Botanical Pesticide Outperforms Synthetic Alternative in Killing Olive Bark Beetle

Summary

A study found that nat­ural gar­lic dis­til­late is highly effec­tive in killing olive bark bee­tles, with close to 100 per­cent mor­tal­ity rates, while not harm­ing the beetle’s pri­mary nat­ural preda­tor, the par­a­sitic wasp Cheiropachus quadrum. This research, pub­lished in the Journal of the Entomological Research Society, high­lights the poten­tial of gar­lic dis­til­late as a safer and more effec­tive alter­na­tive to syn­thetic insec­ti­cides for con­trol­ling olive bark bee­tle infes­ta­tions.

A new study has shown that nat­ural gar­lic dis­til­late can achieve close to 100 per­cent mor­tal­ity against the olive bark bee­tle, despite its syn­thetic equiv­a­lent being rel­a­tively inef­fec­tive. 

The research, pub­lished in the Journal of the Entomological Research Society, also found that the com­pound did not affect Cheiropachus quadrum, the par­a­sitic wasp, which is the beetle’s pri­mary nat­ural preda­tor and an essen­tial bio­log­i­cal con­trol.

Olive bark bee­tles are par­a­sitic insects that can directly affect olive pro­duc­tiv­ity. They spend the win­ter in their adult state, shel­ter­ing in small gal­leries dug into the joints of leaves, fruits or small branches. 

Shortly after prun­ing takes place in the spring, they move towards the par­tially-healed wounds, where the males bur­row beneath the bark, con­struct­ing a cav­ity known as the ​“nup­tial cham­ber.” After mat­ing in this cham­ber, the females cre­ate a gallery that branches off to the left and right, in which they deposit an aver­age of 50 to 100 eggs.

When hatched, the lar­vae feed on the wood, cre­at­ing exten­sive gal­leries by bur­row­ing per­pen­dic­u­lar to the orig­i­nal cav­ity until pupa­tion. 

The new gen­er­a­tion of adults emerges after 40 to 60 days, gen­er­ally between the end of May and July. These adults then move to nearby olive groves to feed and then over­win­ter until the cycle begins again the fol­low­ing spring.

The feed­ing gal­leries of the adults cause the most seri­ous com­mer­cial dam­age. This feed­ing activ­ity par­tially or com­pletely engulfs the branch or twig, weak­en­ing its struc­ture and dam­ag­ing vas­cu­lar tis­sues, which causes leaves, fruit and pro­duc­tive twigs to fall.

Trees that are affected over mul­ti­ple sea­sons can also exhibit retarded devel­op­ment due to the recur­ring dis­rup­tion of nat­ural growth pat­terns. This sub­se­quently affects pro­duc­tiv­ity and com­mer­cial via­bil­ity.

Generally, where the infes­ta­tion is lim­ited, prun­ing and sub­se­quent burn­ing of the infested branches are suf­fi­cient to elim­i­nate the prob­lem. However, weak­ened trees, such as those affected by drought, that are severely attacked by the lar­vae have a lower chance of recov­ery and may even die.

In cases where live adults are present on more than ten per­cent of shoots, insec­ti­cide use is indi­cated. 

Artificial insec­ti­cides, such as pyrethroids, how­ever, have proven to be far less effec­tive against the adult olive bark bee­tle than against its lar­vae. 

In addi­tion, there is grow­ing alarm over their lethal effects on non-tar­get organ­isms, such as bees and other key species.

To assess the sus­cep­ti­bil­ity of both the lar­val and adult stages of the bee­tle to gar­lic dis­til­late, the researchers con­ducted a series of lab­o­ra­tory bioas­says. 

The dis­til­late in ques­tion pri­mar­ily com­prised dial­lyl disul­fide, dial­lyl trisul­fide, methy­lal­lyl trisul­fide and vinyldithiin (1,2‑dithiin and 1,3‑dithiin) mol­e­cules, sev­eral of which have seen indi­vid­ual suc­cess in pre­vi­ous insec­ti­ci­dal tri­als.

The high­est lethal con­cen­tra­tion value of gar­lic dis­til­late was esti­mated at 3.45 mil­ligrams per liter (mg/L) for lar­vae and at 4.41 mg/L for adults. 

The appli­ca­tion of a dose of 8.19 mg/L achieved 100 per­cent mor­tal­ity in both life stages. The mor­tal­ity rate of adults treated with the arti­fi­cial alter­na­tive was sig­nif­i­cantly lower than that caused by gar­lic dis­til­late at 7, 14 and 21 days after treat­ment.

The effects on Cheiropachus quadrum, a par­a­sitic wasp, were also eval­u­ated.

Hymenopteran par­a­sitoids are among the most impor­tant nat­ural ene­mies of bark bee­tles world­wide, and C. quadrum is the pri­mary preda­tor of the olive bark bee­tle, capa­ble of reduc­ing pop­u­la­tions by 30 to 50 per­cent.

The researchers found that, unlike pyrethroid-based insec­ti­cides, the gar­lic dis­til­late being stud­ied had no sig­nif­i­cant effect on the adult wasp, nor on its abil­ity to par­a­sitize bee­tle lar­vae and repro­duce, thus fur­ther increas­ing its suit­abil­ity as an eco-friendly bio-pes­ti­cide can­di­date.

• Junta de Andalucía
• Plantix