February 2016. After a discussion on the potential for biocontrol of the vespid wasps in New Zealand, a member wrote about one particular wasp that has caused much damage to parts of the Pacific – the Erythrina gall wasp. He wrote:
Probably the worst wasp, both culturally and ecologically, has been the Erythrina gall wasp which showed up around tiin the late 1990s or early 2000s. Over the past 15 years has destroyed the culturally important Erythrina variegata var orientalis, which was central to the Samoan shifting agricultural system, about the most common living fence or boundary planting in Tonga, Samoa, New Caledonia, Vanuatu, Rotuma, and Fiji and was central to the traditional yearly calendar which was linked to the yam planting season. It was also a beautiful tree. It has virtually disappeared in Fiji and in parts of Tonga and Samoa and was represented by endemic species in Hawaii. The introduction of a parasitic wasp from East Africa seemed to offer some hope in Hawaii.
A member from Hawaii replied that biocontrol using Eurytoma erythrinae has done well at least in the context of the endemic Erythrina. However, there are still parts of the plant, notably the flowers and seedpods, that can get hit quite heavily. Another species is being considered for release which should reduce populations of the Erythrina gall wasp even further and increase flower seed and yield (of course we still have to contend with the bruchid seed borer!)
In New Zealand, one species, Erythrinax sykesii : Coral Tree – Flame Tree – is an invasive and unwanted species, in particular in the Northern parts of the North Island. (http://whangareiflora.weebly.com/invasive-plants.ht).
Erythrina crist-galli: Common name, Coral Tree. Deciduous tree E. crista-galli which is a large tree with bright red flowers. It was planted freely by early settlers and is one of the widely grown species, and has become an invasive plant.
It appears that New Zealand has species different from the E variegate var. orientalis but sometimes biocontrol agents are not too species specific.
The problem of how to manage the gall wasp was raised. The question of why more was not done to control the wasp was asked. One factor was the rapid spread of the wasp in Pacific island countries, and the other was that many countries viewed Erythrina as an unwanted species as it was host to the caterpillar of the fruit piercing moth. There are many who want to get rid of the trees in places where fruit piercing moth is a problem, but there are also many people who are more concerned with the loss of the Erythrina trees, and suggest that there are other ways of controlling the fruit piercing moth.
The situation in french Polynesia was presented in full (see below). The EGW was detected in Tahiti by the Dept of Ag in 2010 (http://www.li-an.fr/jyves/Talk_Meyer_Quadrastichus_erythrinae_2010.pdf). The EGW killed many Erythrina variegata and attacked the rare endemic and legally protected Erythrina tahitensis, especially those at low elevation. An assessment of the impacts of EGW was conducted by a student (and Dr J-Y Meyer) in 2013 on wild populations of E. tahitensis located at mid-elevation, and by a graduate student from the University of French Polynesia in 2013-2014 on cultivated E. tahitensis plants at low elevation (with our Dept of Agriculture and Dept of Environment). The parasitoid was not introduced for administrative and financial reasons, so only chemical treatments (using imidacloprid by injection or spraying) was done on cultivated plants; there was relatively good success. However, in 2015, there were no further signs of EGW attacks on wild populations of E. tahitensis, and the same was true for some cultivated plants at low elevation; there was no clear explanation for this. There may have been a decrease in the EGW because of a longer dry season, or even an accidental introduction of the parasitoïd which was not recorded in 2014. Monitoring of E. tahitensis in cultivation and in the wild is on-going.
On Guam, the introduction of the gall wasp and the loss of Erythrina is considered fortuitous. A fact sheet on the fruit piercing moth was provided (see below). Today, the problem of the fruit piercing moth has almost disappeared, whereas it caused a major problem before the introduction of the wasp.
Tonga does not have endemic Erythrina species, and the tree is not of economic value, so it is waiting to see the results of the biological control in Hawaii before making a decision to bring in the parasitoid of the wasp.