Evaluating Effects of No-Till Cover Cropping Systems on Indigenous Entomopathogenic Nematodes and Fungi

Abstract

Entomopathogens, entomopathogenic nematodes (EPN) and entomopathogenic fungi (EPF), are promising biocontrol agents for insect pest management. Use of entomopathens in Hawaii is challenged by quarantine restrictions and the failure of introduced entomopathens to persist in the field. This research focused on enhancing indigenous EPN (unknown Heterorhabditis sp.) populations in the field through conservation agriculture. We hypothesized that cover cropping with black oats (Avena strigose) or oil radish (Raphanus sativus ssp. oleiferus) followed by a no-till practice in an Oxisol soil would provide a favorable environment for EPN via provision of an organic mulch, reduced soil disturbance, improved water conservation, attracting alternative insect host and possibly production of herbivore-induced volatiles (HIPVs) from cover crops. Two field experiments, Oil Radish (OR) Experiment and Black Oat (BO) Experiment, both repeated once, were conducted to compare pre-plant treatments of 1) black oat (BO) or oil radish (OR) as cover crops in no-till plots, 2) bare ground (BG) followed by conventional tillage, and 3) conventional tillage followed by soil solarization (SOL) on abundance and infectivity of EPNs using mealworm larva (Tenebrio molitor) as a bait. Data were taken every other week throughout the 3 months of corn (Zea mays) growth following the preplant treatments. Indigenous EPNs recovered were identified by sequencing the ITS region and matched undescribed species of Heterorhabitis in NCBI, labeled as H1, SGgj, and SGmg3. Soil in BO and OR had higher volumetric soil moisture, field capacity, and soil organic matter than BG and SOL (P ≤ 0.05) in all four trials. In addition, the nematode soil food web at termination of the cover crop, and monthly during corn growth had greater % fungivore (F), fungivore to bacterivore (B) plus F ratio (F/F+B), and channel index (P ≤ 0.05) than BG and SOL in Trial I, and higher % omnivores and structure index (P ≤ 0.05) than BG and SOL in Trial II throughout the corn growing season suggested that no-till with black oat residues was in favor for fungal dominated decomposition path ways in Trial I but lead to more structured soil food web in Trial II. None-the-less, EPN infectivity was greater (P ≤ 0.05) in BO than in BG and SOL based on field cage assays in both trials. BO may have provided a habitat that enhances EPN infection. Both multivariate canonical analysis conducted for data from Trial I and Trial II suggested that higher EPN infectivity was associated with soil physical properties related to water conservation and improved soil health conditions (higher EI and SI). In the Oil Radish Experiment, OR increased abundance of Heterorhabditis sp. infective juveniles (IJs) as the age of OR increased suggested that OR provided a favorable environment for EPN reproduction. Oil radish growth consistently resulted in higher EPN abundance in OR than in BG and SOL (P ≤ 0.05) at cover crop termination. However, no-till cover cropping with OR reduce EPNs abundance over time. This could be due to fast decomposition of OR residue that resulted in lack of soil coverage and unfavorable edaphic factors for EPNs. In addition, reduced abundance of herbivorous nematodes and higher occurrence of Metarhizium anisopliae from the larva bait assay in OR plots also suggested that allelopathic compounds released from oil radish (i.e. isothiocyanates) could have biofumigant effects against EPN infectivities. In conclusion, this thesis project suggested that BO cover crop in a no-till system provided favorable edaphic factors for EPN infectivity despite lack of ability to enhance IJs abundance of theses nematodes. On the other hand, OR cover cropping enhanced abundance of EPNs, but did not improve EPNs infectivity. Occurrence of M. anisopliae on larva bait assay suggested no obvious effect of no-till cover cropping with black oats, and a negative effect of oil radish cover cropping on infectivity of EPN. Future research is needed to understand if herbivore induced plant volatiles (HIPVs) are involved in increasing ability of OR to attract EPNs, and if a mixture of OR and BO cover cropping can improve EPN activities in agroecosystems.