The submersed aquatic plant, Hydrilla verticillata (L.f.) Royle (hydrilla) is a serious invasive weed in the southern USA, but in Central African lakes, it is not considered a problem. Previous surveys in Lake Tanganyika identified 2 species of chironomid midges in the genus Polypedilum Kieffer that putatively caused tip-mining damage to hydrilla. We returned to Lake Tanganyika to further investigate the midges and to explore for other natural enemies of hydrilla. High proportions of stems collected at 2 locations south of Bujumbura were found to have bored apical meristems. No insects were observed in the burrows, but large numbers of nymphs of the African burrowing mayfly, Povilla adusta Navas, were dislodged when hydrilla stems were shaken under water. Because P. adusta is known to burrow in living aquatic plants, wood and several inorganic substrates, we hypothesized that P. adusta nymphs caused the boring damage to hydrilla. To test this hypothesis, undamaged hydrilla was exposed to P. adusta nymphs in a series of laboratory experiments. In all experiments combined, 4% of hydrilla stems exposed to P. adusta were damaged, whereas no stems in containers without P. adusta were damaged. Damage was characterized as lateral or vertical boring in apical meristems and, less frequently, in stems. To quantify the level of damage in the field, stems were collected from 2 locations on 6 occasions. The proportions of stems with bored apical meristems ranged from 5–52% and 13–57% of stems were missing apical meristems. Although Povila adusta caused substantial damage to hydrilla in Lake Tanganyika, it should not be considered for biological control because it is not a herbivore, and is known to burrow in a wide variety of substrates. The Polypedilum spp. midges earlier reported to bore into hydrilla apical meristems were not abundant in the field and not found associated with boring damage. The damage earlier attributed to the midges was likely due mostly or entirely to P. adusta. These midges should not be pursued further as candidate biological control agents of hydrilla.
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Vol. 94 • No. 3