INTERACTIVE EFFECTS OF EUROPEAN CORN-BORER (LEPIDOPTERA, PYRALIDAE) TUNNELING AND DROUGHT STRESS ON FIELD CORN WATER RELATIONS

The effects of drought stress and stem tunneling by European corn borer, Ostrinia nubilalis (Hubner), on water relations of field corn, Zea mays L., were evaluated in 1987 and 1988. Using a pressure chamber, leaf and stem water potentials averaged 3.8 and 3.5 bars, respectively, higher in plants from well-watered soils compared with drought-stressed plants. Larval tunneling had no effect on the leaf or stem water potential. A vacuum infiltration technique was developed to examine the effects of these factors on stem hydraulic conductance and provided hydraulic conductance measurements comparable with that predicted with the Hagen-Poiseuille equation. O. nubilalis tunneling reduced the internode hydraulic conductance by up to 63.5% compared with undamaged stems, and slight reductions were also seen with conductance through intact sterns (internode with attached node). The node appeared to mitigate the effects of tunneling on hydraulic conductance in intact stem sections. Soil moisture status had no effects on hydraulic conductance as measured with the vacuum infiltration technique. O. nubilalis tunneling, from a single larva, rendered an average of 36.6% of the medullary xylem vessels nonfunctional and also slightly reduced the functioning of the peripheral xylem vessels. Finally, O. nubilalis tunnel dimensions averaged 783.1 mm3 in volume, and 46.7 mm in length, and 9.2% of the stern area was tunneled. Tunnel dimensions were generally similar in stem sections from plants from fully irrigated and drought-stressed soils. Tunnel measurements related to cavity volume and cross-sectional area destroyed were more closely correlated with hydraulic conductance than were tunnel length or shape. The majority of damage to the plants' vascular system, as reflected by hydraulic conductance, stained vessels, and cavity dimensions, occurred during the first 6 d of tunneling.