Introduction of Samurai Wasp Proving Effective Against Stink Bugs in Italy

Scores of so-called samu­rai wasps will soon be released for the sec­ond year in a row in the heart of north­ern Italy’s Po Valley, as part of a project to limit the spread of the brown mar­morated stink bug.

The tiny Trissolcus japon­i­cus wasp repro­duces by par­a­sitiz­ing stink bugs’ eggs, reduc­ing their abil­ity to repro­duce.

Both insects are native to Asia, but the stink bug has spread widely in sev­eral European coun­tries and some areas of the United States.

“With this year’s samu­rai wasp releases, we will be tar­get­ing the areas where stink bug colonies will most likely be found,” Massimo Bariselli, a phy­topathol­o­gist and ento­mol­o­gist at the Emilia-Romagna regional phy­tosan­i­tary ser­vice, told Olive Oil Times.

“The good news is that the Japanese wasps we had pre­vi­ously released have shown to repro­duce and install them­selves in areas quite far from the areas of release, sug­gest­ing good poten­tial for spread­ing in the ter­ri­tory,” he added.

Fruit grow­ers in north­ern Italian regions, such as Emilia-Romagna, Lombardy and Veneto, have been severely hit by the spread of the stink bug, which dam­ages a wide vari­ety of crops, includ­ing tree fruits and soy­beans.

Since pre­lim­i­nary research has shown a pos­si­ble asso­ci­a­tion between stink bug out­breaks and the olive ​“green drop” dis­ease, olive farm­ers have been espe­cially wor­ried about the spread of the pest.

In the Emilia-Romagna region alone, between 2020 and 2022, local author­i­ties have dis­trib­uted €63 mil­lion in com­pen­sa­tion to farm­ers affected by the insect, while local author­i­ties in Lombardy esti­mate that the stink bug causes €15 mil­lion worth of dam­ages annu­ally to regional agri­cul­ture.

“At the moment, we have about 30,000 stink bugs gath­ered by the tech­ni­cians and farm­ers who bring them to the lab­o­ra­to­ries, which are then used to grow the samu­rai wasp,” Bariselli said. ​“When the time comes, the tubes with the wasps are assigned to the dif­fer­ent farm­ing areas and released.”

Last year in Italy, the samu­rai wasp was released in more than 600 loca­tions, with more than 300 in Emilia-Romagna.

The first evi­dence of a pos­si­ble pos­i­tive impact of the samu­rai wasp’s intro­duc­tion comes from researchers sur­veilling stink bugs’ eggs. According to the data released by the Cimice.net project, the wasps col­o­nized 38 per­cent of the sam­ples.

While these find­ings con­firm the wasps’ abil­ity to tar­get the stink bug, experts in the field warn that the true impact of the bio­log­i­cal war against the insect is extremely dif­fi­cult to mea­sure.

Monitoring the bug pop­u­la­tion is dif­fi­cult since it under­goes sig­nif­i­cant changes each year. The stink bugs thrive at tem­per­a­tures between 30 ºC and 32 ºC and suf­fer from a high mor­tal­ity rate dur­ing the win­ter.

“In a sea­son like the last one, where the fruit pro­duc­tion was more than halved by the freez­ing tem­per­a­tures, where there was no rain­fall in sum­mer and hedges are dry, all stink bugs would have been forced to attack the fruits,” Bariselli said. ​“That cre­ates the incor­rect per­cep­tion that there was a large inva­sion in a sea­son in which the stink bugs could not repro­duce well.”

In other regions, such as Piedmont, the stink bug has been tar­geted by other preda­tors, includ­ing a sep­a­rate species of wasp that also par­a­sitizes the bugs’ eggs – the European Anastatus bifas­cia­tus. These tri­als are tak­ing place on organic farms.

According to Bariselli, Anastatus bifas­cia­tus also effec­tively tar­gets the stink bug. However, unlike the samu­rai wasp, they prey on other insects, so their effi­ciency in cur­tail­ing the stink bug’s spread might be lim­ited.

“Looking at what hap­pens in Italy and what hap­pens in Asia, it seems that Anastatus bifas­cia­tus can affect between 10 and 15 per­cent of the stink bug pop­u­la­tion,” he said.

Chemical treat­ments cur­rently used on the stink bugs have not yielded promis­ing results. They are quite expen­sive and are not envi­ron­men­tally friendly. They also make it more dif­fi­cult for the stink bugs’ preda­tors to sur­vive.

“Chemical war­fare against the stink bug tends to be highly inef­fi­cient,” Bariselli said. ​“This is because the bug rarely lives in the trees and tar­gets a lot of dif­fer­ent plants.”

He added that the stink bugs also tend to sting the fruits and quickly leave, mean­ing they are not exposed to pes­ti­cides for long peri­ods.

“Even if chem­i­cals do not work against the stink bug, heavy dam­age caused by the bug could push some farm­ers to invest heav­ily in that kind of treat­ment to get rid of the insect,” Bariselli added. ​“That will not work.”

According to the data released dur­ing a recent sum­mit on the stink bug and reported by Agronotizie, 24 per­cent of the 47,000 stink bug eggs ana­lyzed in 2021 had been par­a­sitized.

Thirty-eight per­cent of them had been tar­geted by the samu­rai wasp, 31 per­cent by the related Trissolcus mit­sukurii and 22 per­cent by the Anastatus bifas­cia­tus.

“What we are see­ing is the very first evi­dence that the samu­rai wasp might be cur­tail­ing the mar­morated stink bug pop­u­la­tion,” Bariselli said. ​“But that mostly hap­pens in wild areas, where the wasps were released, areas where there are no chem­i­cal treat­ments on the veg­e­ta­tion.”

“The bug is there, but not only there,” he added. ​“We now need to under­stand how much the fruit crops [con­ven­tion­ally treated] can hin­der the spread of the stink bug’s nat­ural preda­tors.”

The cur­rent samu­rai wasp release project in the region will end in 2023.

During the sum­mit, Alessio Mammi, Emilia-Romagna’s regional sec­re­tary of agri­cul­ture, empha­sized how the goal of the first year of the project was to ​“make the samu­rai wasp set­tle in the ter­ri­tory, and that is a tar­get we have acquired.”

“In 2022, we must now step up the release activ­i­ties and spread­ing within and around the most rel­e­vant regional fruit-pro­duc­ing areas,” he added.

Bariselli said he is opti­mistic that a com­bi­na­tion of intro­duc­ing the stink bugs’ preda­tors and other con­tain­ment strate­gies would lead to an improved sit­u­a­tion in the com­ing years.

“Within one or two years, we expect that the pres­sure of the stink bug will be slow­ing down a bit,” he said. ​“It will stay, and it will con­tinue to be a dam­ag­ing pres­ence for farm­ers, but it will have found a new bal­ance within that envi­ron­ment just like other insects already have.”

• Centro Richerche Produzioni Vegetali
• Conferenza delle Regioni e delle Province Autonome
• Idea Web