Can the botanical azadirachtin replace phased-out soil insecticides in suppressing the soil insect pest Diabrotica virgifera virgifera ?

<strong>Can the botanical <em>azadirachtin</em> replace phased-out soil insecticides in suppressing the soil insect pest <em>Diabrotica virgifera virgifera </em>?</strong> <strong>Background</strong> Due to recent bans on the use of several soil insecticides and insecticidal seed coatings, soil-dwelling insect pests are increasingly difficult to manage. One example is the western corn rootworm (<em>Diabrotica virgifera virgifera</em>, Coleoptera: Chrysomelidae), a serious root-feeder of maize (<em>Zea mays</em>). We investigated whether the less problematic botanical <em>azadirachtin</em>, widely used against above-ground insects, could become an option for the control of this soil insect pest. <strong>Methods</strong> Artificial diet-based bioassays were implemented under standard laboratory conditions to establish lethal dose curves for the pest larvae. Then, potted-plant experiments were implemented in greenhouse to assess feasibility and efficacy of a novel granular formulation of <em>azadirachtin </em>under more natural conditions and in relation to standard insecticides. <strong>Results</strong> Bioassays in three repetitions revealed a 3-day LD₅₀ of 22.3 µg <em>azadirachtin</em> per ml which corresponded to 0.45 µg per neonate of <em>D. v. virgifera </em>and a 5-day LD₅₀ of 19.3 µg per ml or 0.39 µg per first to second instar larva. No sublethal effects were observed. The three greenhouse experiments revealed that the currently proposed standard dose of a granular formulation of 38 g<em> azadirachtin </em>per hectare for in-furrow application at sowing is not enough to control <em>D. v. virgifera </em>or to prevent root damage. At 10x standard-dose total pest control was achieved as well as the prevention of most root damage. This was better than the efficacy achieved by <em>cypermethrin</em>-based granules and comparable to <em>tefluthrin</em>- granules, or <em>thiamethoxam</em> seed coatings. The ED₅₀ for suppressing larval populations were estimated at 92 g <em>azadirachtin</em> per ha, for preventing heavy root damage 52 g /ha and for preventing general root damage 220 g /ha. <strong>Conclusions</strong> There seems clear potential for the development of neem-based botanical soil insecticides for arable crops such as maize. They might become, if doses are increased and more soil insecticides phased out, a promising, safer solution as part of the integrated pest management toolkit against soil insects.