The parasitic wasp Tetrastichus planipennisi is native to China and is showing promise as a biocontrol in the U.S. for emerald ash borer. Photo: Stephen Ausmus
The parasitoid wasps from China and Russia, natural enemies of the emerald ash borer (EAB), were released over the past few years in select U.S. states. The parasitoids deposit their eggs or larvae into or on emerald ash borer larva or eggs which then serve as the wasp’s hosts.
The emerald ash borer (Agrilus planipennis) is a destructive wood-boring pest of ash trees (Fraxinus spp.). Native to China, Mongolia, North Korea, South Korea, Japan, Taiwan and the Russian Far East, the EAB was unknown in North America until its discovery in southeast Michigan in 2002. Today, EAB infestations have been detected in 35 U.S. states and the District of Columbia, and in five Canadian provinces.
Early on in the project, the wasps were taken to a quarantine facility, where they were tested to ensure they would attack only ash borers, not other species. Then researchers worked in the lab to produce thousands of wasps, plus their food source — emerald ash borers. Finally, to make sure the wasps connected with their target, scientists carefully synchronized their release into nature with the ash borers’ activity.
At test sites in Michigan and New England, the wasps are starting to make an impact, reducing ash borer populations, according to Jian Duan, research entomologist with the USDA’s Beneficial Insects Introduction Research Unit. Though the pests greatly outnumber the wasps for now, Duan is optimistic. “We still think the natural enemy eventually will catch up with emerald ash borer populations.”
Today, the USDA is working to rear massive numbers of the parasitoid wasps in lab facilities by providing lab-grown emerald ash borers as hosts for their eggs. Despite COVID-19 disruptions, the agency produced over 550,000 parasitoids in 2020 and released them at over 240 sites.
According to Kristine Grayson, associate professor of biology at the University of Richmond in Richmond, Virginia, the goal is to create self-sustaining field populations of parasitoids that reduce emerald ash borer populations in nature enough to allow replanted ash trees to grow and thrive. She notes several studies have shown encouraging early results, but securing a future for ash trees will require more time and research.
“One hurdle is that emerald ash borers grown in the lab need fresh ash logs and leaves to complete their life cycle,” says Grayson, who studies invasive forest insects. “I’m part of a team working to develop an alternative to the time- and cost-intensive process of collecting logs: an artificial diet that the beetle larvae can eat in the lab.”
The food must provide the right texture and nutrition. Other leaf-feeding insects readily eat artificial diets made from wheat germ, but species whose larvae digest wood are pickier. In the wild, emerald ash borers only feed on species of ash tree.
While the emerald ash borer has spread uncontrollably in nature, producing a consistent laboratory supply of these insects is surprisingly challenging – and developing an effective biological control program requires a lot of target insects.
“In today’s global economy, with people and products moving rapidly around the world, it can be hard to find effective management options when invasive species become established over a large area,” notes Grayson. “But lessons learned from the emerald ash borer will help researchers mobilize quickly when the next forest pest arrives.” ●
Read NAI’s related story on EAB control in NAI June-July 2021
U.S.-based researchers have successfully used a naturally occurring fungus to control invasive crazy ants at a state park in Texas.
The tawny crazy ant (Nylanderia fulva) originates from South America and is an invasive pest in the southern United States. In Texas, the ants have been known to swarm and kill animals, including native species – and to take over homes, interfering with electrical devices such as sewage pumps and air conditioners.
The idea for using the fungal pathogen came from observing wild populations of crazy ants becoming infected and collapsing without human intervention.
About eight years ago, Edward LeBrun, a research scientist with the Texas Invasive Species Research Program at Brackenridge Field Laboratory and lead author of a study published in Proceedings of the National Academy of Science, and Rob Plowes, also with the Brackenridge Field Laboratory, were studying crazy ants collected in Florida when they noticed some had abdomens swollen with fat. When they looked inside their bodies, they found spores from a microsporidian, a group of fungal pathogens – a species new to science. Microsporidian pathogens commonly hijack an insect’s fat cells and turn them into spore factories.
