Progress on the development of an IPM programme against insect pests
of pecan nut trees
Dr JL Hatting
ARC-Small Grain Institute
Bethlehem
During the past 3 years, surveys were conducted at three sites (Bull Hill, Tadcaster and Mogogong) in the Hartswater area to characterise the species complex of pestiferous and beneficial insects associated with pecan trees. In total, 12927 insects were collected over 16 bi-monthly surveys. The most prevalent pests represent five families, comprising of stinkbugs (Pentatomidae), snout weevils (Curculionidae), yellow pecan aphid (Aphididae), citrus leafhopper (Cicadelidae) and plant bugs (Miridae). A strong natural enemy complex was also revealed during these surveys, including lady-bird beetles, lacewings, praying mantids and antlions. The entomopathogenic fungus, Beauveriabassiana (Hypocreales: Cordycipitaceae) (Figure 1), was also isolated from several insect cadaverscollected and processed during this project. This fungus holds potential for development as biological insecticide, especially strains isolated directly from pecan-related pests collected within the production area. Several such strains were isolated from stinkbugs, an important group of pestiferous insects. In addition, several insects of unknown association were also recovered, termed ‘neutral’ in the bar graph below (Figure 2).
Figure 1. Stinkbug infected with the entomopathogenic fungus, Beauveriabassiana(Photo: JL Hatting, ARC-SGI)
Figure 2. Percentage (Y axis) pestiferous (red bars), beneficial (green bars) and ‘neutral’ (light blue bars) insects recovered during bi-monthly surveys over three years at three sites.
Based on the significant percentage beneficial species recovered, farmers are urged not to resort to full-cover insecticide applications at this time. Above data were also pooled across sites and time, to show the prevalence of the various insect types over a 12-month cycle (Figure 3). Clearly, the pestiferous species outperform the beneficials during the early part of the season (August to November), but last-mentioned feature strongly during the months of December through June. Notably, the build-up of lady-bird beetles (mainly Hippodamiavariegata) follow aphid populations developing during the late summer, into autumn, with H. variegata peaks noted during the month of June. Given the survey method used (contact-type insecticide applied with a mist blower), the February/April data do not fully represent the aphid populations, as these sap-sucking insects are not immediately affected by the contact insecticide used and/or are easily lost to wind movement following detachment of their mouthparts from feeding site (underside of leaves).
Figure 3. Average number of insects per tree throughout a 12-month cycle (colour codes as in Fig. 1).
In order to investigate the impact of these pest population on nut weight and damage, two field trials were conducted during the 2015/16 season. The trials were conducted at Bull Hill and Tadcaster with treatments applied at various frequencies to represent 1x (Apr), 2x (Feb, Apr), 3x (Dec, Feb, Apr) and 4x (Oct, Dec, Feb, Apr) chemical applications at bi-monthly intervals (Oct, Dec, Feb and March). After harvest, 10% of the nuts from each tree were transferred to ARC-SGI for processing. Nuts were weighed (in shell) and then individually opened, nutmeat weighed, and inspected for damage. The results showed that control trees (serviced by the strong natural enemy component) yielded nuts of statistically equal weight to that of trees sprayed1x to 3x. Moreover, aphid populations, typically peaking during February to April, seem not to affect nut weight as the February and April applications failed to statistically increase nut weight compared to controls. Notably, a significant increase in nut weight was noted from trees sprayed 4x (bi-monthly from October to April). By comparing the effect of 3x versus 4x applications, the October application (only included in the 4x application) seems to have the greatest impact on nut weight.
The question then arises: which insect species were present on the trees during the October surveys? Survey data revealed, amongst others, three primary insect pests, viz., a plant bug (Miridae), sunflower seed bug (Agonoscelis versicolor) and snout weevils (Curculionidae). As there are no nuts yet during October (late bud to pollination phase), the feeding behaviour of these insects need to be investigated to better understand the physiological affect thereof on early-stage nut set and development. Second-year field trials are currently underway, which will also include single-month insecticide applications.
The percentage damage to nutmeatfrom control trees was generally low, averaging 2.32%. Likewise, treatment trees showed nutmeat damage ranging from 1.37 to 3.70%. Of importance, however, is the fact that no nuts were observed with typical stinkbug feeding damage (i.e., dark circular blemish on nutmeat). The damage observed, is described rather as a general darkening of the nutmeat with shrivelled appearance. These observations are supported by trials during 2015/16, where the bark stinkbug, Coenomorpha nervosa, was released onto caged nut clusters at densities of 1 and 3 insects per cluster, with no typical damage observed despite damage levels of 10% (control), 9% (1 insect per cage) and 7% (3 insects per cage). Nutmeat weight was also not affected during these trials with averages of 3.8g (control), 4.1g (1 insect per cage) and 3.8g (3 insects per cage). Conversely, damage of 9% (control) versus 28% (3 insects per cage) and a significant reduction in nutmeat weight (3.8g and 3.1g, respectively) were recorded in 2014/15, but again, no typical damage was recorded.These observations suggest that C. nervosadoes not affect the nut directly as in the case with the two-spotted stinkbug (Bathycoeliadistincta) on macadamia nuts. Indirect feeding on twigs, stems and green husks may be responsible for reduced nutmeat weight, as observed in 2014/15.
In summary, farmers should take note of the ecosystem services rendered by the natural enemy complex in their pecan orchards. Untimely insecticide applications may upset these populations and lead to a resurgence of insect pest species as well as an increased risk of resistance developing against certain insecticides. Following the first year of chemical field trials, a better understanding now exists of the most important insect pests and their impact on nut development. Further field trials are underway and more information will be communicated to farmers in due course.