Tomato spotted wilt tospovirus ingestion by first instar larvae of Frankliniella occidentalis is a prerequisite for transmission.

Tomato spotted wilt tospovirus (TSWV) is, after ingestion by first instar larvae, efficiently transmitted by second instar larvae and adults of the thrips Frankliniella occidentalis in a propagative manner. The developmental stage at which thrips larvae acquire an infectious dose, resulting in adults that can transmit the virus, is further defined. TSWV accumulation and transmission occurred after ingestion by first instar larvae. Second instar larvae failed to acquire and retain TSWV upon ingestion and did not develop into transmitters. No correlation was found between the quantity of TSWV ingested by thrips and their ability to acquire TSWV. Instead, first instar larvae gradually lost their acquisition ability with increasing age, suggesting the development of a barrier preventing TSWV acquisition and, consequently, replication needed for transmission. The finding that TSWV is acquired exclusively by first instar larvae of F occidentalis is of crucial importance for understanding the epidemiology of this devastating virus.