Effects of root nodules and taproots on survival and abundance of Sitona hispidulus (Coleoptera: Curculionidae) on Medicago sativa

ABSTRACT.

• 1 The effects of food resources on populations of larvae of Sitona hispidulus (F.), a root and nodule herbivore of Medicago sativa L., were investigated in 1983 and 1984.
• 2 Density-dependent mortality among first-instar larvae indicated that a lack of available root nodules, the primary resource for first instars, contributed to the 66.2±5.2% and 80.7±4.1% mortalities of first-instar larvae in 1983 and 1984, respectively. Initial densities of larvae entering the soil were 31.7±2.4 and 15.6±1.3 per soil sample (10.5 cm diam. ×15 cm deep) in 1983 and 1984, respectively (±SEM).
• 3 Survivorship and number of first-instar larvae per soil core sample were significantly related to number and biomass of nodules per soil sample. Percentage soil moisture was not consistently correlated with survivorship and number of larvae per soil sample.
• 4 Numbers of second- and third-instar larvae were not consistently correlated with either nodule or taproot biomass.
• 5 Numbers of fourth-instar larvae were associated with taproot biomass, suggesting that taproot surface area may also limit population levels of S.hispidulus. No correlations were found between taproot and nodule biomass on any sampling date indicating that high numbers of fourth-instar larvae associated with larger taproots were not merely due to a greater survival of first instars associated with larger taproots.
• 6 The study suggests that population levels of nodule-herbivores are controlled by the availability of root nodules. A sparse distribution of nodules results in high levels of mortality among nodule-herbivores of Medicago sativa.