Insects associated with pecan trees, Carya illinoinensis: characterisation, impact and control

Dr JL Hatting, ARC-Small Grain, P/Bag X29, Bethlehem, 9700

Project overview (1 October 2017 – 30 September 2018)

During this fiscal year, two principal objectives were pursued (1) compilation of an Insect Field Guide (incorporating all survey data gathered from December 2013 to June 2017) and (2) expanding surveys to include three sites, Hartswater (Magogong), Prieska and Modimolle. Survey data from South Africa have shown that only one of the ‘true’ pecan insect pests, naturally associated with pecan in North America, is present locally (i.e., yellow pecan aphid, Monelliopsis pecanis). Ongoing surveys are thus needed as pre-emptive approach in alerting the industry to new incursions as soon as noted. Following, a brief overview of each research area.

1. Insect Field Guide

During the past four years, bi-monthly surveys were conducted at three sites (Bull Hill, Tadcaster and Magogong) in the Hartswater area to characterise the species complex of pestiferous and beneficial insects associated with pecan trees. In total, 21 surveys were conducted with 47 444 insects collected. This information was packaged in the form of an Insect Field Guide, launched during the 2018 SAPPA AGM in Cradock. The guide provides a brief overview of the insect Life Cycle, the two basic insect Feeding Mechanisms (piercing-sucking versus biting-chewing), followed by information on the Identification, Seasonal occurrence, Damage and Control (where applicable) of the most prevalent species.

Fig. 1. Cover and title page of the 2018 Insect Field Guide

Information on the natural enemies associated with these insects is also provided, thereby aiding the farmer to recognise their presence in support of Conservation Biological Control; especially within a perennial crop environment. Correct identification of the insects associated with pecan remains the backbone of any successful control strategy, albeit humanly induced or naturally aided. The guide is in the form of a clip file, enabling future additions/updates to be made to the text and photos, as new research findings materialise within the industry.

Under the insects with piercing-sucking mouthparts, 5 pest types are discussed: stink bugs (Pentatomidae), aphids (Aphididae), leafhoppers (Cicadellidae), soft scales (Coccidae) and plant bugs (Miridae). Although found to be the most prevalent among the stinkbugs encountered, the bark stink bug, Coenomorpha nervosa (Fig. 2), failed to inflict typical round feeding lesions to pecan nuts when caged on nut clusters. Moreover, comparisons of in-shell and meat weight, between insect-challenged versus control cages, showed no statistical differences. Therefore, the presence of this insect in pecan orchards should not be viewed as an immediate threat and response through area-wide insecticide applications is discouraged.

Fig. 2. Adult bark stink bugs (Photo: JL Hatting, ARC-SG)

The guide also highlights the indirect effect due to feeding of certain insects with piercing-sucking mouthparts; i.e., the production of so-called ‘honeydew’ (a sugary substance) utilised by some fungi as growth medium. The blackish fungal growth developing on pecan leaves is commonly referred to as ‘sooty mold’, and may affect photosynthesis if excessively present. Ants may also feed on this substance (often directly from the insect) and their presence could deter the natural enemies of these pest insects.

Under the insects with biting-chewing mouthparts, 8 pest types are discussed: longhorn beetles (Cerambycidae), ambrosia beetles (Curculionidae: Scolytinae), snout beetles (Curculionidae), leaf beetles (Chrysomelidae), chafers (Scarabaeidae), darkling beetles (Tenebrionidae), false codling moth (Tortricidae) and African bollworm (Noctuidae). Among this group, notable damage can be caused by the fig-tree borer, Phryneta spinator, although not widely distributed within the pecan production areas. An exotic invader to South Africa is the polyphagous shot hole borer, Euwallacea sp., noted also in pecan trees in the Jan Kempdorp area during 2017. As this insect and its symbiotic fungus infests a wide range of host plants, a national attempt at containing the insect is currently underway, led by the Forestry and Agricultural Biotechnology Institute (FABI). The guide includes a

short description and provides a link to a web portal for detailed information. Adults of a destructive chafer, Anomala ustulata, were recovered from light traps operated in the Vaalharts area, but also directly from young pecan trees. The guide includes several photos of this pest and producers are alerted to its damage potential, especially die-back of affected shoots.

