Susceptibility of the Taro Beetle,Papuana uninodis(Coleoptera, Scarabaeidae) to Two NewBacillus popilliaeIsolates fromPapuanaspp.

Abstract

Two morphological types ofBacillus popilliae,causal agent of the milky disease, have been isolated from taro beetles (Papuanaspp, Coleoptera: Scarabaeidae).B. popilliaefromP. woodlarkiana woodlarkiana(Papua New Guinea) was a type A1 with a small sporangium (4.1×1.6 μm) and a large spore (2.1×1.4 μm) and parasporal body (1.8×1.2 μm) that sometimes overlap.B. popilliaefromP. uninodisandP. woodlarkiana laevipennis(Solomon Islands) was a type B2 with a small sporangium (2.8×1.3 μm), a small eccentric spore (1.1×0.7 μm), and no parasporal body. The infectivity of theseB. popilliaetoPapuana uninodislarvae was compared with twoB. popilliaesamples fromPopillia japonicain injection tests. The hemolymph ofP. uninodissupported the germination and growth of isolates fromPapuanaandP. japonica.Results were similar in third instars and adults. Highest infection (spores present) and mortality was caused by the isolates fromPapuana:mortality reached almost 100% 4 weeks after injection of the B2 typeB. popilliaewith 40% of larvae and 52% of adults infected. Injection of type A1 caused lower mortality but a similar percentage infected. Of two A1B. popilliaefromP. japonica,one caused a mortality comparable to type A1 fromPapuanabut lower infection; an older isolate resulted in low mortality and only one infected larva.B. popilliaetype A1 fromP. woodlarkianawas produced in the Solomon Islands by injection of spores inP. uninodis.Thirty four percent of the injected larvae and 31% of the adults produced spores with an average yield of 3.2 and 0.8 × 109spores/insect, respectively. Oral application of a single dose of 107spores of theB. popilliaeisolates fromP. uninodisorP. japonicadid not cause infection and similarly inoculation of the food with spores ofB. popilliaetype B2 did not result in infections. However, when different rates were applied to the food of second- and third-instarP. uninodis,theB. popilliaetype A1 fromP. woodlarkianacaused up to 15% infection and concentration-related mortality.