Chemical Ecology and Management of Yellowmargined Leafbeetle Microtheca ochroloma Stal (Coleoptera: Chrysomelidae)
Type of Degreedissertation
Entomology and Plant Pathology
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Cruciferous vegetable production is an important industry in Alabama and other parts of the southern United States (U.S.). Many farmers in the region grow various kinds of cruciferous crops (e.g., turnip, radish, mustard, napa cabbage, cabbage, collards, arugula, and Japanese leafy vegetables, such as mizuna and mibuna) as mixed cropping systems in the spring and fall using organically acceptable practices. The yellowmargined leaf beetle, Microtheca ochroloma Stål (Coleoptera: Chrysomelidae) is the most damaging pest of organic cruciferous crop production in the region. The goals of this project are to investigate the ecology of M. ochroloma and develop alternative and organically acceptable management practices, in particular biorational insecticides and attractant-based strategies for managing M. ochroloma in the southern U.S. In chapter II, I investigated the mechanism of host plant selection and preference in M. ochroloma. The host plants investigated were napa cabbage (Brassica rapa subsp. pekinensis cultivar Minuet F1), collards (Brassica oleracae var. acephala cultivar champion), cabbage (Brassica oleracea var. capitata cultivar Farao F1), and turnip (Brassica rapa var. rapa cultivar purple top white globe). The results showed that turnip and napa cabbage are highly preferred by M. ochroloma while cabbage and collards are less preferred host plants. In chapter III, I examined the headspace volatile profiles of four host plants that were tested in host preference study in chapter II using in situ dynamic headspace collection and analytical techniques. The headspace volatile profiles of the two highly preferred host plants(turnip and napa cabbage) were different from that of the less preferred host plants (cabbage and collards), suggesting that host preference is likely mediated by differences in volatile profiles of the host plants. Further analysis by GC-EAD showed that one peak, which was unique to the highly preferred host plants, elicited significant GC-EAD activity in female beetles. This compound (a novel isothiocyanate) was later identified by GC-MS as a putative host plant attractantfor M. ochroloma. In chapter IV, I carried out laboratory experiments to evaluate the susceptibility of larvae and adults of M. ochroloma to some botanical and microbial insecticide formulations using leaf-dip bioassays. Insecticides evaluated included OMRI (Organic Material Review Institute) approved formulations such as PyGanic® (pyrethrum), Entrust® (spinosad), Mycotrol O® (Beauveria bassiana strain GHA), and NOFLY® (Paecilomyces fumosoroseus strain FE 9901). Others were MBI-203 (an experimental organic formulation of Chromobacterium subtsugae) and BotaniGard® 22WP (a conventional formulation of Beauveria bassiana strain GHA). The insecticides were first evaluated at the field recommended rate against M. ochroloma larvae and adults, followed by multiple-concentration assays to determine the LC50 (median lethal concentrations) and LT50 (lethal time to kill 50% of test insects) for promising formulations. At the field recommended rate, all tested formulations were toxic to the larvae compared to the untreated control, whereas only Entrust® and PyGanic® were effective against the adults. These two most effective formulations caused 100% mortality to the larvae and adults just after 24 h of exposure. The LC50 values of Entrust® and PyGanic® were 200 ×and 15 × less than the actual field recommended rate, respectively. MBI-203 was effective against the larvae (100% mortality after 5 days) but not the adults. All three entomopathogenic fungal formulations, Mycotrol®, NOFLY®, and BotaniGard®, caused significantly higher larval mortality than the untreated control after 5 days of exposure, butnone was effective against the adults. In chapter V, I conducted field experiments over four growing seasons (2007-2010) in Alabama to evaluate some botanical and microbial insecticides evaluated in Chapter IV against M. ochroloma in organically grown crucifer crops. Insecticides evaluated included OMRI approved formulation such as PyGanic®, Aza-Direct®, Entrust®, Mycotrol O®, and NOFLY®. Others were Novodor® (Bacillus thuringiensis subspecies tenebrionis) and two experimental organic formulations, Tick-Ex (Metarhizium anisopliae strain F52) and MBI-203. The insecticides were applied as stand-alone treatments at recommended field rates on a weekly schedule. The results showed that Entrust® consistently performed well in suppressing M. ochroloma adults, larvae and crop damage. PyGanic® was the second best treatment. PyGanic® or NOFLY™ can be applied in rotation with Entrust® for effective management of M. ochroloma. Entrust® applied weekly or in alternation with PyGanic® and NOFLY™ provided acceptable control of M. ochroloma in organic crucifer production. The findings of this study have given the base line information towards the development of attractant-based management strategies, such as detection and monitoring, attract and kill, and push-pull strategies, against M. ochroloma in organic crucifer vegetable production.