The impact of parasitoids on diamondback moth in Europe: a life table approach

Diamondback moth, Plutella xylostella (Linnaeus) (Lepidoptera: Plutellidae), was first recorded in North America from Europe about 150 years ago and can be a significant pest of canola in Western Canada. Because parasitism of P. xylostella in Canada is generally low, the introduction of one or more additional exotic parasitoids from Europe is being considered to increase the suppression of P. xylostella populations. Life table studies to determine the impact of parasitoids on diamondback moth populations in Europe were conducted in northwestern Switzerland in 2014–2016. Net reproductive rates were found to be less than one in seven out of eight life tables, suggesting that P. xylostella populations in Switzerland are mostly driven by immigration and recolonisation. In total, seven primary parasitoid species and one hyperparasitoid were associated with diamondback moth. Pupal parasitism by D. collaris reached up to 30%, but because generational mortality was mainly driven by abiotic mortality factors and predation of larvae, the overall contribution of pupal parasitism was low (< 6%). In regions of Canada, where P. xylostella may have increasing populations and low larval mortality, the addition of D. collaris may be a promising approach. Life table studies across Canada are necessary to determine the need for such intervention.

Population- and Species-Based Variation of Webworm–Parasitoid Interactions in Hogweeds (Heracelum spp.) in the Netherlands

In three Dutch populations of the native small hogweed (Heracleum sphondylium L. [Apiales: Apiaceae]), and one of the invasive giant hogweed (H. mantegazzianum Sommeier & Levier [Apiales: Apiaceae]), interactions between a specialist herbivore, the parsnip webworm (Depressaria radiella), and its associated parasitoids were compared during a single growing season. We found host plant species-related differences in the abundance of moth pupae, the specialist polyembryonic endoparasitoid, Copidosoma sosares, the specialist pupal parasitoid, Barichneumon heracliana, and a potential hyperparasitoid of C. sosares, Tyndaricus scaurus Walker (Hymenoptera: Encyrtidae). Adult D. radiella body mass was similar across the three small hogweed populations, but moths and their pupal parasitoid B. heracliana were smaller when developing on giant than on small hogweeds where the two plants grew in the same locality (Heteren). Mixed-sex and all-male broods of C. sosares were generally bigger than all-female broods. Furthermore, adult female C. sosares were larger than males and adult female mass differed among the three small hogweed populations. The frequency of pupal parasitism and hyperparasitism also varied in the different H. sphondylium populations. These results show that short-term (intra-seasonal) effects of plant population on multitrophic insects are variable among different species in a tightly linked food chain.

Chemical investigations of volatile kairomones produced by Hyphantria cunea (Drury), a host of the parasitoid Chouioia cunea Yang

In tritrophic ‘plants–herbivores–natural enemies’ systems, there are relatively few reports concerning the role(s) of kairomones in pupal parasitism. Chouioia cunea Yang (Hymenoptera: Eulophidae), an endoparasitic chalcid wasp, parasitizes pupae of the fall webworm (Hyphantria cunea Drury). The role of host-related kairomones was investigated using electroantennogram (EAG) and behavioral techniques. Chemicals from some host stages (pupae) and host by-products (frass), induced arrestment behavior of female parasitoids, while chemicals from prepupae, were inactive. Gas chromatography–mass spectrometry analysis of volatiles collected from pupae, frass and prepupae using solid-phase microextration revealed seven compounds with carbon chain lengths ranging from C4 to C20. All of the chemicals elicited significant EAG responses in C. cunea. Y-tube olfactometer bioassays demonstrated a significant positive response of mated female C. cunea to 1-dodecene. These data provide a better understanding of the host location mechanisms of pupal parasitoid.

Some Biological and Behavioral Studies of the Tetrastichus sp. (Hymenoptera: Eulophidae), a Pupal Parasitoid of Brontispa longissima Gestro (Coleoptera: Chrysomelidae)

