The performance of Melolontha grubs on the roots of various plant species

Melolontha grubs are polyphagous and are adapted to feeding on plants of varying nutritional value. Our research sought to investigate whether host plant quality affects first-instar grub development, weight gain, or mortality. Ten plant taxa of the families Polygonaceae, Brassicaceae, Asteraceae, Fabaceae, and Pinaceae were tested. The quality of each plant species was estimated based on the root content of phenols, condensed tannins, soluble sugars, starch, nitrogen, and carbon. Retarded development, high mortality, and low weight gain were observed in grubs feeding on roots of Fagopyrum esculentum, Brassica rapa subsp. rapifera, and B. napus, whereas Tanacetum vulgare, Trifolium repens, and Lupinus polyphyllus roots proved to be the most beneficial for larval performance. Weight gain was positively correlated with the concentration of soluble sugars and starch in the plant roots. Starch was also positively correlated with the percentage of molted grubs. The revealed positive correlations may be explained by the fact that nonstructural sugars constitute an energy source that is essential for grub movement in the soil. Plant species that negatively affect cockchafer grubs may be used in integrated plant protection against these pests in agriculture, horticulture, and, to some extent, in forestry, e.g., in nurseries and postagricultural lands.