Representing a dramatic change in the way scientists would genetically modify crops - one that could allow these crops to respond more readily to factors like changing climate - a research program under development in the United States proposes introducing genetic changes into already-planted fields, using infectious viruses. The approach - distinct in its targeting of crops already planted - is referred to as HEGAAs. In this Policy Forum, Richard Guy Reeves and colleagues highlight concerns associated with this approach in the way that a high-profile funder, the Defense Advanced Research Projects Agency (DARPA), envisions it. Specifically, the DARPA-funded approach for dispersing genetically modified viruses directly into fields mandates that these viruses be delivered not through traditional means like overhead sprays, which are more easily monitored and controlled, but by insects, citing lack of spraying equipment among farmers as the rationale. "It would have been perfectly possible for the DARPA work program to have proposed the development of HEGAAs to be deployed using agricultural spraying equipment," say the authors, "without the involvement of insects." Their opinion is that DARPA's approach reflects "an intention to develop a means of delivery of HEGAAs for offensive purposes," such as defense. If such an approach comes to be, Reeves and colleagues say, "easy simplifications" could be used to generate a new class of biological weapons - weapons that would be extremely transmissible to susceptible crop species due to insect dispersion as the means of delivery. This perceived intent, Reeves and colleagues say, is of critical consideration in regard to international treaties that prohibit the use and development of certain weapons. The authors call for more transparency from DARPA as this program develops, as well as for more opportunities for public deliberation on this paradigm-shifting strategy.