Nuclear polyhedrosis virus as biological control agent of Spodoptera exigua

Several aspects of the control of the beet armyworm, Spodopteraexigua (Hübner) (Lepidoptera: Noctuidae) in greenhouse crops with nuclear polyhedrosis viruses (NPVs) (Baculoviridae, subgroup A) were studied.

Beet armyworm behaviour was observed in various crops. The distribution of egg batches (Chapter 2) was found to be similar in chrysanthemum, tomato, gerbera and geranium. Most eggs were laid on the underside of leaves within 10 cm of the soil surface. In chrysanthemum moths deposited more eggs on very young than on older plants. No correlation was found between size of egg batches and plant species, plant age or position within in a crop.

After hatching beet armyworm larvae gradually moved upwards to the top of chrysanthemum plants (Chapter 3) and then predominantly fed on the upper foliage layers. On tomato, however, larvae did not move upwards and mainly fed on the lower leaves. On chrysanthemums most feeding of the younger larvae occurred at the underside of foliage while the upper epidermis remained intact. Larvae dispersing from a single egg batch of 35 eggs damaged about 90 small and 50 tall chrysanthemum plants during their development. The five successive larval stages contributed 0.1%, 0.4%, 4%, 20% and 75% to the total foliage consumption, respectively.

Five nuclear polyhedrosis viruses infectious for beet armyworm larvae were compared for their potential as biological control agent (Chapter 4). Restriction endonuclease patterns of the DNA showed that three of the isolates, collected from deceased beet armyworm larvae in the Netherlands, are closely related with Mamestrabrassicae nuclear polyhedrosis virus (MbMNPV). Therefore they were named MbMNPV-NL80, MbMNPV-NL82 and MbMNPV-NL83. These isolates are not closely related with AutographcalifornicaMNPV (AcMNPV) and Spodopteraexigua MNPV (SeMNPV), both originating from the USA.

Comparison of the biological activity of these five MNPVs (Chapter 4) showed that the SeMNPV has a greater biological activity against beet armyworm larvae than MbMNPV-NL80, - NL82, -NL83 and AcMNPV. The LD-50 values of the five MNPVs against second instar larvae were 3, 26, 14, 17 and 18 polyhedra, respectively. SeMNPV, therefore, seemed to be the most suitable candidate for biological control of the beet armyworm.

Bioassays carried out with the droplet-feeding method showed that larvae became less susceptible to SeMNPV with increasing age (Chapter 5). The LD-50 values for the five subsequent larval instars were 4, 3, 39, 132 and 11610 polyhedra. The LT-50 values increased from ca. 3.5 days for first instars to almost 6 days for fifth instar larvae.

Production of SeMNPV (Chapter 6) was carried out in late fourth instar larvae reared on semi- synthetic diet. A maximum amount of virus, 1 to 2 x 10 ⁹polyhedra/larva, was produced in individually-reared larvae, after seven days of incubation at WC, with an inoculum of 7.5 x 10 ⁴polyhedra/cm ²diet surface. Virus yield was reduced to ca. 9 x 10 ⁸polyhedra/larva when production was carried out in groups of 400 and 600 larvae per container.

Biological activity of virus harvested from living and from deceased larvae was similar. Microbial contaminants, predominantly bacteria, in the produced virus batches numbered 1-6% of the number of polyhedra. Vertebrate pathogens were not present in any of the produced virus batches.

The effect of high-volume handspraying (HV), low-volume spinning disc (LV-SD) and ultra-low- volume electrodynamic (ULV-ED) application systems on the efficacy of larval control with SeMNPV was studied on chrysanthemum crops (Chapter 7). Virus applications with the LV-SD system generally showed a slightly better control efficacy than was achieved with the HV system. The efficacy of ULV-ED applications was much lower except on a very short crop, where control comparable with that of the LV-SD system was obtained.

The efficacy of SeMNPV for control of beet armyworm larval populations was studied in chrysanthemum, gerbera, kalanchoe and tomato crops (Chapter 8). Applications of 1 x 10 ⁸polyhedra/m ²resulted in 95-100% larval mortality on each of the crops. Virus applications caused comparable levels of mortality in populations of early and late instar larvae. Reduction in feeding damage to the crops, however, was more pronounced when the larvae were early instars at the time of virus application. Single applications with the virus resulted in higher levels of larval mortality as double applications with 0.1% methomyl or 0.1% diflubenzuron.

In the general discussion (Chapter 9) the results from the various studies, commercial aspects and the current situation in Dutch greenhouse industry were evaluated. This lead to the final conclusion that there is good potential for the practical use and commercial development of SeMNPV as a control agent of beet armyworm larvae in Dutch greenhouses.