Observations on the method of transmission of internal boll disease of cotton by the cotton stainer‐bug

Results are presented to show that all the rules of proof for insect transmission of Nematospora by Dysdercus spp. have now been satisfied: viz. a close association of the insect with diseased plants in nature; visitation of healthy plants by the insect under conditions suitable for transmission of the disease; the presence of the pathogen in or on the insect in nature or following visitation to a diseased plant; and experimental production of the disease by insect visitation under controlled conditions. Hence it is claimed that Dysdercus spp. are proved vectors of internal boll disease of cotton. Fungal material as spores or mycelium is carried as an external contaminant on the mouth‐parts, although it is located in the deep stylet pouches, where protection is afforded and spore germination is possible. The fungus is cast off with the exuviae during moulting, but recontamination from exuviae or other sources in the environment occurs. The transmission is hence purely mechanical with the insect obligatory only in its function as a syringe to inject the fungus which is otherwise unable to reach its substrate. Insect and fungus can develop independently and the insect is apparently unharmed by the presence of the fungus. Nematospora spores also reach the intestine but do not appear to remain viable there and do not germinate. The fungus is believed to enter and leave the insect by the same route. The spores are well adapted in shape to reach the stylet pouches of the insect. No alternative morphological form of Nematospora gossypii within the insect has been found. The results do not lead to methods of control of the disease beyond those in practice‐trapping of insects, destruction of alternative host plants, and the alteration of time of planting to avoid insect attack.