Parasitism by beneficial insects such as Diadegma insulare, shown here along with one diamondback moth larvae, were surprisingly high. Credit: Lloyd Dosdall

High rates of parasitism on diamondback moth larvae

Key result: This study continued work initiated by earlier research which showed that parasitism of diamondback moth larvae and pupae can be relatively high early in the season. Canola producers are encouraged to carefully monitor populations, waiting a few days after nominal thresholds are reached to provide beneficial insects an opportunity to control larvae and to determine whether insecticide applications
are necessary.

Project title, Principal investigator: “Improved integrated crop management with beneficial insects,”
Juliana Soroka, AAFC Saskatoon Research and Development Centre.

Funding: Alberta Canola, MCGA, SaskCanola, CCC, AAFC, GF2

This project successfully investigated the phenology of diamondback moth larvae and its parasitoids in four eco-regions of Saskatchewan at three different times of the canola-growing season during 2012, 2013 and 2014.

Larvae of the moth were most numerous in the most southerly eco-region surveyed, and parasitism was highest in the Boreal Transition Eco-region running along the Black and Dark Gray soil zones early in the season (June).

Two parasitoid species were found to regularly parasitize diamondback moth larvae in the field, on occasion at levels high enough to exert considerable control of pest numbers. The parasitoid to emerge from diamondback moth larvae in the greatest number was a parasitic wasp, Diadegma insulare in the ichneumonid family, followed by the braconid wasp Microplitis plutellae. Another parasitoid was also present, later identified with the help of the National Identification Service as Diolcogaster claribita, which is the first report of this wasp in Saskatchewan.

Parasitism levels reached 45 per cent at some sampling times, indicating that biological control can have a considerable effect on diamondback moth populations on the Prairies. High rates of natural parasitism of diamondback moth found in this study were a surprise to researchers and producers.
Although not definitive, variation in DNA profiles of early and late-collected moths suggest that overwintering of diamondback moth may be occurring, but further research is necessary to confirm this.

Published results

Considerable information obtained on the effects of temperature on the development of both diamondback moth and D. insulare was reported in the Environmental Entomology journal*. Laboratory studies quantified, for the first time, low and high temperature developmental parameters of the principal parasitoid D. insulare at 2.1 and 34.0°C, respectively.

The research found that the low temperature threshold for diamondback moth development was lower than previously determined. The low temperature threshold was therefore lowered by several degrees to 2.1°C.

Other laboratory studies found that fluctuating temperatures, more representative of actual field conditions, affected diamondback moth and beneficial insect development differently than did constant temperatures of the same average value as the fluctuating temperatures. This points out the importance of natural conditions in determination of insect development.
In addition, the discovery that expression of a heat shock protein in both diamondback moth and D. insulare suggest that this is a viable means of tracking arthropod response to changing climates.

The project has generated information that will have both practical and theoretical utility. Identification of the natural enemies of diamondback moth present in Prairie canola fields will aid in finding ways to increase their efficacy. The research will aid in the development of a Dymex model to predict the responses of both insects to irregular patterns of global climatic change.

*New Threshold Temperatures for the Development of a North American Diamondback Moth (Lepidoptera: Plutellidae) Population and Its Larval Parasitoid,Diadegma insulare (Hymenoptera: Ichneumonidae); M. H. Bahar, J. J. Soroka, L. Grenkow, L. M. Dosdall; Environmental Entomology Oct 2014, 43 (5) 1443-1452; DOI: 10.1603/EN14055