Transient endophytic colonization of melon plants by entomopathogenic fungi after foliar application for the control of Bemisia tabaci Gennadius (Hemiptera: Aleyrodidae)

Three Beauveria bassiana and two Metarhizium brunneum strains, two of which GFP-transformed, were evaluated for virulence against fourth-instar nymphs of the sweet potato whitefly Bemisia tabaci (B. tabaci nymphs, BTN), with the following results. (1) Immersion of infested melon leaves in suspensions ranging from 10(5) to 10(8) conidia ml(-1) yielded an LC50 ranging between 3.4 x 10(4) and 2.2 x 10(7) conidia ml(-1). (2) Temporal colonization of the leaf tissues infested with BTN 96 h after inoculation was observed in both sprayed leaves (SL) (ranging between 98.0 and 40.0 %) and leaves of the same plant that were not directly exposed to the fungal treatment [i.e., leaves that were not sprayed (LNS)] (62.0-12.0 %). Total nymphal mortality ranged between 83.9 and 100.0 % and between 66.3 and 87.9 % in SL and LNS, respectively. Interestingly, from 0 to 16 % nymph cadavers from LNS showed fungal outgrowth. (3) The adaxial surfaces of infested melon leaves were brushed with 10(8) conidia ml(-1) suspensions to ascertain the translaminar fate of the fungi in the leaves that resulted in nymphal mortality, which ranged from 53.4 to 96.0 %. As before, mortality with fungal outgrowth was detected only in the B. bassiana treatments as a result of the different leaf colonization patterns of the two fungal species revealed by histological examination. Destruxin A was present in 43.0 % of the melanized nymphs on the leaves treated on their adaxial surfaces with the M. brunneum isolate. The effect of transiently colonized leaves must be considered to estimate the true acute impact of field sprays containing entomopathogenic fungi on B. tabaci and other sucking insect pests.