Tawny crazy ants swarm on a cobweb spider. Photo: Mark Sanders.
It’s not clear where the pathogen came from, perhaps from the tawny crazy ants’ native range in South America or from another insect, but LeBrun and his colleagues started finding the pathogen in crazy ants at sites across Texas. Observing 15 local populations for eight years, the team found that every population that harboured the pathogen declined – and 62 percent of these populations disappeared entirely.
LeBrun theorizes that perhaps the colonies collapsed because the pathogen shortens the lifespan of worker ants, making it hard for a population to survive through winter. Whatever the reason, it seems to be a crazy-ants-only problem. Unrelated to other microsporidia that infect ants, the pathogen appears to leave native ants and other arthropods unharmed, making it a seemingly ideal biocontrol agent.
The team deployed the pathogen this way after LeBrun got a call from Estero Llano Grande State Park in Weslaco, Texas, in 2016. The park was losing its insects, scorpions, snakes, lizards and birds to tawny crazy ants. Baby rabbits were being blinded in their nests by swarms of the acid-spewing ants.
Using crazy ants they had collected from other sites already infected with the microsporidian pathogen, the researchers put infected ants in nest boxes near crazy ant nesting sites in the state park. They placed hot dogs around the exit chambers to attract the local ants and merge the two populations. In the first year, the disease spread to the entire crazy ant population in Estero. Within two years, their numbers plunged. Now, they are nonexistent and native species are returning to the area.
The researchers have since eradicated a second crazy ant population at another site in the area of Convict Hill in Austin. The researchers planned to test their new biocontrol approach this spring in other sensitive Texas habitats infested with crazy ants.
Crazy ants have decimated Christmas Island's red crabs.
Photo: Pete Green
Wasp may help control crazy ants Meanwhile, scientists in Australia say they could introduce another biocontrol agent into the rainforests of Christmas Island, as crazy ants continue to kill its iconic red crabs.
Crazy ants are believed to have reached the shores of the remote island in the 1990s and have killed millions of the crabs since then. The ants spray acid into the crabs' eyes to blind them, and then into their joints to immobilize them.
The red crabs are famed throughout the world for their yearly migration to the sea, which turns Christmas Island roads into red carpets.
In 2016, scientists introduced a Malaysian micro-wasp to try and deprive the crazy ants of one of its food sources – honeydew, which is produced by an insect called the yellow lac scale.
"The wasp spread under its own steam across the island very well and has done a terrific job of controlling the yellow lac scale," said Pete Green, associate professor from La Trobe University who has been working with the Christmas Island National Park to eradicate the ants. "But what we really wanted was to gain control of the ants themselves by controlling the food produced by that yellow lac scale and that's been patchy."
According to Australia’s ABC News, scientists are now grappling with the idea of introducing one or two more micro-wasps into the jungle on Christmas Island. Scientists' first choice is an African micro-wasp, which is a pest in the Australian agriculture sector. ●
CABI scientists have led new research that highlights the safety of a classical biological control agent against the invasive fruit fly Drosophila suzukii, commonly called spotted wing drosophila, a frugivorous insect native to Eastern Asia that was accidentally introduced to the Americas and Europe in the 2000s where it rapidly spread.
Field cage releases of the parasitoid G1 Ganaspis cf. brasiliensis carried out in two regions of Switzerland in August 2021 supports findings from previously conducted laboratory-based experiments and the low risk for non-target effects on native Drosophila spp. The study, carried out with colleagues from the Repubblica e Cantone Ticino, Agroscope, and the Institute of Agricultural Sciences (IAS) of ETH Zurich, revealed that larvae of the target species D. suzukii feeding in fresh fruits was readily parasitized, and of 957 emerging parasitoids, only one was from larvae of the non-target species Drosophila melanogaster feeding on decomposing fruits.
