be needed to achieve the maximum benefit from this key agent where this weed is invasive and especially over the 350 river miles of the Rio Grande River along the Texas-Mexico border.