PHYSIOLOGICAL AND BEHAVIORAL EFFECTS OF SECONDARY PLANT COMPOUNDS ON WESTERN CORN ROOTWORM DIABROTICA VIRGIFERA VIRGIFERA

Efat  F. ABOU‑FAKHR

The western corn rootworm (WCR), Diabrotica virgifera virgifera LeConte (Coleoptera: Chrysomelidae), is an important economic pest in the U.S. Larvae of the WCR are the most damaging stage of the insect, but the adults have an important role in locating their host plant corn, Zea mays, by migrating among corn fields after the corn silks are dry and by locating oviposition sites for the survival of the progeny. Different groups of secondary plant compounds, as benzoxazines and Elavonoids are present in different parts of a corn plant.

Corn silks were found to elicit elect rophysiolgical responses on the antennae of adult western corn rootworm, but a very specific form of corn silk, brown silk, was found to elicit the highest electroantennogram (EAG) responses. Extracts of brown silk that are EAG active were isolated by silica‑gel chromatography (with Florisil^R) and gas‑liquid chromatography. Compounds that elicited (EAG) activity were consistently present, one nonpolar compound that eluted in the hexane Florisil^R fraction, and one or more medium polarity compounds that eluted in the 25% dichloromethane:hexane Florisil^R fraction. This is the first report of EAG responses of WCR to a plant material, specifically corn silks.

Benzoxazines are secondary plant compounds known to be present in corn leaves and roots. Detection of the level of benzoxazines present in a nondiapausing strain of WCR was done by preparing trimethylsilyl derivatives of benzoxazines into more stable compounds that were analyzed by gas chromatography‑mass spectrometry. 2-Hydroxy-7-methoxy-2H-1,4-benzoxazine-3(4H)-one (HMBOA) was the most abundant benzoxazine present in all stages of the insect . The presence of a larger amount of 2,4-dihydroxy-7-methoxy-2H-1,4 -benzoxazine‑3 (4H) -one (DIMBOA) in adult males of WCR than adult females indicates the possibility of a sexual selectivity for metabolizing this compound by the WCR. The presence of MBOA in the adults 'of WCR that did not feed on any corn might Indicate that the insect has acquired the ability to sequester this compound.

6‑Methoxy‑2‑benzoxazolinone (MBOA), the decomposition product of DIMBOA, was studied for its toxicity or deleterious effect on the first and second instars of diapausing and nondiapausing strains of WCR. MBOA was found not to be toxic to the larvae of WCR, and it did not have any effect on the feeding behavior of the insect. The non‑toxic effect of MBOA is important because of the known semiochemical role of this benzoxazine to WCR.