Asian Bug May Be Cause of ‘Green Drop’ in Olive Trees, Researchers Find

An exper­i­ment con­ducted by a group of Italian agron­o­mists may have found a link between the brown mar­morated stink bug – also known as the Asian bug – and a new olive tree dis­ease known as ​‘green drop,’ a con­nec­tion pre­vi­ously made by local pro­duc­ers.

“During the last few sea­sons, we received reports, based on empir­i­cal obser­va­tion, from col­leagues who had noticed the coex­is­tence of these two fac­tors,” Michele Dell’Oro, one of the lead researchers on the project along with Matteo Ghilardi and Giandomenico Borelli, told Olive Oil Times.

Since 2017, pro­duc­ers in north­ern Italy have noticed some green olives falling pre­ma­turely, well before verai­son has taken place. During the pre­vi­ous crop year, ​‘green drop’ was among the rea­sons for north­ern Italy’s dras­tic pro­duc­tion decrease.

‘Green drop’ describes the anom­alous fall of the green olives, which have also devel­oped necrotic spots. The fruit detach­ment force of the olives is also reduced, so that a small exter­nal stim­u­lus is suf­fi­cient to cause the fruit to fall.

Starting from the post-fruit set­ting phase, the dis­ease spreads until the period of full hard­en­ing of the stone, affect­ing, in many cases, all the fruits on the tree.

A more detailed obser­va­tion of the olives affected, through radial dis­sec­tion, demon­strates the pres­ence of necro­sis within the tis­sues of the endo­carp, with the devi­tal­iza­tion of the embryo in for­ma­tion. Once the stone has fully hard­ened, ​‘green drop’ slows down sig­nif­i­cantly.

“At first, some oper­a­tors attrib­uted this symp­to­ma­tol­ogy, that was unevenly dis­trib­uted, to a fun­gal dis­ease,” Dell’Oro said. ​“However, anti-fun­gal treat­ments that were applied proved to be inef­fec­tive in con­tain­ing the prob­lem. At the same time, the fre­quent pres­ence of the brown mar­morated stink bug was recorded in the olive groves.”

According to the Italian National Institute for Environmental Protection and Research (ISPRA), last year the brown mar­morated stink bug dam­aged about 300 types of crops in the north of the coun­try, result­ing in a loss of €600 mil­lion ($708 mil­lion).

Native to Asia and first sighted in Italy in 2012, the Asian bug has been included in the European and Mediterranean Plant Protection Organization (EPPO) alert list due to its wide polypha­gia – habit of feed­ing on many dif­fer­ent types of foods – and poten­tial inva­sive­ness.

“According to reports, the Asian bugs were seen per­form­ing trophic activ­ity on the olive trees (namely, they stung the fruits). Leccino seems to be the most sen­si­tive vari­ety, but other vari­eties also were affected,” Dell’Oro said. ​“At that point, a debate arose and we decided to carry out an exper­i­men­tal test that does not pre­tend to be defin­i­tive but, rather, it is a pre­lim­i­nary work with the aim to stir up the inter­est of the sci­en­tific com­mu­nity.”

The exper­i­men­ta­tion was con­ducted in three farms in the province of Lecco, on Leccino trees between the ages of 15 and 35, all of which were in full pro­duc­tion and prop­erly man­aged accord­ing to the poly­conic vase grow­ing sys­tem.

After the fruit set­ting phase, when fruits reach five mil­lime­ters (0.20 inches) in diam­e­ter, some small fruit-bear­ing branches were iso­lated and enclosed in spe­cially-made bags, using an anti-insect net. Before posi­tion­ing the bags, the work group treated the branch­lets with an insec­ti­cide, pyrethrin, in order to exclude the pres­ence of other insects.

After installing the bags, the researchers intro­duced eight Asian bugs in dif­fer­ent stages of devel­op­ment, both young and adults, in half of the bags.

Each bag was iden­ti­fied with a unique code and the branches were con­stantly mon­i­tored dur­ing the trial period, in order to pre­vent break­ages, or any error, dur­ing the exper­i­ment. The test branches were removed in late July and mid-August when the fruits were in the phase of stone hard­en­ing.

“At the end of the trial period, we col­lected the bags and counted the fallen olives,” Borelli said. ​“We con­sid­ered all of them, then not exclud­ing a per­cent­age of phys­i­o­log­i­cal drop in both types of repli­ca­tion. Then, we col­lected the data, cat­a­loged and cre­ated charts, while a sta­tis­tics expert eval­u­ated the reli­a­bil­ity of the data.”

The results showed that the per­cent­age dif­fer­ence of fallen olives between the bags with bugs and those with­out bugs was sig­nif­i­cant.

“Most of the branch­lets that hosted the bugs showed a drop of 100 per­cent,” Ghilardi said. ​“We only found one case with a fig­ure of less than 90 per­cent (84 per­cent), while in the branch­lets with­out the bugs there was a much lower per­cent­age of drop, with fig­ures rang­ing from 15 to 55 per­cent, in the worst cases.”

The aver­age per­cent­age of fallen fruits was 98 per­cent in the bags with bugs, about 39 per­cent in those with­out bugs.

During the trial, the group also observed what was hap­pen­ing in the rest of the olive grove and recorded a symp­to­ma­tol­ogy sim­i­lar to that observed on the tested olive trees.

They had empir­i­cal con­fir­ma­tion of what was found last year, how­ever with fewer bugs and fruit drops, yet in some cases with severe dam­age in the absence of treat­ments aimed at con­tain­ing the Asian bug.

“The results of the trial showed that it could be a direct or indi­rect action of the Asian bug,” Dell’Oro con­cluded. ​“Namely, the bug directly, or a fun­gal dis­ease caused by the bug, could lead to the fruit fall. These are just pre­lim­i­nary results, but we want to stim­u­late a com­par­i­son and dis­cus­sion with the objec­tive to find a solu­tion.”