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Leaf surface compounds and alighting and egg laying behaviour of WCR (Western Corn Rootworm)

WCR (Western Corn Rootworm) egg laying is the behavioural step which determines the site where the larvae will feed the next year. We tested the hypothesis that primary metabolites present on the maize surface, particularly the elder growth stages, could influence host acceptance and stimulate egg laying. In a first step we localized the parts of the plants on which information could be taken. For that we observed the insect abundance in a Hungarian maize field, at three periods of the day and at two months. The different plant parts were chemically analysed. Then the organ on which the insect is the majority of the time were sprayed with water to collect metabolites from its surface and the washing were then tested on Hungarian WCR egg laying in laboratory. Pre-egg laying and egg laying behaviour were observed to define which insect body parts are used for detection and the eventual influence of washings on behaviour. We also looked for gustatory like sensilla on body parts, which are used in the perception of gustatory cues.

All the free amino acids, soluble carbohydrates, and sugar alcohols are present on the two different corn hybrid surfaces tested. During the reproductive stages the metabolite quantities and ratios found on the plant surface may be due to metabolites coming from the plant tissues and also to metabolites issued from pollen metabolites and microorganisms metabolism linked to the presence of these substances. Metabolite quantities and proportions discriminate plant growth stages, organs and growth year. In general the total metabolite quantities of each chemical group increase on the leaf surfaces with the growth stage, except for soluble carbohydrates.

Among the sugars, sucrose dominates on leaves; especially in the elder stages, glucose proportions progressively increase whereas sucrose ones diminishes. Sugar alcohols vary progressively from young to older stages, quebraquitol diminishes and myo-inositol increases. Among free amino acids proline and glutamine increase particularly within the growth stages.

At the reproductive stages, the composition of leaf surfaces is partly influenced by the presence of pollen. The comparison between leaf surfaces washed and not washed before the collect by spraying show that generally the plants not washed have more free amino acids (all of them) and particularly GABA, proline, alanine, serine, glutamine, more sugars except sucrose and more sugar alcohols particularly myo-inositol.

When looking at the washing composition in metabolites of the different stages the best correlation between single metabolite of the washings and egg laying was obtained with glutamine (R² = 0.87). When testing an artificial blend composed of 28 metabolites present in the washings of young plants the same activity as the leaf surface washing (deterrence and inhibition) was observed. PCA analyse based on the compositions of metabolites on the maize surfaces on both years discriminates the elder stages which are stimulant from the inhibitors younger ones. Later growth stages were very near to soybean, which had both a neutral effect on WCR egg laying.

The maize leaf washings had an effect on WCR egg laying, varying with the maize growth stage and were similar on the hybrids tested. The activity of artificial metabolite blend revealed that there was a high probability that the activity of leaf surface washings is due to a blend of primary metabolites.

Maize leaf surface water washings definitely stimulate WCR egg laying. Their activities vary with the maize growth stages. From VT to R3 stages there is a bigger egg laying stimulation than on V8. The difference is all the more big as V8 has an inhibitor effect (less acceptance and less eggs per females). The V8 inhibitor effect could be due to very low quantities of metabolites under the threshold that is needed to induce acceptance of substrate for egg laying.

It is too early to define which metabolites are concerned, however there is a blend of metabolites, which is active. Few correlations with individual substances and egg laying were shown. The synergism and antagonism between substances in their activity needs more years to be demonstrated. Proline, glutamine and sugars are probably much concerned in the blend stimulant effect.

The observations on insect behaviour corroborate the fact that the insect takes information on the plant by scanning the surface with its ovipositor and walking a lot on plant. The egg laying site and site where the stimulation is perceived for egg laying are different and several hours may be observed between scanning and egg laying. Deposit of eggs in the soil and at the bottom of boxes is linked to tigmotactism and negative geotropism. It is independent from the chemical cues present on the substrate. Gustatory sensilla present on ovipositor and legs confirm the possibility for WCR of taking information by contact with the plant surface.

Reported by

Institut National de la Recherche Agronomique
Route de Saint-Cyr RD 10
78026 Versailles
France
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