The African bollworm, Helicoverpa armigera, is a well-known pest in South Africa, and now known to also infest pecan. During initial nut expansion that follows fertilization and embryo development, small developing nutlets are targeted by young larvae. The larva creates a characteristic round entry hole in the nutlet, feeding on its contents and arresting all further development. Unfortunately, some confusion exists with the use of its common name in South Africa. Several published sources, including insecticide labels, incorrectly refer to this insect as the American bollworm. This confusion arose when New World populations of the American corn earworm, Helicoverpa zea, and Old World populations of the cotton bollworm, Heliothis armigera, were incorrectly viewed as one cosmopolitan species. These two species are however distinct. Hardwick (1965) moved these two species to the genus Helicoverpa. Helicoverpa armigera was originally described from Europe and its worldwide distribution is depicted in the CABI Invasive Species Compendium (see https://www.cabi.org/ISC/), as is that of H. zea. The common name, African bollworm, is therefore the most fitting name for H. armigera and has been widely adopted by the entomological fraternity in southern Africa. The misnomer, American bollworm, should be strongly discouraged to avoid further confusion.

Reference

Hardwick DF, 1965. The corn earworm complex. Memoirs of the Entomological Society of Canada, 40: 1-247.

The guide underscores the fact that pecan orchards in the Vaalharts region are characterised by a prolific complex of beneficial species, peaking during the latter part of the pecan season. Prevalent types encountered include several species of ladybird beetles (Coccinellidae), especially Hippodamia variegata (Fig. 3), lacewings (Chrysopidae), hover flies (Syrphidae), praying mantises (Mantodea), antlions (Neuroptera), tiger beetles (Carabidae) and spiders (Araneae).

Fig. 3. Adult ladybird beetles, Hippodamia variegata, on harvested pecan nuts (Photo: JL Hatting, ARC-SG)

Chemical field trials, conducted by ARC/SAPPA over the 2015/16 and 2016/17 seasons in the Vaalharts region with a broad spectrum contact-type insecticide, showed no significant increase in nut weight among the chemical treatments (up to four applications per season), compared to controls. Based on such data, pecan producers should be wary of resorting to full pesticide cover applications, as the impact to beneficial species could lead to pest resurgence and/or the development of resistance. Rather, a Conservation Biological Control approach is strongly advised. Producers are also reminded of the popular article “Ken jou (vriendelike) vyand”, published in three parts to cover insect pathogens (SA Pecan, Lente 2015, Vol. 72, pp. 9 – 11), predators (SA Pecan, Somer 2016, Vol. 73, pp. 30 - 32) and parasitoids (SA Pecan, Winter 2016, Vol. 74, pp. 35 - 37).

2. Expanded surveys

The guide also points out that the South African pecan industry has, to date, not been challenged by the principal pecan pest species encountered in North America (region of origin for C. illinoinensis). For this reason, local surveys have been expanded to cover (from January 2018) three regions: Hartswater (Magogong area), Prieska and Modimolle. Through a pre-emptive approach, new incursions into South Africa may be noted more rapidly and control interventions initiated. Important species include the pecan weevil, Curculio caryae (Coleoptera: Curculionidae), black pecan aphid, Melanocallis caryaefoliae (Hemiptera: Aphididae), pecan twig girdler, Oncideres cingulata (Coleoptera: Cerambycidae), pecan phylloxera, Phylloxera devastatrix (Hemiptera: Phylloxeridae), pecan nut casebearer, Acrobasis nuxvorella (Lepidoptera: Pyralidae), hickory shuckworm, Cydia caryana (Lepidoptera: Tortricidae), fall webworm, Hyphantria cunea (Lepidoptera: Erebidae), pecan leaf scorch mite, Eotetranychus hicoriae (Acari: Tetranychidae), brown stink bug, Eushistus servus (Hemiptera: Pentatomidae), pecan budmoth, Gretchena boliana (Lepidoptera: Tortricidae), walnut caterpillar, Datana integerrima (Lepidoptera: Notodontidae), ambrosia beetle, Xylosandrus crassiusuculus (Coleoptera: Scolytidae), flatheaded apple tree borer, Chrysobothris femorata (Coleoptera: Buprestidae) and obscure scale, Melanaspis obscura (Hemiptera: Diaspididae). The local pecan industry should be aware of these species and any such incursions reported immediately to the Department of Agriculture, Forestry and Fisheries (DAFF), ARC or SAPPA.