Putting numbers on what stands between RNAi and effective insect control

For most insect pest species tested so far, it has proven difficult to achieve a commercially relevant activity level using an insecticidal RNA-based biocontrol. Multiple loss mechanisms have been described, including external factors, such as the environment and feeding behavior, and internal factors related to the physiology of the insect. Generally, differences in RNAi efficiency are described only qualitatively, as it is not straightforward to obtain exact numbers and compare between different delivery methods, feeding behaviors etc.

In this work, the differences were quantified more precisely to increase our understanding of physiology-related loss mechanisms. Four species from diverse orders were selected for 1) being candidate targets for an RNA-based biocontrol product, and 2) showing a varying degree of RNAi efficiency. Using both injection and feeding delivery methods, lethal dose and lethal time were accurately specified and linked to the precise amount taken up using a feeding tracker. In addition, the QuantiGene Plex method, routinely used to quantify dsRNA expression levels in GM plants, was adapted to detect dsRNA in the insect body. A similar robust quantification method was developed to compare degradation rates in the insects’ hemolymph and gut. Putting all this data together increased our understanding of the limitations of exploiting RNAi in these commercially relevant pests, and revealed some interesting discrepancies between RNAi efficiency and known loss mechanisms.