Effectiveness of entomopathogenic nematodes in an alginate gel formulation against lepidopterous pests

An improved calcium alginate gel formulation was developed and tested as a carrier for entomopathogenic nematodes against Spodoptera littoralis and Helicoverpa armigera larvae. Mortality of 100% teas caused in 4th instar larvae of the two insects by feeding them on 1000 infective juveniles (1J) g(-1) of Steinernema carpocapsae (ALL strain) in the gel for 24 h. Exposing 2nd to 5th instars of H. armigera and 3rd to 6th of S. littoralis to 500 IJ g(-1) of S. carpocapsae (ALL strain) resulted in 70-100% larval mortality. Mature larvae were less susceptible to the nematodes. Mortality of larvae exposed to 500 IJ g(-1) of S. carpocapsae (ALL strain) ranged from about 45-550 at 4 h to 90-95% at 48 h. Fourth instar larvae fed for 24 h with 250 IJ g(-1) of nematode strains in gel showed in S. littoralis ranges of susceptibility in the following descending order: S. feltiae (IS-7 strain) = S. carpocapsae (DT strain) = S. feltiae (IS-6 strain) > S. carpocapsae (Mexican strain) = S. carpocapsae (ALL strain) = Heterorhabditis bacteriophora (HP-88 strain) = H sp. (IS-5 strain) > S. riobravae (Texas strain); in H. armigera the rating was: S. feltiae (IS-7 strain) = H. bacteriophora (HP-88 strain) > S. carpocapsae (ALL strain) = S. feltiae (IS-6 strain) = Heterorhabditis sp. (IS-5 strain) > S. carpocapsae (Mexican strain) > S. riobravae (Texas strain). The number of nematodes per larval cadaver increased with mortality rates. In greenhouse tests at 28 +/- 2degreesC and 90% relative humidity, gel discs containing 500 IJ g(-1) of nematodes were pinned to leaves of potted plants of cotton (Gossypium hirsutum) (Acala SJ2) and the plants were offered to S. littoralis larvae. Larval mortality of 89 +/- 12.7% was caused by S. feltiae (IS-7 strain) and most of the plant leaves were protected against the larvae by the nematodes. In the control, larval mortality was 3.3 +/- 0.050 and the plants were almost completely defoliated. Possibilities of using the gel-nematode formulation to protect sheltered crops against insect pests are discussed.