Biology and control of green vegetable bug, Nezara viridula (L.) (Heteroptera: Pentatomidae) in process sweet corn, Zea mays (L.) in the East Cape Region, New Zealand

Abstract

Nezara viridula (L.) (Heteroptera: Pentatomidae), the green vegetable bug (GVB) is a serious pest of process sweet corn Zea mays (L.) in the East Cape region of the North Island, New Zealand. In the past seven years the damage attributed to this insect has increased dramatically.

Field observations and adult dissections over two and a half years determined the number and the timing of green vegetable bug generations. GVB had a maximum of two generations, the first of which invaded the region's sweet corn crops, causing economic damage. Reproduction was continuous within the region from early summer until autumn although the insects' phenology differed between sites.

GVB occupied a large number of hosts over the year but exhibited a small host range in some months. GVB reproductive activity was compared between a cool, wet season (1996/7) and a hot dry one (1997/8). Analysis of heat accumulations suggested that high temperatures experienced in the 1997/8 season caused an increase in oviposition and/or survival rates.

Sampling techniques were developed for GVB in process sweet corn. The relative efficiencies of different sampling methods were compared between individual sweet corn plants. Results showed 90% of the GVB present were distributed 0.2 m below and up to 0.3 m above the base of the primary sweet corn cob (ear), 'the cob zone'. Plants sampled using an intrusive method examining all cob surfaces including beneath the silk detected 79% of GVB present on individual plants with a minimum sampling time of 3-5 seconds per plant.

Significantly greater numbers of GVB were recorded in the 24-row crop headland than in the centre of the crop (P<0.001) in five crops sampled over a range of pest infestations. Pronounced edge effects were observed within all crop headlands except at the most severe infestation level that resulted in total crop loss. A preliminary sampling system was developed sampling five plants in each of the second, fourth and sixth rows inside the crop headland around the entire perimeter of a crop. The method was used to compare infestations both within and between three process sweet corn crops over three growing seasons. These results and observations made in 137 other sweet corn crops during this time showed GVB invaded crops rapidly in sometimes large numbers immediately after crops entered the reproductive growth phase.

Field estimates of the relationship between pest number and damage in five different process sweet corn crops showed one adult GVB may damage between 2.00 (± 0.2) and 6.79 (± 0.61) cobs. A field sleeve cage experiment showed the feeding activity of one adult GVB over one week equalled the process threshold of 5.0 damaged kernels per cob from R1 (silk) until the harvest date. The sweet com growth stage most susceptible to damage from GVB feeding was R3 (milk) approximately 14 days from harvest. Results of both the field correlations and the sleeve cage experiment demonstrated that damage may occur very late in the life of a crop and that the economic injury threshold for this pest is very low.

The distribution and impact of Trissolcus basalis (Wollaston) (Hymenoptera: Scelionidae), an egg parasitoid of the GVB were evaluated in the upper North Island of New Zealand between 1995 and 1998. Two surveys and three augmentative release trials were conducted in crops of Zea mays (L.) sweet corn, Cyphomandra betacea (Cav.) Sendt., tamarillo and domestic vegetable gardens in Auckland, Northland and in the East Cape region of the North Island.

Using sentinel egg rafts obtained from culture, the parasitoid was monitored for its presence and impact, measured as percent parasitism throughout a growing season. Although not present at all sites, the parasitoid was found to be well distributed throughout the regions. Average percent parasitism over a growing season was < 20% across all monitored sites. Percent parasitism followed a general trend of increase from spring through until late summer/autumn. At several sites, parasitism reached 60-70% on occasion but this was always late in the season. Survey results included the first New Zealand record of Acroclisoides sp. (Hymenoptera: Pteromalidae), a hyperparasitoid previously reported to attack T. basalis in Australia. Average maximum rates of hyperparasitism were 10.4%.

Throughout the trial work, the parasitoid was recorded only where GVB was observed to be actively breeding. Results indicated that the parasitoid preferred weeds and vegetable crops/gardens over tree crops. Augmentative release trials in sweet corn crops suggested that successful augmentation is possible although the effects are short-lived. Overall, these results suggest that Trissolcus basalis is a useful parasitoid of GVB but it does not provide a commercially acceptable level of control. Present rates of parasitism are much lower than those reported following the introduction of the parasitoid into New Zealand 50 years ago.

The efficacy of insecticides for the control of GVB were tested in laboratory and field trials. Bioassays of 11 insecticides against field collected adults were used to select two insecticides for a field experiment conducted in a process sweet corn crop (cv. Punch) near Tolaga Bay, in the East Cape region. Lambda-cyhalothrin (as 200 ml Karate®/100 litres water/ha) and methamidophos (as 1 litre Tamaron™/100 litres water/ha) were tested against a water control. Both insecticides demonstrated efficacy, with respective mortality rates of 87% and 78% compared with 2% in the control.

GVB flew into the experimental crop in large numbers over a 48-hour period prior to the commencement of the trial. Economic thresholds were exceeded in two of the three control plots within seven days of GVB first invading the crop, leaving only a very small 'window' for insecticide application. Success or otherwise of these insecticides is therefore likely to be dependent on the timing of the application as well as the efficacy of the products themselves.

The use of trap crops to reduce GVB damage to sweet corn was investigated in a series of three field experiments. In the first season, small plots (2.7 m by 10m) of white mustard, Sinapis alba (L.) with pea, Pisum sativum (L.) were sown along a crop border and compared with sweet corn alone. In the second season, black mustard Brassica nigra (L.) was sown at 14-day intervals and compared with a sweet com control, to examine how the maturity of the trap crop affected numbers of GVB trapped. GVB were contained almost exclusively within the trap crop in both small plot experiments until the sweet corn was harvested.

A field scale experiment demonstrated the effectiveness of black mustard as a trap crop to protect larger areas of sweet corn from GVB. Percentages of damaged sweet corn cobs in the outside rows of fields protected by a trap crop were 0% and 1% respectively compared with 11% and 22% in control fields. In all three experiments, GVB populations were much higher on the trap crops compared with the sweet corn. Trap cropping is recommended as an effective strategy to manage this insect. Options for cultivating or spraying the trap crops to reduce GVB survival are discussed. Results of these experiments are discussed in respect of the short, medium and long-term control of GVB in New Zealand.