Influence of humidity and a surfactant-polymer-formulation on the control potential of the entomopathogenic nematode Steinernema feltiae against diapausing codling moth larvae (Cydia pomonella L.) (Lepidoptera: Tortricidae)

Abstract

The codling moth (Cydia pomonella L.) is a serious pest of pome fruit. Diapausing cocooned larvae overwinter in cryptic habitats in the soil or in the bark of infested trees. The entomopathogenic nematode Steinernema feltiae (Filipjev) (Rhabditida: Steinernematidae) is used to control diapausing codling moth larvae. The objective of this study was to define environmental conditions favouring the performance of the nematodes. Cocooned larvae were more susceptible than non-cocooned larvae. Susceptibility of pupae was low. To determine the influence of decreasing water activity (a w -value) on the activity of the nematodes, mortality of codling moth larvae and Galleria mellonella L. were tested in sand-sodiumpolyacrylate mixtures of variable water activity. S. feltiae was able to infect both insects at a w -values [0.9. Cocooned larvae of both insects died at lower a w -values than non-cocooned larvae. Mortality of cocooned larvae did not further increase after half an hour of exposure to nematodes, whereas the mortality of non-cocooned larvae increased with increasing exposure time. LC 50 and LC 90 considerably decrease with increasing RH. The negative influence of the relative humidity (macro environment) was less important than the effect of the water activity in the bark substrate (micro environment). The micro environment can be manipulated by applying S. feltiae with higher volumes of water. A surfactant-polymerformulation significantly increased nematode efficacy and can buffer detrimental environmental effects.

Effect of humidity and a superabsorbent polymer formulation on the efficacy ofHeterorhabditis zealandica(Rhabditida: Heterorhabditidae) to control codling moth,Cydia pomonella(L.) (Lepidoptera: Tortricidae)

“…A clear distinction should be made between moisture-levels pertaining to the macro-environment (surrounding orchard humidity) and those pertaining to the micro-environment (the cryptic habitat where larvae, for example, reside underneath loose pieces of bark on the tree), as conditions may differ considerably between the two environments concerned (Navaneethan et al 2010). Water activity (awvalue) gives an indication of the amount of available water on the bark surfaces of trees, and thus of the effectiveness of nematodes in this type of micro-environment, as nematodes required a water film for propulsion (Koppenhöfer 2007).…”

“…Water activity (awvalue) gives an indication of the amount of available water on the bark surfaces of trees, and thus of the effectiveness of nematodes in this type of micro-environment, as nematodes required a water film for propulsion (Koppenhöfer 2007). Navaneethan et al (2010) documented the first investigation of the influence of water activity on nematode efficacy, using S. feltiae.…”

“…The developmental stage of codling moth that is best suited for control with nematodes is the diapausing larval stage, which occurs between late summer and early spring in such cryptic habitats as underneath loose pieces of bark or in pruning wounds on trees (Lacey and Unruh 1998;Navaneethan et al 2010). Eliminating the stage concerned would protect fruit from damage during the following growing season (Lacey and Unruh 1998; Lacey, Arthurs, Knight, and Huber 2007).…”

“…Due to the aboveground location of diapausing codling moth larvae, the suggested time of application is during autumn or early spring, when the whole population of codling moth consist of diapausing larvae (Lacey et al 2006a(Lacey et al , 2010Navaneethan et al 2010) Low temperatures and low levels of relative humidity (RH) have been proven to be the most limiting factors in the use of these nematodes for the control of codling moth. In colder fruit production regions, cold-adapted species such as Steinernema feltiae could be applied, since their application has been proved to be effective at temperatures below 15°C (Grewal, Gaugler, and Wang 1996; Lacey and Unruh 1998).…”

“…Genetically improved nematode isolates can partially help to overcome this problem (Mukuka et al 2010), although manipulation of the habitat where the nematodes will be applied can offer a practical solution (Webster 1973). Improved nematode formulation, through the addition of certain hydroscopic additives such as gels and certain adjuvants, has been proven to allow for the partial prevention of the rapid desiccation of nematodes (Glazer 2002;Navaneethan et al 2010;Lacey et al 2010). …”