For thousands of years, large locust swarms have caused severe problems in agriculture. People fight against current outbreaks by using chemical pesticides or an insect fungus known as Green Muscle™. While chemical pesticides may be harmful for humans and non-target species, the sporulation of the fungus takes a long period of time and requires conditions of high humidity that are not always found in the field. In this study, we tested the toxicity of a linseed oil/bicarbonate emulsion against gregarious desert locusts and screened for plant essential oils that enhance its toxicity. Finally, we combined three essential oils to develop a novel formulation that is effective against the desert and migratory locust species after single spray treatment. Within 24 h, this formulation caused a mean mortality rate of 80% and 100% of desert and migratory locusts, respectively. Its toxicity is based on a synergistic effect resulting from the combination of caraway, orange peel and wintergreen oils. In addition, we tested this botanical pesticide on two beetle species regarded either as alternative or beneficial non-target species. The first species, mealworm beetles, did not suffer from the spray treatment and behaved normally after 8 days. In contrast, 67.7% of ladybird adults died in the same time span. Interestingly, the growth of wheat seedlings was almost unaffected by spraying this botanical pesticide. These results suggest this botanical pesticide can be used as a strong agent against desert and migratory locusts, but needs to be used with care to minimize unwanted side effects on the ecosystem. Keywords Locust swarm • Linseed oil • Essential oils • Mealworm beetles • Ladybird beetles • Pest management • Botanical pesticide Key message • In many countries, locust outbreaks still constitute a big problem for farmers. Current pest management is potentially harmful for humans. • We developed a novel botanical pesticide against locusts that consisted of natural plant oils. • We identified orange peel, caraway and wintergreen oils to be effective against locusts when added to a linseed oil/bicarbonate emulsion. • Our novel botanical pesticide had a synergistic toxic effect. • This formulation is highly effective against two gregarious locust species and is considered harmless for humans. Communicated by M.B. Isman.

Gregarious desert locusts constitute very destructive agricultural pests. They aggregate and form collectively moving swarms that devastate vegetation and reduce crop production. To combat gregarious locusts, a bio-pesticide formulation that contains linseed oil as the main component was described recently. Since linseed oil is rich in fatty acids, some of which function as necromones that indicate injury or death in various insect species, we investigated the influence of linseed oil on the aggregation behaviour of sexually mature gregarious desert locusts. For this reason, we performed a series of aggregation experiments with six individuals of the same sex and brushed the wings of one individual (target individual) with linseed oil. The time the oil brushed target males spent close to any other individual was reduced in 76% of trials (average reduction of 18%), whereas the time target females spent in groups with members of the same sex did not alter. These results suggest that linseed oil may act as a bioactive agent that has the potential to disrupt swarm formation.

In forest clearings of the Malaysian rainforest, chirping and trilling Mecopoda species often live in sympatry. We investigated whether a phenomenon known as stochastic resonance (SR) improved the ability of individuals to detect a low-frequent signal component typical of chirps when members of the heterospecific trilling species were simultaneously active. This phenomenon may explain the fact that the chirping species upholds entrainment to the conspecific song in the presence of the trill. Therefore, we evaluated the response probability of an ascending auditory neuron (TN-1) in individuals of the chirping Mecopoda species to triple-pulsed 2, 8 and 20 kHz signals that were broadcast 1 dB below the hearing threshold while increasing the intensity of either white noise or a typical triller song.Our results demonstrate the existence of SR over a rather broad range of signal-to-noise ratios (SNRs) of input signals when periodic 2 kHz and 20 kHz signals were presented at the same time as white noise. Using the chirp-specific 2 kHz signal as a stimulus, the maximum TN-1 response probability frequently exceeded the 50% threshold if the trill was broadcast simultaneously. Playback of an 8 kHz signal, a common frequency band component of the trill, yielded a similar result. Nevertheless, using the trill as a masker, the signal-related TN-1 spiking probability was rather variable. The variability on an individual level resulted from correlations between the phase relationship of the signal and syllables of the trill. For the first time, these results demonstrate the existence of SR in acoustically-communicating insects and suggest that the calling song of heterospecifics may facilitate the detection of a subthreshold signal component in certain situations. The results of the simulation of sound propagation in a computer model suggest a wide range of sender-receiver distances in which the triller can help to improve the detection of subthreshold signals in the chirping species.