Strategies to Enhance Efficacy of Entomopathogenic Nematodes against Diamondback Moth (Plutella xylostella) and Imported cabbageworm (Pieris rapae)

dc.contributor.advisor Wang, Koon-Hui K-H.W Budhathoki, Sabina
dc.contributor.department Tropical Plant Pathology 2021-09-30T18:21:08Z 2021-09-30T18:21:08Z 2021 M.S.
dc.subject Plant pathology
dc.subject Agriculture
dc.subject Entomology
dc.subject trap crop
dc.subject adjuvant
dc.subject cabbage
dc.subject intermittent sprinkler irrigation
dc.subject kai choi
dc.subject Steinernema feltiae,
dc.title Strategies to Enhance Efficacy of Entomopathogenic Nematodes against Diamondback Moth (Plutella xylostella) and Imported cabbageworm (Pieris rapae)
dc.type Thesis
dcterms.abstract Diamondback moth (DBM), Plutella xylostella, and imported cabbage worm (ICW), Pieris rapae, are the most destructive insect pests of cruciferous crops worldwide. Whereas various control measures against ICW are viable, DBM management in Hawaii is challenged by the development of insecticide resistant populations. This thesis focuses on exploring IPM strategies to enhance the efficacy of entomopathogenic nematodes (EPN) against DBM and ICW. Three approaches compatible with organic farming were examined to enhance the efficacy of foliar application of EPNs by integrating EPN sprays with 1) trap cropping, 2) intermittent sprinkler irrigation, and 3) using adjuvants comply with organic certification. For the trap cropping approach, three 2 × 2 (trap crop × EPN) factorial field trials were conducted by using kai choi (Brassica juncea) as trap crop and Steinernema feltiae for EPN sprays supplemented with 1.6 ml/L (or 20 fl oz/acre) on head cabbage (Brassica oleraceae var capitata) and kale (Brassica oleraceae var acephala). On head cabbage, trap cropping by kai choi reduced the abundance of DBM by 46%, ICW by 73%, DBM damage by 45% and ICW damage by 33%, respectively. On the other hand, EPN reduced DBM number in trap crop plots and ICW in no trap crop plots only. Effects of trap cropping were less on kale compared to that observed on cabbage. In the first kale trial, trap crop suppressed 50% of DBM abundance and 19% of leaves with DBM damage, whereas in the second trial, trap cropping only reduced 13% of leaves with ICW damage. EPNs was not effective against number and damage of both ICW and DBM in Kale Trial I, but it suppressed DBM numbers by 100% in Trial II (P ≤ 0.05) soon after application. Overall, trap cropping did not improve the efficacy of EPN spray. EPN only suppressed DBM or ICW successfully when average pest pressure throughout a crop was below 0.5/plant. For the intermittent sprinkler irrigation approach, two 3 × 2 (sprinkler irrigation regime × EPN +/-) factorial-split plot cabbage field trials were conducted. The three main plots of intermittent sprinkler irrigation regimes were: 1) 5-min sprinkler irrigation twice at dusk (6:00 and 8:00 pm) (Sd), 2) 5-min sprinkler irrigation from 8:00 am to 4:00 pm at 2-hour intervals and twice at dusk (6:00 pm and 8:00 pm) (SDd), and 3) no sprinkler irrigation (NS). Each main plot was split into subplots of foliar EPN (1.25 million IJs/ha) or no EPN (EPN-) applications. In Trial I, SDd and Sd decreased DBM damage on head cabbage by 19% compared to the NS but they had no effect in Trial II. SDd also reduced ICW damage by 88% in Trial I and by 45% in Trial II. In terms of insect abundance, SDd reduced ICW numbers on head cabbage by 86% compared to the NS control in Trial I and by 58% in Trial II. However, SDd and Sd did not affect DBM numbers. Unfortunately, no interaction occurred between sprinkler irrigation and EPN application, indicating that intermittent sprinkler irrigation also did not improve the performance of EPN on these pests. To explore the third approach to improve EPN performance, two greenhouse experiments were conducted to test the efficacy of different adjuvants in enhancing the persistence of Steinernema feltiae MG-14 and its suppression against DBM. The adjuvant treatments tested were 3.9 ml/L (i.e. 50 fl oz/100 gal) of 1) Oroboost®, 2) Kinetic® and 3) Exit® compared to a 4) water control with EPN only where S. feltiae IJs were exposed to high heat in a greenhouse for 0, 30, 60, 120 and 180 min. Only Oroboost® extended the survival rates of S. feltiae for 2 hours. In fact, Kinetic® and Exit® showed a sign of toxicity against EPN. Oroboost® at 3.9 ml/L added to the EPN spray was more effective than Oroboost® at 1.6 ml/L (adjuvant rate used in the field trials as described above) in reducing the DBM population on cabbage plants 2 days after EPN application in a greenhouse. A laboratory and a greenhouse experiment were followed up to test dosages of S. feltiae at 0, 0.625, 1.25 and 2.5 IJ/cm2 mixed with Oroboost® at 3.9 ml/L against DBM larvae and pupae. Both experiments showed that 0.625 IJ/cm2 was as effective as the commercial (1.25 IJ/cm2 equivalent to 125 mil IJs/ha) and high (2.5 IJ/cm2 equivalent to 250 mil IJs/ha) dosages of S. feltiae in killing and infecting DBM. Interestingly, both larva and pupa of DBM were equally susceptible to S. feltiae infection. In summary, kai choi was more effective as a trap crop against DBM and ICW damage when intercropped with cabbage than with kale. Effect of kai choi as a trap crop in a kale cropping system was sporadic, mostly effective against DBM damage but not against ICW. The intermittent sprinkler irrigation (ISI) regimes tested in this thesis were only effective and rather significantly against ICW when ISI was in the daytime (> 80% in one trial and > 45% in another). ISI only reduced DBM damage by < 20% regardless of day or dusk ISI. More ISI regimes later at night need to be tested against DBM. The EPN efficacy tests conducted in the laboratory or greenhouse clearly showed that the high commercial EPN rate is not warranted but use of adjuvant is imperative. Future trap cropping field trials need to re-evaluate efficacy of S. feltiae with 62.5 mil IJs/ha supplemented with 3.9 ml Oroboost®/L. None-the-less, this project provided promising progress towards non-pesticide-based approaches against DBM and ICW that can help to mitigate insecticide resistance problem for organic farmers in Hawaii.
dcterms.extent 100 pages
dcterms.language en
dcterms.publisher University of Hawai'i at Manoa
dcterms.rights All UHM dissertations and theses are protected by copyright. They may be viewed from this source for any purpose, but reproduction or distribution in any format is prohibited without written permission from the copyright owner.
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