Efficacy of entomopathogenic nematodes against potato tuber moth, Phthorimaea operculella (Lepidoptera: Gelechiidae) under laboratory conditions

Biocontrol Science and Technology (Impact Factor: 0.73). 01/2013; 23(2):146.

ABSTRACT The susceptibility of potato tuber moth, Phthorimaea operculella (Zeller)
(Lepidoptera: Gelechiidae) to native and commercial strains of entomopathogenic
nematodes (EPNs) was studied under laboratory conditions. Native strains
of EPNs were collected from northeastern Iran and characterised as Steinernema
feltiae and Heterorhabditis bacteriophora (FUM 7) using classic methods as well
as analysis of internal transcribed spacer (ITS) and D2/D3 sequences of 28S
genes. Plate assays were performed to evaluate the efficiency of five EPN strains
belonging to four species including Steinernema carpocapsae (commercial strain),
S. feltiae, Steinernem glaseri and H. bacteriophora (FUM 7 and commercial
strains). This initial assessment with 0, 75, 150, 250, 375 and 500 IJs/ml
concentrations showed that S. carpocapsae and H. bacteriophora caused the
highest mortality in both larval and prepupal stages of P. operculella, PTM.
Thereafter, these three strains (i.e. S. carpocapsae, H. bacteriophora FUM 7 and
the commercial strains) were selected for complementary assays to determine the
effects of soil type (loamy, loamy�sandy and sandy) on the virulence of EPNs
against the second (L2) and fourth instar (L4) larvae as well as prepupa. A soil
column assay was conducted using 500 and 2000 IJs in 2-ml distilled water.
Mortality in the L2 larvae was not affected by the EPN strain or soil type, while
there was a significant interactive effect of nematode strains and soil type on
larval mortality. The results also showed that EPN strains have higher efficiency
in lighter soils and caused higher mortality on early larvae than that in loamy soil.
In L4 larvae, mortality of PTM was significantly influenced by nematode strain
and applied concentrations of infective juveniles. The larval mortality induced by
S. carpocapsae was higher than those caused either by a commercial or the FUM
7 strain of H. bacteriophora. Prepupa were the most susceptible stage.

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    ABSTRACT: There are few insecticidal options for potato tuber moth (PTM), Phthorimaea operculella Zeller (Lepidoptera: Gelechiidae), control late in the growing season. We evaluated the PTM granulovirus (PoGV) and Bacillus thuringiensis Berliner ssp. kurstaki (Btk) for season-long control of PTM on potato foliage in 2006 and 2007. Compared to untreated controls, 10 weekly applications of PoGV (10¹³ occlusion bodies/ha) reduced PTM populations in replicated 1-m³ field cages by 86-96% on pre-harvest foliage and 90-97% on tubers added to cages shortly before harvest. Infection rates of 82-95% of L4 larvae by PoGV were noted within individual larval cohorts. Equivalently timed Btk treatments (1.12 kg product/ha) were significantly less effective at population suppression, with a 36-76% reduction in larvae recovered from tubers added to cages. A PoGV/Btk alternation was significantly more effective than Btk alone and as effective as PoGV in 2007, but not in 2006. There was some evidence that reduced rate PoGV treatments (10% rate or 50% application frequency) were less effective than the standard program. There were no treatment effects on percentage of tubers growing in the ground that were infested at harvest, which remained comparatively low at [less-than or equal to]8.1%. Bioassays were conducted to evaluate the residual activities of foliar deposits. Early-season applications were highly effective for the first 24 h (greater-than-or-equal93% mortality) with a steady decline in activity over 10 days. A second application, applied later in the season, showed similar patterns, although in this case Btk was less persistent than PoGV, whereas both agents provided significant larval mortality compared with controls over 14 days. Both PoGV and Btk provide alternatives to manage field infestations of PTM prior to harvest, thus reducing the risk of tuber infestations in storage.
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