Andrew Keddie

Biological control of the diamondback moth, Plutella xylostella: A review

Abstract The diamondback moth (DBM), Plutella xylostella (L.) (Lepidoptera: Plutellidae), is one of the most destructive cosmopolitan insect pests of brassicaceous crops. It was the first crop insect reported to be resistant to DDT and now, in many crucifer producing regions, it has shown significant resistance to almost every synthetic insecticide applied in the field. In certain parts of the world, economical production of crucifers has become almost impossible due to insecticidal control failures. Consequently, increased efforts worldwide have been undertaken to develop integrated pest management (IPM) programs, principally based on manipulation of its natural enemies. Although over 130 parasitoid species are known to attack various life stages of DBM, most control worldwide is achieved by relatively few hymenopteran species belonging to the ichneumonid genera Diadegma and Diadromus, the braconid genera Microplitis and Cotesia, and the eulophid genus Oomyzus. DBM populations native to different regions have genetic and biological differences, and specific parasitoid strains may be associated with the specific DBM strains. Therefore, accurate identification based on genetic studies of both host and parasitoid is of crucial importance to attaining successful control of DBM through inoculative or inundative releases. Although parasitoids of DBM larvae and pupae are currently its principal regulators, bacteria-derived products (e.g., crystal toxins from Bacillus thuringiensis) and myco-insecticides principally based on Zoophthora radicans and Beauveria bassiana are increasingly being applied or investigated for biological control. Viruses, nematodes and microsporidia also have potential as biopesticides for DBM. When an insect pest is exposed to more than one mortality factor, there is the possibility of interactions that can enhance, limit, or limit and enhance the various aspects of effectiveness of a particular control tactic. This paper reviews the effectiveness of various parasitoids and entomopathogens against DBM, interactions among them, and their possible integration into modern IPM programs.

Maize stem borer colonization, establishment and crop damage levels in a maize-leucaena agroforestry system in Kenya

The effect of resource concentration on the population of stem-borers of maize in the maize-leucaena agroforestry system was evaluated. The studies covered six cropping seasons from October 1992 to August 1995, and were conducted at Mtwapa and Amoyo in coastal and western Kenya, respectively. Treatments included monocropped and intercropped (maize, leucaena) plots, weeded and unweeded plots, mulched and unmulched plots, and three spacing (1.5, 2.25 and 3.0 m) regimes for leucaena hedgerows. Maize was interplanted at the onset of the rainy season. Abundance of adult and larval/pupal stages of maize stem borers, oviposition preference, foliar damage, borer entry/exit holes, maize plant mortality and stem breakage because of borer damage were significantly greater in the maize monocrop than in the maize-leucaena intercrops. The reduced pest numbers were associated with reduced yield loss per plant, and the 3.0 m leucaena hedgerow spacing plots yielded more than the maize monocropped plots despite having 25% fewer initial maize plants.

Larval survival, host plant preferences and developmental responses of the diamondback moth Plutella xylostella (Lepidoptera: Plutellidae) on wild brassicaceous species.

The diamondback moth Plutella xylostella (L.) (Lepidoptera: Plutellidae) is an important pest of cultivated brassicaceous crops worldwide. The host plant preferences, developmental biology and survival and longevity of P. xylostella are relatively well understood on commercial crop species; however, its relationship with brassicaceous weeds is poorly known. Sinapis arvensis L., Erysimum cheiranthoides L. and Capsella bursa‐pastoris (L.) Medicus are among the most common brassicaceous weeds worldwide and can serve as important bridge hosts of P. xylostella. In this study, preference and performance of P. xylostella were compared on these weed species. In free‐choice situations, females deposited 5.5 and 18.8 times more eggs on S. arvensis than on E. cheiranthoides and C. bursa‐pastoris, respectively. Survival from neonate to pupa and from pupa to adult was highest on S. arvensis and E. cheiranthoides and lowest on C. bursa‐pastoris. Development was fastest, foliage consumption was greatest, pupae and silk cocoons were heaviest, adult body masses and longevities were highest and forewings were largest for both females and males when reared as larvae on S. arvensis. Realized fecundity of new generation adults was highest for individuals reared on S. arvensis compared to those reared on E. cheiranthoides or C. bursa‐pastoris. Relative growth rates of pupae and adults were highest on S. arvensis, suggesting that this plant species is a high‐quality host for P. xylostella compared with other species tested. Potential impacts of these wild brassicaceous species on P. xylostella populations are discussed.

Gut contents in molting lepidopteran larvae: a source of error in nutritional studies

It is generally assumed that larval insects allow the gut to clear of all food prior to each molt (Waldbauer, 1968; Kogan, 1986; Ayres & MacLean, 1987). This premise has been important in the standardization of insect starting weights in growth experiments; the food in feeding larvae may account for up to 50 ~o of the dry weight of phytophagous insects (Wightman, 1981; Kogan, 1986). The only published reference that we found of a lepidopteran species which does not clear its gut during molting was Spodoptera eridania (Cram.) (Soo Hoo, 1962 in Waldbauer, 1968). The finding that Paratrytone melane Edwards larvae also consistently contain a substantial amount of food in the midgut during and following a molt (Barbehenn, 1989) prompted us to make a survey of molting lepidopteran larvae in order to generalize about the prevalence of this putatively rare phenomenon. We also present a method for estimating the correct initial dry weights (DW) of larvae of species which retain food in their guts during a molt.

Flight activity and dispersal of the cabbage seedpod weevil (Coleoptera: Curculionidae) are related to atmospheric conditions.

