Study Suggests Stink Bug Caused Mysterious Fruit Drop in Italy

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.

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.

• Journal of Integrated Pest Management