Investigating the identity, biology, economic damage and distribution of olive seed wasps in cultivated olives in the Western Cape, South Africa
Objectives & Rationale
The objectives are to (1) confirm taxonomic identity of seed wasp/s infesting commercial olives in the Western Cape, (2) study the life cycle and seasonal occurrence of olive seed wasps (OSW), (3) determine OSW economic damage potential, (4) determine cultivar susceptibility to OSW, and (5) to determine the geographical distribution and abundance of OSW in all olive growing regions. This information is required to develop monitoring and control strategies.
Methods
Total DNA was extracted from six reference specimens each of Eupelmus spermophilus and Eurytoma oleae to obtain reference sequences and develop a DNA-based method for identification of these OSW species. OSW seasonal activity was monitored with yellow sticky traps. Fallen olives, collected from trial sites at Stellenbosch and Agter-Paarl, were examined for OSW infestation. Total yield for each data tree was recorded at harvest and yield loss due to OSW infestation calculated. The geographic survey was expanded to include areas like Citrusdal, Tulbagh, Riversdale and the Little Karoo. Cultivated and wild olives (where available) were collected to determine how widespread OSW infestation is.
Key Results
Reference sequences were obtained for Eupelmus spermophilus and Eurytoma oleae. Sequence alignment and preliminary phylogenetic reconstruction showed that the two species are sufficiently diverged at the COI sequence level to allow for confident DNA-based identification. Seasonal occurrence followed a similar pattern to the previous season and adult OSW were active in orchards during the time that cultivated olives were susceptible to attack. Although OSW numbers on traps were generally lower in 2016/17 than during 2015/16, the percentage yield loss due to OSW infestation was significantly higher: 14% vs 0% at Stellenbosch and 12% vs 1.79% at Paarl for the 2016/17 and 2015/16 seasons, respectively. The geographic survey showed that OSW was present in most areas sampled. Some areas where no OSW was found in 2016, showed significant levels of infestation during 2017, while other orchards where significant levels of OSW infestation was observed in 2016, showed no infestation at all in 2017.
Conclusion/Discussion
A DNA-based identification method was developed for Eupelmus spermophilus and Eurytoma oleae and research now focuses on developing a method that does not require sequencing, which will allow for more rapid and cost-effective identification. Sticky traps at wild olive trees and in orchards are useful to indicate when OSW females are active, particularly at the stage when cultivated olives are susceptible to attack, but OSW numbers on traps do not reflect levels of olive infestation accurately. Variation in the numbers of OSW on sticky traps, yield of wild and cultivated olives and levels of OSW infestation over the past two seasons underscore the importance of repeating these observations for another season before drawing conclusions regarding the impact of OSW on cultivated olives. Some farms included in the geographic survey in both 2015/16 and 2016/17 showed marked differences in OSW infestation levels between the two seasons. For this reason, the survey in February/March 2018 should include farms surveyed for the first time in 2017, particularly those where no OSW was found, before definite conclusions regarding the areas where OSW occurs, can be drawn.