Woolly Apple Aphid our pest of the season.
By Dr. Gulu Bekker
Eriosoma lanigerum (Hemiptera: Aphididae), better known in the South Africa apple industry as Woolly Apple Aphid (WAA), is a highly problematic apple pest. It is also commonly known as apple root aphid or “appelbloedluis”. In South Africa, WAA is rarely found on other host plants, but it has been recorded in peaches and pears. A native to eastern North America, it regularly uses elm trees as alternate hosts. However, in South Africa, the entire life cycle is completed on apples.
This interesting organism can reproduce parthenogenetically (asexually), in other words, the female eggs self-fertilise and don’t need to be fertilised by a male. This means that one individual female can start a new colony. In addition, WAA have aerial and subterranean phases, with overlapping generations. Normally a subterranean population is present throughout the year where they feed and develop on the roots of the apple tree, however in spring, a proportion of individuals (crawlers) from the root colonies start to move upward into the aerial part of the tree and settle in leaf axils and also in pruning wounds, where they form colonies and secrete a woolly white substance, hence its name. It is also at this stage where most growers note the presence of the pest.
In some cases, WAA can just be a nuisance and in other cases a serious risk to growers, as it can be detrimental to apple production or even put the tree’s survival in jeopardy. The WAA feed on the roots, causing galls to form, which can then form a crust of root material on the surface or just below the soil surface. There are two negatives to gall formation:
- it impairs water penetration to the roots, and
- below the galls, it can hinder the uptake of water and nutrients by the roots, by forming a barrier.
In the tree itself, the colonies forming in the pruning wounds and leaf axils, also forms galls (as a result of feeding) and this often destroys developing buds in the leaf axils. When infestations are high, WAA can also settle in the stem and calyx of the fruit and even crawl into the core of the apple, resulting in the downgrade of the fruit.
During this past season, the majority of apple growers in the industry, listed WAA as one of their major pest concerns, due to its wide spread incidence. Some growers, said it was of little concern or even non-existing. This raises the question as to why the majority of apple growers have problems with WAA versus those that didn’t. Many farmers commented that it was just a bad WAA year, possibly referring to the climate.
However, climate varies between the different apple growing regions of South Africa. Other factors such as soils, topography, cultivars and rootstock also vary between regions. Although climate definitely might have played a role in this seasons WAA incidence across the industry, there seems to be another golden thread or common denominator in the industry related to the WAA problem. It was noted that farms with a good mulch, cover crop and WWA resistant rootstocks, have either very little concern with WAA or don’t even see it. There are obviously always exceptions to the rule, where some growers have “susceptible rootstocks”, but with mulches and cover crops, there are no WWA concerns. These observations have led to the initiation of a research project by Hortgro and Stellenbosch University, to determine the effects of different mulches and cover crops on the suppression of WAA, as well as the role of rootstocks on the incidence of WAA .
During this past season, the majority of apple growers in the industry, listed WAA as one of their major pest concerns, due to its wide spread incidence. Some growers, said it was of little concern or even non-existing. This raises the question as to why the majority of apple growers have problems with WAA versus those that didn’t. Many farmers commented that it was just a bad WAA year, possibly referring to the climate. However, climate varies between the different apple growing regions of South Africa. Other factors such as soils, topography, cultivars and rootstock also vary between regions. Although climate definitely might have played a role in this seasons WAA incidence across the industry, there seems to be another golden thread or common denominator in the industry related to the WAA problem. It was noted that farms with a good mulch, cover crop and WWA resistant rootstocks, have either very little concern with WAA or don’t even see it. There are obviously always exceptions to the rule, where some growers have “susceptible rootstocks”, but with mulches and cover crops, there are no WWA concerns. These observations have led to the initiation of a research project by Hortgro and Stellenbosch University, to determine the effects of different mulches and cover crops on the suppression of WAA, as well as the role of rootstocks on the incidence of WAA .
Furthermore, farms following less intensive chemical programs, and moving towards a more biological crop protection approach have noticed the increased presence and workings of the most effective natural enemy of WAA, the parasitic wasp, Aphelinus mali. This wasp can be severely disrupted by the use of chemicals. In 2003, Heunis and Pringle screened 14 insecticides and 10 fungicides, against A. mali, and found that most of the chemicals were highly toxic to the adult wasps. However, the mummies were somewhat resistant to the insecticides. Aphelinus mali normally lays a single egg inside the WAA. When it’s egg hatches it feeds on the insides of the WAA, eventually killing it. The parasitoid larvae then pupates inside the body of the aphid, turning it into a blackish mummy. The exit holes from the mummy, caused by the eclosion of the wasp from the pupae, is evident of a successful completion of the life cycle of this parasitic wasp.
With the phasing out of Imidacloprid (which is widely used as a winter soil application, and should only be applied to trees younger than 15 years old), many growers are concerned about how they are going to manage WAA. Growers definitely don’t need to be concerned as there is an effective alternative. RealIPM works with farmers on developing a bespoke integrated approach for each farmer that includes good monitoring, mulches and cover crops, as well as the use of entomopathogenic fungi together with biopesticides and selective insecticides. This approach works alongside the excellent work of naturally occurring beneficial insects such as the Aphelinus mali wasp. Please visit our website to find out more about our approach and products and to contact one of our highly competent technical advisors, for a free consultation.
References
Insects of Cultivated Plants and Natural Pastures in Southern Africa. Gerhard L. Prinsloo and Vivienne M. Uys (Eds). Entomological Society of Southern Africa, PO Box 13162, Hatfield, 0028, South Africa, 2015. pp. 786. ISBN 978-0-620-60841-1.
Heunis, J.M. and Pringle, K.L., 2003. The susceptibility of Aphelinus mali (Haldeman), a parasitoid of Eriosoma lanigerum (Hausmann), to pesticides used in apple orchards in the Elgin area, Western Cape Province, South Africa. African entomology, 11(1), pp.91-95.