Cowpeas leave a message in the soil, and the next plant uses it to call wasps

The mechanism runs in five steps ^[15]. A larva of the American serpentine leafminer tunnels into a cowpea leaf ^[5]. The plant changes what its roots put into the soil ^[3]. Those root chemicals cultivate bacteria ^[3]. The bacteria direct a later cowpea grown in that soil to release (Z)-3-hexenyl acetate ^[3]. A parasitoid wasp arrives ^[9]. Only the third step is portable in principle, and it is the one this account leaves anonymous: the bacteria are described by what they do, not by what they are ^[18].

The wasp's side of the deal is thinner than the plant's. Neochrysocharis formosa reproduces by laying eggs inside leafminer larvae, which the hatchlings then eat ^[7]. Here the volatile pulled wasps toward plants with no herbivore on them ^[10], so the recruits landed where there was nothing to parasitise ^[16]. The reported outcomes stop at attraction ^[19], and what a wasp does after an empty visit is what separates a control tactic from a redistribution of wasps.

Earlier work had already established that plants shape their root-zone microbes and that leaf-chewing insects alter those communities ^[12]; the new part is the return trip, soil back to leaf to wasp ^[1]. Michelle Afkhami of the University of Miami, who was not involved, reads the result as soil partners keeping alive a protection that would otherwise fade ^[13], the contrast being aboveground alarm chemistry, thought to dissipate once the pest stops eating ^[11]. Zeng-Rong Zhu of Zhejiang University, who led the work, says it is not known how long the microbial protection lasts or whether other kinds of plants use it ^[2]. The durability claim, in other words, rests on a soil transfer with no clock on it ^[17].

Demand is not the missing piece. Liriomyza trifolii is native to the southeastern United States and the Caribbean and is now found in parts of Asia, Europe and Canada ^[4]; its larvae tunnel through the leaves of cowpeas, eggplants and beans among others ^[5]; Zhu says leafminers can "destroy all the leaves if we don't use any measures to control them" ^[6]. Afkhami's other example of microbial defence, fungi that coat leaves with toxic alkaloids, works where the damage is ^[14]. This one works from somewhere else entirely ^[14], which is both the reason it is interesting and the reason it is hard to sell. Until someone isolates and grows the bacteria, the deliverable is a volume of dirt with a history, and dirt does not ship.