March 2007. Rats are a problem on the weather coast (south) of Guadalcanal in many villages. Farmers are complaining of the damage to their sweet potato gardens and are asking for help to control the rats. Entire gardens are reported to have been destroyed and people are facing food shortage.
One member observed, the damage in the photo (not shown) looks very similar to that observed in sweet potato during the drought associated with the 1981-82 El Nino at Karimui (1100 m altitude) in Simbu Province, Papua New Guinea. Villagers said it was caused by rats (and/or bandicoots, but there are no bandicoots in Solomon Islands, of course). This raises two questions: is the damage seen on the weather coast a common, usual, problem, or is it exceptional this year? Have climatic conditions been unusual?
Rat damage has been described before in Solomon Island. See e.g. Golllifer, DE (nd) Rat damage to sweet potatoes in the British Solomon islands. Unpublished report, 11 pages. Listed in: Reilly, P (1985) Solomon islands bibliography of agriculture and forestry. Surrey, Land Resources Development Centre, v + 331 p. This describes use of a wire fence (for trial purposes), baits and costs. Rats preferred two of the 25 sweet potato cultivars used in trials.
In the Solomon Islands’ trials, rat damage on sweet potato roots was correlated with yield. Rats (R rattus and R exulans) were controlled with a cereal-based bait containing warfarin at a concentration of 0.04 % w/w applied at 12.8 kg/ha in cut bamboo sections at 90 and 105 days after planting. Roots were harvested at 120 days. From the exclusion trials the damage was assessed at c.25% of root yield. There was a negative correlation between tuber yields and rainfall at Dala (1969-74).
Jim Wilson (Pest Control Officer) who was stationed at Noumea with SPC during the 1970’s may have left some publications on rat damage to sweet potatoes at the SPC headquarters.
It is possible to make an effective, but slow-working, rat poison from leaves of Gliricidia sepium, The poison is toxic to all warm-blooded animals, including young children and pet chickens and dogs, so it must be used with care. Also, it must be prepared daily and be consumed every day for several days to perhaps 2 weeks to be effective. There are several sources of information for preparation of the poison. In brief, when the young leaves of Gliricidia are pounded and mixed with rice or other bait, certain chemicals in the young leaves are converted by bacteria decomposing the leaves into chemicals similar to the one in most commercial rat poison (brodifacoum). These naturally occurring chemicals are less toxic than brodifacoum, so larger amounts must be consumed.
In Sri Lanka a trap is used: It’s a long box made of wire mesh with a door and an auto shutting mechanism which works when the rat gets inside to feed on a bait (e.g. a piece of half burnt coconut kernel).