Parasitism performance and fitness of Cotesia vestalis (Hymenoptera: Braconidae) infected with Nosema sp. (Microsporidia: Nosematidae): implications in integrated pest management strategy.

The diamondback moth (DBM) Plutella xylostella (L.) has traditionally been managed using synthetic insecticides. However, the increasing resistance of DBM to insecticides offers an impetus to practice integrated pest management (IPM) strategies by exploiting its natural enemies such as pathogens, parasitoids, and predators. Nevertheless, the interactions between pathogens and parasitoids and/or predators might affect the effectiveness of the parasitoids in regulating the host population. Thus, the parasitism rate of Nosema-infected DBM by Cotesia vestalis (Haliday) (Hym., Braconidae) can be negatively influenced by such interactions. In this study, we investigated the effects of Nosema infection in DBM on the parasitism performance of C. vestalis. The results of no-choice test showed that C. vestalis had a higher parasitism rate on non-infected host larvae than on Nosema-treated host larvae. The C. vestalis individuals that emerged from Nosema-infected DBM (F1) and their progeny (F2) had smaller pupae, a decreased rate of emergence, lowered fecundity, and a prolonged development period compared to those of the control group. DBM infection by Nosema sp. also negatively affected the morphometrics of C. vestalis. The eggs of female C. vestalis that developed in Nosema-infected DBM were larger than those of females that developed in non-infected DBM. These detrimental effects on the F1 and F2 generations of C. vestalis might severely impact the effectiveness of combining pathogens and parasitoids as parts of an IPM strategy for DBM control

Developmental time of <i>C. vestalis</i> reared on <i>Nosema</i>-infected DBM.

<p>(<b>A</b>) Means (± SE) development time (days) of egg-larvae, pupae of F1 <i>C. vestalis</i> on uninfected and <i>Nosema</i>-infected <i>P. xylostella</i> larvae. (B) Means (± SE) development time (days) of egg-larvae, pupae of F2 <i>C. vestalis</i> on uninfected <i>Pllalute xylostella</i> larvae. Different letters above error bars indicate significant difference (Student’s t-test, <i>P</i><0.05).</p

Egg production by control and <i>Nosema</i>-infected <i>C. vestalis</i>.

<p>Mean (±SE) number of eggs produced by <i>C</i>. <i>vestalis</i> females (F1) emerged from infected DBM larvae (A) and infected <i>C</i>. <i>vestalis</i> females (F2) emerged from healthy DBM larvae (B). Different letters above error bars indicate significant difference (Student’s t-test, <i>P</i><0.05).</p

Morphometric (mean) characteristics of <i>Cotesia vestalis</i> developed within uninfected and <i>Nosema</i>-infected <i>P. xylostella</i> larvae.

<p>Morphometric (mean) characteristics of <i>Cotesia vestalis</i> developed within uninfected and <i>Nosema</i>-infected <i>P. xylostella</i> larvae.</p

Egg stage of <i>C. vestalis</i> devolved from control and infected DBM.

<p>(A) Eggs dissected out of <i>C</i><b><i>.</i></b><i> vestalis</i> female. (B) Close-up of a typical egg (control) showing the peduncle at the front end of the egg (C) an egg from F1 <i>C. vestalis</i> (B) an egg from F2 <i>C. vestalis.</i> Magnification 40×.</p

Mean % parasitism, cocoons number and weight, number of adults and percentage of emergence of F1 <i>C. vestalis</i> developed within uninfected and <i>Nosema</i>-infected DBM larvae.

<p>Mean % parasitism, cocoons number and weight, number of adults and percentage of emergence of F1 <i>C. vestalis</i> developed within uninfected and <i>Nosema</i>-infected DBM larvae.</p