ABSTRACT The cabbage seedpod weevil, Ceutorhynchus obstrictus (Marsham) (Coleoptera: Curculionidae), is an invasive pest of canola (Brassica napus L. and Brassica rapa L.) in western Canada. Under current climatic conditions, C. obstrictus is spreading from established populations in southwestern Alberta at ≈55 km/yr. We studied the influence of climatic conditions on C. obstrictus flight behavior in 2007 and 2008 and eastward dispersal from the western border of Saskatchewan from 2002 to 2007. Positive linear relationships between increases in mean temperature and flight height and between greater mean maximum temperature and expanded dispersal distances were significant. Increases in relative humidity were associated with reduced flight heights and dispersal distances. We developed models that predict the relationships of temperature and relative humidity with flight height and with dispersal distance. We also discuss implications for C. obstrictus dispersal under current climatic conditions and in the context of predicted climate change.

Antixenosis and antibiosis resistance to Ceutorhynchus obstrictus in novel germplasm derived from Sinapis alba × Brassica napus.

Abstract Introgression of cabbage seedpod weevil, Ceutorhynchus obstrictus (Marsham) (Coleoptera: Curculionidae), resistance from Sinapis alba L. to susceptible Brassica napus L. (Brassicaceae) has produced genetic lines resistant to the weevil in replicated field trials. In the current study, weevil feeding and oviposition on S. alba and on resistant novel lines developed by crossing S. alba × B. napus were less frequent than on susceptible germplasm. Development times were greater and biomass was less when larvae were reared on resistant lines or S. alba. Oocyte development was faster in post-diapause springtime adult female weevils caged on susceptible plants than in those on a resistant line, S. alba, or an early-season food host, Thlaspi arvense L (Brassicaceae). Our results suggest that antixenosis resistance and antibiosis resistance are expressed by resistant lines. These results and previous chemical analyses of these lines also suggest that resistance is potentially influenced by attractive and (or) feedings-timulant effects of 2-phenylethyl glucosinolate and antifeedant or toxic effects of 1-methoxy-3-indolylmethyl glucosinolate.

Diadegma insulare development is altered by Plutella xylostella reared on water-stressed host plants

Natural enemies of herbivores function in a multitrophic context, and their performance is directly or indirectly influenced by herbivores and their host plants. Very little is known about tritrophic interactions between host plants, pests and their parasitoids, particularly when the host plants are under any stress. Herbivores and their natural enemies’ response to plants under stress are diverse and variable. Therefore, in this study we investigated how diamondback moth, Plutella xylostella (L.), reared on water‐stressed host plants (Brassica napus L. and Sinapis alba L.) influenced the development of its larval parasitoid, Diadegma insulare (Cresson). No significant differences were observed in development of P. xylostella when reared on water‐stressed host plants. However, all results indicated that water stress had a strong effect on developmental parameters of D. insulare. Development of D. insulare was delayed when the parasitoid fed on P. xylostella, reared on stressed host plants. Egg to adult development of D. insulare was faster on non‐stressed B. napus than non‐stressed S. alba followed by stressed B. napus and S. alba. Female parasitoids were heavier on non‐stressed host plants than stressed counterparts. Furthermore, the parasitoid lived significantly longer on stressed B. napus. However, body size was not affected by water treatment. Most host plant parameters measured were significantly lower for water‐stressed than non‐stressed treatments. Results suggest that development of this important and effective P. xylostella parasitoid was influenced by both water stress and host plant species.

Selective effects of floral food sources and honey on life-history traits of a pest-parasitoid system

Many parasitoids and their herbivorous hosts forage on the same floral resources in agroecosystems. Floral resources that benefit natural enemies without supporting pests can improve the efficacy of biological control agents. Here, we report the results of a study on selective effects of floral and non‐floral food sources on the life‐history traits of the parasitoid Diadegma insulare (Cresson) (Hymenoptera: Ichneumonidae) and its host Plutella xylostella (L.) (Lepidoptera: Plutellidae). Under standard laboratory conditions, insects were exposed to four flowering plant species, 10% honey solution, and water. All food sources increased the longevity of the herbivore and its parasitoid by as much as four‐ to nine‐fold, respectively, compared to the water control diet. Diadegma insulare survived the longest on Thlaspi arvense L. (Brassicaceae) and P. xylostella on Sinapis arvensis L. (Brassicaceae). However, none of the food sources tested was beneficial to the parasitoid alone, though Lobularia maritima L. (Brassicaceae) was found to selectively favor the longevity of D. insulare. Diadegma insulare adults were heavier when fed on S. arvensis, whereas feeding on the honey solution led to higher body weights of P. xylostella. In conclusion, we demonstrated that floral and non‐floral food sources varied in their suitability and acted differently on life‐history traits of a host–parasitoid system. The selective characteristics of nectar‐producing plants and their influence on the herbivore‐natural enemy combination can, therefore, be employed to increase their impact in integrated P. xylostella management.

Incorporation of Novel Ceutorhynchus obstrictus-resistant Canola Genotypes Into Mixed Cropping Strategies: Evidence for Associational Resistance

Canola genotypes resistant to the cabbage seedpod weevil, Ceutorhynchus obstrictus (Marsham) (Coleoptera: Curculionidae), have recently been developed through introgression of Sinapis alba L. to Brassica napus L. Several lines express antixenosis and antibiosis resistance and have been shown to be less attractive to weevils in visual and olfactory behavioral bioassays. This paper details a small-plot study that assessed the effects on distribution dynamics of weevil adults and larvae of interspersing susceptible among resistant genotypes relative to monocultures over two growing seasons. Results indicate that mixes reduced weevil numbers and oviposition in pods of susceptible genotypes. These results are consistent with associational resistance.