Study Suggests Stink Bug Caused Mysterious Fruit Drop in Italy

The brown marmorated stink bug has been identified in Italian and Greek olive groves. Increasing evidence now links its presence to an early fruit drop in northern Italy.

Brown marmorated stink bug
By Simon Roots
Oct. 31, 2024 16:31 UTC
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Brown marmorated stink bug

In recent years, reports of pre­ma­ture fruit drop and sub­se­quent pro­duc­tion losses in north­ern Italian olive groves have steadily increased.

Numerous causes have been posited, from extreme weather and cli­mate change to uniden­ti­fied fun­gal infes­ta­tions, inva­sive pests or com­bi­na­tions of all of the above.

However, stud­ies over the past year strongly sug­gest that the brown mar­morated stink bug (Halyomorpha halys) is the pri­mary dri­ver.

See Also:Invasive Sheep Devastate Olive Groves in Eastern Spain

H. halys, native to China, Japan, the Korean penin­sula and other Asian regions, is believed to have been acci­den­tally intro­duced to North America in the late 1990s.

Since then, it has become a sig­nif­i­cant agri­cul­tural pest. Highly polyphagous, the bug spreads eas­ily to new food sources and, by 2010, was caus­ing annual losses of at least $37 mil­lion in apple crops alone.

The insect’s spread to and through Europe has fol­lowed a sim­i­lar pat­tern. It is believed to have been intro­duced to the con­ti­nent via Switzerland and reached the United Kingdom by 2021.

It has also reached Turkey, where it has been stated that the bug has already caused a 20 per­cent drop in hazel­nut yield in the province of Artvin, losses that are expected to rise to 50 per­cent or $1 bil­lion in dam­ages.

A 2023 study pub­lished in the jour­nal Insects aimed to char­ac­ter­ize the dam­age inflicted by H. halys on olive fruits and its impact on pre­ma­ture fruit drop and fruit qual­ity.

The study was con­ducted in north­ern and cen­tral Italian olive groves, using nat­ural obser­va­tions and con­trolled field exper­i­ments.

The field exper­i­ments ana­lyzed the impacts of H. halys at two of the fruit’s devel­op­men­tal stages: pre-pit-hard­en­ing and post-pit-hard­en­ing.

The results showed that high den­si­ties of H. halys caused a sig­nif­i­cant increase in pre­ma­ture fruit drop dur­ing the pre-pit-hard­en­ing stage, which was less pro­nounced dur­ing the post-pit-hard­en­ing stage.

Chemical analy­sis revealed sig­nif­i­cant changes in the phe­no­lic com­po­si­tion of affected olives, with dam­aged fruits hav­ing higher con­cen­tra­tions of phe­no­lic com­pounds such as oleu­ropein.

These phe­no­lic com­pounds play a role in the plant’s defense mech­a­nisms, sug­gest­ing that H. halys feed­ing trig­gers these mech­a­nisms by induc­ing stress in the tree.

These ele­vated phe­no­lic lev­els may affect the yield and qual­ity of olive oil, as phe­no­lic com­pounds are cen­tral to olive oil’s fla­vor and health ben­e­fits.

A fur­ther study, pub­lished in the Journal of Economic Entomology in June 2024, sought to deter­mine the cause of pre­ma­ture fruit drop across mul­ti­ple olive groves in north­ern Italy. The researchers from the Universities of Verona and Padova began by exam­in­ing fallen olives for evi­dence of fun­gal infec­tions or insect-feed­ing activ­i­ties.

Fungal species were iso­lated from both healthy and dis­lodged olives. However, the study found no sig­nif­i­cant dif­fer­ences in the fun­gal pres­ence in healthy and dis­lodged olives.

In addi­tion, most of the species iden­ti­fied were com­mon endo­phytes with which the olive tree has a gen­er­ally mutu­al­is­tic rela­tion­ship, indi­cat­ing that fun­gal infec­tions were not respon­si­ble for the observed olive drop.

In con­trast, the tri­als demon­strated a strong cor­re­la­tion between the num­ber of stink bugs present and the extent of pre­ma­ture fruit drop.

In keep­ing with the results of the 2023 study, the most sig­nif­i­cant dam­age was caused dur­ing the early stages of fruit devel­op­ment before the olive pits had fully hard­ened.

The stink bugs’ feed­ing was found to have caused seed necro­sis, which in turn led to the tree shed­ding fruit pre­ma­turely. The team con­cluded from these results that H. halys was the pri­mary dri­ver of this early olive drop, with the high­est infes­ta­tions result­ing in the most sig­nif­i­cant fruit loss.

The researchers rec­om­mended that affected farm­ers adopt inte­grated pest man­age­ment strate­gies, incor­po­rat­ing phys­i­cal exclu­sion and tar­geted insec­ti­cide use.

Given the ever-increas­ing spread of this and other inva­sive pest species, they also stressed the need for future research into more sus­tain­able and scal­able con­trol meth­ods.



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