Skunk vine, Paederia foetida L. (Rubiaceae), is an invasive weed native to Asia but a problem in Florida and other areas of the southern USA, and in Hawaii (Pemberton & Pratt 2002). In the southern states of the USA it is mostly a problem in natural areas, whereas in Hawaii the plant is a problem in both natural areas and agriculture. Skunk vine is both a Category I weed (Florida Exotic Pest Plant Committee 2009) and a Florida Noxious Weed (USDA, Natural Resource Conservation Service).
Biological control research began with field surveys in Asia in 1997 (Pemberton & Pratt 2002). A lace bug, Dulinius conchatus Distant (Tingidae), and a leaf beetle, Sphenoraia rutilans Hope (Chrysomelidae) have undergone host specificity testing but neither demonstrated the narrowness of host range needed in a skunk vine biocontrol agent (Pemberton et al. 2005; Pemberton & Witkus unpublished data). Two Trachyaphthona spp. flea beetles were identified as promising biological control candidates during surveys in Japan (Pemberton & Pratt 2002). Both species have larvae that feed on roots and adults that consume leaves. The biology and preliminary host ranges of Trachyaphthona sordida (Baly) and T. nigrita Ohno were studied in Japan (Okamato et al. 2008).
Adult host range testing on T. nigrita was conducted in our Fort Lauderdale quarantine laboratory during 2007 and 2008, with field-collected beetles shipped by Japanese cooperators during each Jun of those years. Adult host range testing on field-collected T. sordida beetles was done in 2009 after Japanese cooperators shipped the beetles during Jun of that year. Adults of both species were set up in wooden screened sleeve cages, 50 cm × 50 cm × 53 cm, and fed cut vines of the control plant, Paederia foetida, and its invasive relative Paederia cruddasiana Pain. Paederia cruddasiana, native to Nepal, Burma, China, and Thailand (Puff 1991), is a naturalized weed in Miami-Dade County, Florida. The beetles mated readily and laid eggs throughout the sleeved rearing cages.
The most likely potential non-target plants of these beetles are the many native and economic members of the Rubiaceae in Florida. The host range testing for T. nigrita and T. sordida initially concentrated on 3 subfamilies within the Rubiaceae. Test plants were native and ornamental members of this family selected to represent different taxonomic tribes. Host range testing on plants outside of the Rubiaceae consisted of species selected to represent related orders including the Cornales, Gentianales, Lamiales, and the Solonales. In total, 34 species in 7 families and 5 orders were tested.
Host range tests on both beetles were conducted with a no-choice test design, and each individual test plant was exposed to 3 beetles for 1 month. Control plants of P. foetida and P. cruddasiana were exposed to 3 beetles for 1 month. Because research on T. nigrita spanned 2 seasons, each experiment was replicated 5 times. Research on T. sordida was conducted for 1 season so each test was replicated 3 times. Host range testing for T. nigrita and T. sordida was conducted on branches of whole potted plants in the quarantine greenhouse. Clear acrylic tubes, 15.2 cm long × 3.8 cm diameter (6″ × 1.57″ diameter), each with 2 mesh ventilation holes, were placed on the end of a small branch or stem of each test plant. A slit sponge bung was placed around the base of the stem and up into the bottom of the tube. The stem and bung juncture was wrapped tightly with parafilm. Another bung was used to seal the top of the tube. Due to the saltatory nature of the flea beetles, an aspirator was used to gently transfer 3 tiny beetles through the bung slit and into the tubes and onto each test branch. The 3 beetles consisted of 1 mating pair and another beetle of undetermined gender due to the difficulty in determining gender in this flea beetle. Beetles were monitored for 1 month, and at the end of each week, all leaves with feeding damage were clipped, pressed and then scanned into jpeg files to enable feeding to be measured with SigmaScan® Pro.
The most important finding of the research was the nearly equal or greater feeding by both Trachyaphthona species on the native Florida test plant Diodia virginiana L. (Table 1). Trachyaphthona nigrita adults ate a mean of 3.75 ± 1.1 mm2 leaf material of D. virginiana, compared to 4.76 ± 1.53 mm2 of leaf material on P. foetida, and 3.31 ± 0.57 mm2 on P. cruddasiana. Trachyaphthona sordida adults consumed a mean of 3.23 ± 0.5 mm2 leaf material on D. virginiana, compared to 1.61 ± 0.82 mm2 on P. foetida, and 1.38 ± 0.63 mm2 on P. cruddasiana. Moreover, D. virginiana belongs to a taxonomic group within the Rubiaceae different than that of skunk vine, i.e., the tribe Spermacoceae rather than the tribe Paederieae for skunk vine. In addition, T. nigrita caused what is considered to be more than test feeding on Spermacoce tetraquetra A. Rich., another member of the Spermacoceae. Trachyaphthona nigrita adults consumed a mean of 0.56 ± 0.16 mm2 of S. tetraquetra, compared to 1.61 ± 0.82 mm2 on P. foetida, and 1.38 ± 0.63 mm2 on P. cruddasiana. To prevent harm to native and important economic members of the Rubiaceae, the feeding of these flea beetles would need to be limited to plants in the skunk vine tribe, the Paederieae.
HOST RANGE TESTING OF TRACHYAPHTHONA NIGRITA AND TRACHYAPHTHONA SORDIDA ADULT FLEA BEETLES SHOWING PLANTS THAT RECEIVED FEEDING. THREE ADULT BEETLES WERE TESTED FOR 1 MONTH FOR EACH SPECIES. DIFFERENCES IN TOTAL AMOUNTS OF FEEDING ARE DUE TO 5 REPLICATES CONDUCTED OVER 2 SEASONS WITH T. NIGRITA, VERSUS 3 REPLICATES WITH T. SORDIDA. FLORIDA NATIVES ARE IN BOLD.
It may be possible that the larvae of these beetles could have greater specificity than the adults. Two factors influenced our decision to cease research on these Trachyaphthona species and to shift to other candidate insects of skunk vine. Most importantly, we have not successfully reared these univoltine beetles, which are, like many root-feeding flea beetles, difficult to culture in captivity. Secondly, our previous testing with the leaf beetle Sphenoraia rutilans found that it could complete its development both on Diodia virginiana and Spermacoce tetraquetra, indicating that these plants have an attractiveness and vulnerability to some Paederia feeding insects.
Trachyaphthona nigrita and Trachyaphthona sordida, 2 potential biological control agents of Paederia foetida L. (Rubiaceae), were collected from Japan and brought into quarantine for adult host specificity testing. Testing indicated that they fed significantly on Florida native plants in the tribe Spermacoceae, a different tribe from that to which the target weed Paederia foetida belongs (Paederieae). These beetles, therefore, lack the appropriate level of host specificity, and this eliminates them from further consideration as potential biological controls of Paederia foetida, skunk vine.
We thank Junichi Yukawa, Fukuoka University (retired), and his students for collecting the Trachyaphthona beetles in Japan and shipping them to Florida. Rachel Taylor provided technical assistance and Luke Kasarjian grew the test plants. The South West Florida Water Management District and the Florida Department of Environmental Protection funded this research.