Cycas micronesica is a tropical cycad species that is endemic among several western Pacific islands (Hill, 1994). Paleoenvironmental investigations document its prevalence in Guam for more than 9000 years (Athens and Ward, 2004). This tree has enjoyed a dominant position in horticulture and agroforestry settings of the region for centuries (Barratt, 2003; Edwards, 1918; Safford, 1905).
Dihammus marianarum (Cerambycidae) is a native stem borer (Marler and Muniappan, 2006). Larvae of this beetle bore tunnels in the cortex of Cycas micronesica stems (Fig. 1A). In the absence of a stressor that initially decreases plant health, the beetle does not attack the trees. When tree health declines, subsequent signs of borer attack are unambiguous with mucilage and frass building up on stem surfaces (Fig. 1B) and around the base of the stem on the soil surface. No other local plant species are known hosts for this stem borer.
Several 21st century insect invasions have threatened Guam’s Cycas micronesica and removed the horticultural appeal of the cycad species. The cycad leaf miner Erechthias sp. (Tineidae) was first reported in 2003 when it was restricted to the southern habitats of the island (Marler and Muniappan, 2006). Caterpillars of this microlepidopteran insect tunnel in leaflets exclusively on hardened, mature Cycas leaves (Fig. 2A–B). The armored scale Aulacaspis yasumatsui (Diaspididae) invaded Guam in 2003 (Marler, 2012; Marler and Muniappan, 2006), and its predator Rhyzobius lophanthae Blaisdell was purposefully introduced in 2005 (Moore et al., 2005). Damage to host plants occurs during a lengthy sessile stage during which waxy covering protects the insects. The butterfly Chilades pandava (Lycaenidae) was found in northern Guam in 2005 (Moore et al., 2005). The caterpillar stage of this butterfly requires soft, expanding Cycas tissue as food. The three herbivore insects established readily and then spread throughout the island, and the predator was transported by local biologists to new localities of armored scale outbreaks until it had established throughout the island. Additionally, exotic termites that attack Cycas micronesica stem tissue are located in some northern habitats (Marler et al., 2011).
The threats to the plant inflicted by these insect pests have been augmented by feral ungulate damage, and this nexus of threats has led to greater than 90% plant mortality in less than one decade (Marler and Lawrence, 2012). The previously healthy Cycas micronesica populations and their sustainable relationship with the native stem borer have been acutely disrupted by the recent pest invasions. Infestation levels of the armored scale and damage by the leaf miner, the butterfly, and the stem borer on Cycas micronesica trees were recorded from 2004 until 2013 to determine the patterns of infestation among the pests. Results may be useful for informing horticultural or conservation management decisions.
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