“There are places he’s gone to in this last exploration that I will never send him to again…A lot of people here do a lot of really good work, but Mohsen, for state pay, he risks his life for plants like wiliwili.”
With those words, state Department of Agriculture entomologist Darcy Oishi sang the praises for Mohsen Ramadan, an exploratory entomologist with the department whose valiant efforts to save Hawai`i’s wiliwili trees are starting to pay off.
In 2005, the invasive erythrina gall wasp (Quadrastichus erythrinae) swept through Hawai`i like a wildfire, killing or crippling countless native wiliwili trees (Erythrina sandwicensis) and non-native erythrina trees statewide in about six months. The wasps lay their eggs in the trees’ leaves and stems, causing them to eventually shrivel and die. Erythrina trees here — which are commonly used for landscaping, windbreaks, and for various cultural purposes – seemed doomed, but Ramadan’s travels through Africa yielded a possible savior: Eurytoma erythrinae, a Tanzanian parasitoid wasp and the most dominant natural enemy of the erythrina gall wasp.
Ramadan collected E. erythrinae from Africa in 2006. In November 2008, after conducting tests to ensure that the wasp would not prey on non-target insects here, the DOA unleashed it, first on O`ahu, then at sites on Lana`i, Kaua`i, Maui, Moloka`i, and Hawai`i. To date, nearly 4,000 E. erythrinae have been released.
According to Oishi, who provided a status report on the biocontrol effort at the annual Hawai`i Conservation Conference last July, the DOA established monitoring four sites each on the islands of Kaua`i, O`ahu, Maui, and Hawai`i, encompassing up to ten trees per site.
Oishi said that by May, most of the monitoring sites showed that E. erythrinae were making a significant dent in gall wasp populations and trees were beginning to recover. He added that his department has not observed any effects on non-target species.
At the Koko Crater and the Lili`uokalani botanical gardens on O`ahu, Oishi said the DOA found that by May, E. erythrinae had probed (which eventually leads to the gall wasp’s death) or replaced more than 50 percent of the gall wasp populations. Likewise, the Kamehameha Golf Course and Lahaina Luna on Maui and Pu`u Wa`awa`a Forest Reserve and South Point on Hawai`i also showed “good activity” by May, he said.
On Kaua`i, Oishi said that trees at Makauwahi showed some evidence of recovery as early as March, but trees at the National Tropical Botanical Garden showed “not much recovery at all,” a possible result of pesticide use. Oishi explained that chemical treatments that may have been applied to control the erythrina gall wasp seem to have also inhibited the establishment of E. erythrinae.
In all, Oishi said that the rate of parasitism in the field was about 75 percent, adding that where there are high densities of E. erythrinae, they attack the gall wasps’ eggs, larvae, and pupae. In low-density areas, E. erythrinae attacks only mature larvae and pupae.
Oishi said it appears E. erythrinae is spreading on its own and that a second, unknown parasitic wasp has been found attacking erythrina wasps at several sites. The parasite has also been found attacking, to a limited extent, E. erythrinae.
Although E. erythrinae is now established on the larger Hawaiian islands, Oishi said “it’s still too early to determine the long-term effects on trees.” The DOA has recently completed host-specificity studies of another erythrina parasitoid, Aprostocetus nitens, “just in case Eurytoma erythrinae doesn’t do what we need it to do,” he said. With an average longevity of 101 days, A. nitens should stay in the field longer than E. erythrinae, which has an adult longevity of about 40 days, he said. A third biocontrol agent, A. exertus, may also be tested, depending on fund availability.
Wekiu Bug Faces Climate Change Impacts
The highly specialized wekiu bug (Nysius wekiuicola) lives only at the summit of Mauna Kea, above 11,500 feet, scavenging frozen insects swept mauka by the wind. The bugs live exclusively in the first foot or so of loose rock of the summit’s cinder cones and escape from being frozen themselves by burrowing deep into the ash layer.
According to wekiu researcher and conference presenter Jesse Eiben, climate change-induced warming could affect their survival. By studying the bugs in a lab, Eiben was able to determine that wekiu grow more slowly and live longer at colder temperatures. While he said he didn’t expect warming to kill the bugs directly, Eiben noted that the summit’s cold temperatures immobilize prey. If it warms, prey could live longer and with less moisture, available food resources might change, he said. In addition, Eiben said the bugs might be crowded out. A loss of 1,000 feet of habitat due to warming would reduce the wekiu’s habitat by 40 percent.
— Teresa Dawson
Volume 20, Number 4 October 2009
Leave a Reply