What is more important to host-seeking entomopathogenic nematodes, innate or learned preference?

Authors: Gaffke, Alexander; Romero, Maritza; Alborn, Hans

Submitted to: Agriculture Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 9/11/2023
Publication Date: 9/13/2023
Citation: Gaffke, A.M., Romero, M.I., Alborn, H.T. 2023. What is more important to host-seeking entomopathogenic nematodes, innate or learned preference?. Agriculture Journal. 13(9):1802. https://doi.org/10.3390/agriculture13091802.
DOI: https://doi.org/10.3390/agriculture13091802

Interpretive Summary: Entomopathogenic nematodes, small soil dwelling round worms that infect and kill pest insects, are known to provide significant pest control in many agricultural systems. However, the effectiveness of the nematodes can depend on many factors. Some nematode species are much better at controlling certain pests, like white grubs and mole crickets, while others can learn from the environment to kill whatever pests are causing damage to the crop. By understanding how nematodes make these choices, farmers can achieve better pest control while utilizing fewer pesticides. Scientist at the USDA-ARS Center for Medical, Agricultural and Veterinary Entomology located in Gainesville, Florida examined ways to manipulate this behavior for better pest control. The research indicated a complex interaction between what the nematodes learn and their innate preferences for certain insects, with some instances increasing control of the insects, while other tests resulted in reduced control. These results form the basis of our understanding of how to maximize efficacy of these biological control agents against targeted ground-dwelling pest insects. As a component of an integrated pest management program, nematodes can contribute to reduction of reliance on pesticides as well as enhance capability of organic food production.

Technical Abstract: Entomopathogenic nematodes (EPNs), small soil-dwelling non-segmented roundworms, are obligate parasites of insects and commonly used in agriculture for biological control of insect pests. For successful reproduction, EPNs must identify, move towards, and successfully infect a suitable insect host in a chemically complex soil environment. While EPNs can have an innate host insect preference, they can also develop strong preference for non-host related chemical signals due to prior exposure. The manipulation of these innate and learned preferences could therefore result in increased biological control services of EPNs in agriculture. The importance of these innate vs learned preferences were therefore investigated. Separate cohorts of the EPN Steinernema diaprepesi were raised on two insects, Galleria mellonella and tenebrio molitor, for multiple generations until the nematodes exhibited a preference for the host they were reared on. Subsequently, the two strains of nematodes were imprinted on three plant-produced terpenoids of agricultural significance: pregeijerene, beta-caryophyllene, and alpha-pinene. After exposure to the plant compounds, the behavior of the EPNs were assayed in sand column, duel-choice assays where the two host insects were presented with and without the plant compounds. When trained IJs were exposed to the preferred host (G. mellonella or T. molitor) with pregeijerene, beta-caryophyllene, or alpha-pinene, and a non-preferred host, the infection rates of the preferred host were unchanged for the pregeijerene treatment, while infection rates were suppressed by the presence of beta-caryophyllene, and alpha-pinene. However, when the plant compounds were combined with the non-preferred host, all treatments resulted in increased infection of the non-preferred host compared to the preferred host. These results indicate that learned preference is neither subordinate nor superior to innate preference, and infection rates can vary with compound exposure and the insect host.