Efficacy of the repellent N,N-diethyl-3-methyl-benzamide (DEET) against tabanid flies on horses evaluated in a field test in Switzerland.

The research article focuses on a field test conducted in Switzerland to evaluate the efficiency of the insect repellent N,N-diethyl-3-methyl-benzamide (DEET) against tabanid flies, a common annoyance to horses. DEET’s repression of these flies is addressed due to its potential impact on decreasing the transmission of Equine Infectious Anemia (EIA).

Objective and Context of the Study

• The primary objective of the study was to establish a protocol for field tests that can effectively measure the repellency of DEET, a substance commonly used to deter insects, against tabanid flies on horses for a duration of up to four hours.
• This study is set against the backdrop of the ongoing spread of Equine Infectious Anemia (EIA) within Europe, a disease mechanically transmitted by tabanid flies and poses significant risks to horses.

Methodology and Trial Design

• The field test was conducted over two consecutive days between July and August 2015.
• The sample involved four horses from three different farms that participated in the pre-test and main test.
• The horses were subjected to DEET treatment, specifically in a 15-17% oil-water emulsion.
• Two and a half hours post the application of the repellent, the horses were made to sweat by lunging.
• Counts and photographs of tabanid fly infestations were gathered at intervals of three and four hours post-application, focusing on the area spanning from the horse’s head to the flank, belly, and the first third of the foreleg.

Key Findings

• When not treated with DEET, a horse was found to host up to 29 tabanid flies simultaneously.
• For horses treated with DEET, that number was significantly reduced to a maximum of four flies.
• In 50% of the horses treated with DEET, no tabanid flies were observed, indicating a 100% repellency in those cases. Additionally, all DEET-treated horses showed lower tabanid fly counts than those unexposed to the repellent, barring one exception at the four-hour mark.
• The repellent’s efficiency was found to be at least 80% and 71% – three and four hours post-application, respectively.

Additional Observations and Conclusions

• A fly trap revealed the presence of not just tabanid flies (Tabanus brominus and Haematopota pluvialis) but also other unspecified arthropods.
• The study effectively managed to simulate practical conditions and quantify the number of tabanid flies – a step necessary to demonstrate the efficiency of DEET as a repellant in horses.