According to lead researcher, Dr. Lukas Seehausen, based at CABI in Switzerland, released parasitoids had the choice to parasitize either D. suzukii larvae in fresh fruits, blueberries or elderberries, or the non-target native species D. melanogaster in decomposing fruits, which is their natural habitat.
“The results were unequivocal in that parasitism of D. suzukii larvae feeding in fresh fruits was on average 15 percent, whereas only one parasitoid emerged from D. melanogaster feeding on decomposing fruits, which is a mere 0.02 percent parasitism. The results achieved under semi-field conditions supports findings from previous laboratory experiments that the parasitoid G1 G. cf. brasiliensis is highly specific to D. suzukii larvae feeding in fresh fruits and parasitism of the closely related D. melanogaster naturally feeding on decomposing fruits is very rare.”
In their conclusion, the scientists note that with the first releases of G. cf. brasiliensis in Italy in 2021, a recent acceptance of the application for releases of the same parasitoid in the US, and the submission of an application in Switzerland in February 2022, the research starts to be implemented into practice. ●
Drosophila melanogaster fruit fly on a flower
Spotted wing drosophila (Drosophila suzukii) is an invasive fruit pest.
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Little penguins in Victoria (Australia) will be among the native species to benefit from a new biocontrol solution to tackle the invasive coastal weed sea spurge, which was released in late March in Port Campbell National Park by Australia’s national science agency, CSIRO, and Parks Victoria.
CSIRO researchers found that the fungus Venturia paralias specifically attacks sea spurge (Euphorbia paralias), which threatens nesting sites of native species including little penguins (Edyptula minor), as well as impacting on the wider coastal ecosystem.
CSIRO scientist Dr. Gavin Hunter said sea spurge is problematic for nesting shorebirds, including penguins, as the weed can alter sand dune structure and displace vegetation which could negatively impact nesting sites of shorebirds. The weed also has a sap that can cause irritation to animals as well as humans.
The fungus was originally found on the Atlantic and Mediterranean coast of France causing leaf and stem lesions on sea spurge plants. The fungus was isolated from these diseased plants and initial tests to explore its host range were performed in France. Following positive results from these tests, the fungus was imported to CSIRO’s quarantine facility in Canberra and studied extensively.
A prolific seed producer, a mature sea spurge plant can produce up to 20,000 seeds per year and can grow anywhere on the beach above the high-water mark, taking over sand and dune vegetation. It is an introduced plant from Europe that has invaded coastal ecosystems from Geraldton north of Perth in Western Australia through to the mid north coast of New South Wales and around Tasmania’s coastline. ●
The fungus Venturia paralias infects sea spurge leaves and from there colonizes stems, causing stem lesions, and in severe infections, can lead to stem collapse. Photo: CSIRO
LAINCO, in collaboration with the Agriculture Service of the Balearic Islands Ministry of Agriculture, Fisheries and Food, has started field trials on almond trees in Mallorca with SENSATIO, a natural solution against Xylella fastidiosa.
After five years of study, in December 2021 Spain-based LAINCO obtained the patent for SENSATIO, which is based on eucalyptus essential oil.
Xylella fastidiosa is a bacterial disease of plants. It is transmitted exclusively by xylem fluid-feeding sap insects, and can infect more than 500 species of plants, causing leaf scorch, wilt and die-back, and eventually plant death. There is no known cure for the disease.
Olives and almond trees are among the crops affected by this bacterium. According to a study by the University of the Balearic Islands (UIB), Xylella fastidiosa is responsible for the disease that affects 80 percent of almond trees in Mallorca. It arrived on the island almost three decades ago, with the introduction of infected Californian almond trees. The study confirmed that, in Mallorca, almond leaf scab is caused by two different subspecies of Xylella fastidiosa: fastidiosa and multiplex.
The trials are being carried out via endotherapy, an alternative method of phytosanitary treatment of trees with low environmental impact. The system consists of injecting SENSATIO into the tree trunk, which is translocated through the xylem towards the apex of the plant.
The first applications of SENSATIA are being applied this month (June) in almond trees and will continue for six months. ●
