Applying voltage and leakage current in green wood

A recent KiCad project of mine is intended to kill garden slugs. The idea is to apply pulses (about 18V) lasting 1-2 seconds at about a 1/8 duty cycle. It is applied to a pair of AWG20 wires spaced about 1 cm apart, like small railroad tracks. These are laid down across the top of the garden soil.

I have a bunch of straight apple twigs so I cut 10 cm long pieces of that and drilled a pair of perpendicular holes through each (think of railroad ties) and ran the wire through those holes. The idea of the extra length of the twigs is to encourage everything to lay flat on the ground.

First application of the whole thing in our cucumber garden is today. But even though I can measure > 1 Meg ohm between the pair of wires, I get high current (> 1 Amp). This immediately trips the overcurrent protection which I incorporated in the pulser electronics. That overcurrent threshold is 1 Vbe across two 8.2 ohm resistors in parallel, so roughly 150 mA.

I am not surprised to see some leakage current and that would not be a serious problem. But I am surprised to see so much leakage current in spite of high resistance according to a digital multimeter. I guess that the I-V curve between the wires passing through some of the twigs appears to be very non-linear. Has anyone ever tried to do something crazy like this? Using natural, not kiln dried (from trees in your yard) wood as a crude electrical insulator. I guess I need to be more careful with the material of my “railroad ties.”