Olive seed wasps in cultivated olives – Possible Threat

Previous research on olive fruit fly showed that many olives thought to be infested with fruit fly were actually infested with indigenous olive seed wasps. At least five seed wasp species have been identified in wild olives (Olea europaea L. subsp. cuspidata) in the Western Cape. Consequently, a team of researchers from ARC Infruitec-Nietvoorbij, the Department of Genetics at Stellenbosch University, and Palermo University in Italy con-ducted research to determine which of these species attack cultivated olives, and whether any of them pose an economic threat to the olive industry. The research was funded by SA Olive, the NRF Research and Technology Fund and the Agricultural Research Council.

Here are the research outcomes:

Identification of olive seed wasps in cultivated olives

Adult wasps can be identified by morphological features. A molecular method for rapid identification of Eupelmus spermophilus and four other potential seed wasp species was developed by the project team. This is particularly useful for identification of larvae and pupae that cannot be identified using morphological features. Observations over three years confirmed that of the five species E. spermophilus (Fig. 1) is the primary seed wasp attacking cultivated olives in the Western Cape.

Figure 1. Olive seed wasp (Eupelmus spermophilus) female (A) and male (B)

Seasonal occurrence and distribution

Yellow sticky traps are useful to indicate when olive seed wasp (OSW) males and females are active in wild olives and in orchards, but they are not reliable indicators of OSW infestation levels in cultivated olives. Sticky trap results showed that OSW adults are active in orchards during spring and summer when developing olives are available, and in February/March when adults emerge from infested olives, but not during winter. Field observations confirmed that OSW does not overwinter in cultivated olives. Adult OSW are active in wild olives all year round. Unlike cultivated varieties, wild olives have more than one flowering cycle during the year, largely depending on the availability of water. As a result, wild olives at different phenological stages are present all year round. This ensures the regular availability of young olives with soft kernels (seeds) in which OSW eggs can be laid. Olive seed wasp overwinters and re-produces in wild olives during times of the year when there are no cultivated olives available.

All the areas in the Western Cape where olives are cultivated in the vicinity of wild olives, had OSW present. They primarily attack cultivars with smaller fruit, mostly the oil cultivars. They do, however, occasionally attack larger table olive cultivars (such as Manzanilla), if the fruits of these cultivars are still small enough when female olive seed wasps are ready to lay eggs.

Economic damage

The OSW female lays an egg in the soft kernel of the young olive fruit, before the pit hardens and the fruit pulp is too thick (>1.5 mm) for her ovipositor to reach the kernel. The larva and pupa develop inside the kernel (Fig. 2A) until the adult seed wasp emerges (Fig. 2B), leaving a small exit hole with a clear edge that becomes corky after some days (Fig. 3).
Many olives infested by OSW fall from the tree before ripening, resulting in direct yield loss. Some infested olives, with or with-out exit holes, remain on the trees to be harvested and processed. To date, there is no evidence that these infested olives have a negative impact on oil quality, but olives with emergence holes are unsuitable for processing as table olives.
Yield of the sample trees and yield loss due to dropped fruit infested with OSW varied greatly between the two field trial sites and seasons (Table 1). Yield loss due to OSW was highest at both sites during 2017, but negligible during 2016 and 2018. Olive seed wasp infestation in dropped olives and yield loss did not correlate consistently with weather data.

Figure 2. Olive seed wasp pupa and black meconium in pit (A) and adult emerging from the seed (B)

Conclusion

Olive seed wasp infestation only poses a risk to cultivated olives grown in the vicinity of wild olives, which act as overwintering sites and sources of infestation. While OSW does not pose a significant threat to the industry as a whole, it can have a significant impact on yield where cultivated olives grow in close proximity to wild olives. The sporadic nature of economically significant OSW infestation complicates decision-making regarding control, since it was found that sticky traps and weather data are not reliable indicators of potential infestation levels. The presence of abundant wild olives near olive orchards during the preceding summer and autumn can result in large numbers of OSW emerging early in the following growing season when cultivated olives are susceptible to OSW oviposition. This can result in economically significant OSW infestation and yield loss. Currently, no chemical control is registered for OSW in cultivated olives.

For further information, contact Dr Elleunorah Allsopp at allsoppe@arc.agric.za or
021 809 3007.