Quick Dips

Bug Gut Chemistry

It's a good thing those annoying insects buzzing around the countryside are so small -- they've got cast-iron guts full of chemicals as strong as oven cleaner to help them digest their food! HortResearch scientists are studying how pest digestion works, as bio-insecticides are almost all absorbed in this way, and beneficial insects such as bees need to be able to stomach the insecticides designed to kill their troublesome counterparts.

Dr Heather Gatehouse says that digestion is largely carried out by enzymes which are sensitive to pH levels, that is acidity or alkalinity. The ability to measure, quickly and accurately, the pH in the gut of an insect is fundamental to the understanding of insect digestion. Gut pH varies in insects from more acidic than vinegar through to nearly as causticly alkaline as oven cleaner, says Gatehouse.

With large insects, the dissected gut can be sectioned, the gut contents suspended in water and the pH read with a commercial pH electrode, but most insects are much too small for this. The technique also kills the insect, so scientists cannot then follow changes in pH after treatments such as starvation, or feeding of specific compounds to the insect.

In collaboration with Massey University's Professor Paul Callaghan, HortResearch's Insect Science Group has been able to follow changes in gut pH in living insects using magnetic resonance microscopy, a similar technique to that used to image patients in hospital. Using this technique, they have found that armyworm larvae gut pH vary in different parts of the gut and change when feeding on insect-specific toxins.

Changes in pH in the gut of the one centimeter-long brown house moth larvae have also been recorded, distinguishing four anatomical regions with different pH. Larvae were fed with a range of pH indicator dyes and colour changes in the gut were noted. The group has just completed a study of the midgut pH of two species of bee.

In future they hope to develop methods to follow pH changes in the gut of even the smallest aphids and mites. One possible technology uses a laser to detect and quantify the presence of pH-sensitive fluorescent compounds below the surface of the sample under the microscope.