When bad things happen, sometimes the consequences are long-lasting. Guam’s recent invasion by the insect pest called Aulacaspis yasumatsui would make it on anyone’s list as a bad thing. The invasion and its potential devastating consequences were predicted in 2000 by researcher Thomas Marler. So when the 2003 invasion occurred, it should have come as no surprise.
Guam’s invasion marked the first time this scale insect invaded a new geographic region that had a native Cycas species. At the time of the invasion, Cycas micronesica was the most abundant tree on Guam. The A. yasumatsui insects, which require Cycas trees for food, must have felt like kids in a free candy store, with a smorgasbord at their disposal.
The consequences were acute at first, then relentless as time passed. Plants started dying within months, beginning with the seedlings then progressing to the juvenile plants and finally to the mature trees. The insect jumped to Rota in 2007 and immediately presented scientists with a déjà vu situation in Rota’s Cycas micronesica population.
The immediate consequences on Cycas micronesica plant health and mortality were to be expected. But as the invasion began revealing all of its nuances Marler noticed that forest sites where high density plants had been defoliated or killed were not behaving as a normal forest gap. “Typically the existing seed bank and suite of small seedlings capitalize on a new forest canopy gap with rapid growth to fill the new opening,” said Marler. “The gaps created by scale-induced leaf and tree death were remaining barren.”
Marler employed the established protocol of using activated carbon to adsorb residual biologically derived compounds that were suspected of lingering in the soils. The soil treatment stimulated plant growth as predicted, and the results pointed to a phenomenon that ecologists call a legacy effect. In this case, organic compounds in the dead Cycas micronesica tissue that was killed by Aulacaspis yasumatsui were leaving behind a legacy that hindered seed germination and seedling growth of the other forest plant species.
The study revealed an example of how cascading effects of an invasive insect pest can negatively affect species other than the insect’s host plant. This case study is an ideal example of how an insect invasion event can lead to unexpected ripple effects that damage the ecosystem in ways that could not be predicted.
Marler, T.E. and N. Dongol. 2013. Do phytotoxic compounds in soils after scale-infested Cycas micronesica litter deposits explain reduced plant growth? HortScience 48:1571-1573.
Watson GW, Marler TE. 2014. Does cycad aulacaspis scale play a direct role in causing soil phytotoxicity? Communicative & Integrative Biology 7:e278811-278813.
Funded by US Forest Service