The Tetrastichus sp. was identified as a pupal parasitoid indigenous to the Philippines. The total development period of Tetrastichus sp. from egg to adult was 16 days under laboratory conditions. The average incubation, larval and pupal periods were 3.5, 4.8, and 7.4 days, respectively. The average adult longevity was 5.5 (male) and 6.4 (female). Upon emergence of an adult wasp from the host pupa, mating took place shortly and the female look for suitable host pupa of Brontispa longissima for oviposition. Tetrastichus sp. is an endogregarious parasitoid. From one parasitized pupa of Brontispa longissima, around 6 - 66 individual wasps emerged with a mean of 22.4 while parasitized pupae collected from the field yielded 4 - 21 individuals with a mean of 12.7. Majority of the wasps emerged in the morning (70%) while 30% emerged in the afternoon. Sex ratio of male and female was 1:5.4 in laboratory condition while 1:3.7 under field condition with females outnumbered the males. Tetrastichus sp. was observed in some cases to reproduce parthenogenetically. Longevity of Tetrastichus sp. can be enhanced to a mean of 7.3 and 6.0 days by feeding on sugar and honey, respectively while water and coconut pollen resulted in longevity of 3.3 and 2.4 days, respectively. Adults without food lived only one day. Parasitism efficiency ratio of 10 parasitoids: 10 host pupae resulted in 90% parasitism. One to two day old pupa of B. longissima was preferred by Tetrastichus sp. for oviposition with a mean of 86.7% parasitism. Percent pupal parasitism in the field ranged 25.7 - 42.3% with an average of 27.6% in selected areas in Mindanao. Tetrastichus sp. was found to effectively control B. longissima under field condition. The study indicated that Tetrastichus sp. is an effective control agent of B. longissima and could be used in the field to prevent economic damage of the palms by the pest.

Native range assessment of classical biological control agents: impact of inundative releases as pre-introduction evaluation

Diadromus pulchellus Wesmael (Hymenoptera: Ichneumonidae) is a pupal parasitoid under consideration for introduction into Canada for the control of the invasive leek moth, Acrolepiopsis assectella (Zeller) (Lepidoptera: Acrolepiidae). Since study of the parasitoid outside of quarantine was not permitted in Canada at the time of this project, we assessed its efficacy via field trials in its native range in central Europe. This was done by simulating introductory releases that would eventually take place in Canada when a permit for release is obtained. In 2007 and 2008, experimental leek plots were artificially infested with pest larvae to mimic the higher pest densities common in Canada. Based on a preliminary experiment showing that leek moth pupae were suitable for parasitism up to 5–6 days after pupation, D. pulchellus adults were mass-released into the field plots when the first host cocoons were observed. The laboratory-reared agents reproduced successfully in all trials and radically reduced leek moth survival. Taking into account background parasitism caused by naturally occurring D. pulchellus, the released agents parasitized at least 15.8%, 43.9%, 48.1% and 58.8% of the available hosts in the four release trials. When this significant contribution to leek moth mortality is added to previously published life tables, in which pupal parasitism was absent, the total pupal mortality increases from 60.1% to 76.7%. This study demonstrates how field trials involving environmental manipulation in an agent's native range can yield predictions of the agent's field efficacy once introduced into a novel area.

Effects of intercropping on maize stemborers and their natural enemies

The effects of maize–cowpea intercropping on three lepidopteran stemborers (Chilo partellus (Swinhoe) C. orichalcociliellus (Strand) and Sesamia calamistis Hampson) and their natural enemies were studied in Kenya. Oviposition was not affected by intercropping, but significantly fewer larvae and pupae were found in the intercrop. Egg parasitism was significantly higher in the intercrop reaching as high as 80% and was positively density dependent. Larval and pupal parasitism remained between 3 and 20%, but neither of these were affected by intercropping or host density. The main predators, ants and wandering spiders, were not augmented by intercropping, but an inverse relationship in abundance was found between these two predator groups. It is concluded that maize intercropped with cowpea has only limited potential as a method of controlling the key pests in maize.

LARVAL AND PUPAL PARASITISM OF THE FOREST TENT CATERPILLAR, MALACOSOMA DISSTRIA HÜBNER (LEPIDOPTERA: LASIOCAMPIDAE), IN ALBERTA, CANADA

Larval and pupal parasitoids were studied from collections made in endemic, outbreak, and post-outbreak populations of forest tent caterpillar (FTC), Malacosoma disstria, at 21 sites in Alberta, Canada, from 1989 to 1994. The parasitoid complex included 18 identified species. Aleiodes malacosomatos (Mason) (Hymenoptera: Braconidae) was common in low density populations and was the only species to attack FTC larvae prior to the fourth instar. Leschenaultia exul (Townsend) (Diptera: Tachinidae) was the most abundant larval parasitoid in endemic populations and after the 2nd year of outbreak. Although prevalent early in outbreaks, Patelloa pachypyga (Aldrich and Webber) (Diptera: Tachinidae) declined in older outbreaks possibly because it completes larval development later than other tachinids, leaving it vulnerable to competition from aggressive pupal parasitoids. I hypothesize that early initiation of oviposition allows L. exul to be more successful than P. pachypyga in low density populations because caterpillars have dispersed before the latter species initiates oviposition. I recorded lower pupal parasitism by Arachnidomyia aldrichi (Parker) (Diptera: Sarcophagidae) in Alberta than has been recorded in other regions, although it was still the most abundant parasitoid reared in this study. Parasitism by A. aldrichi was significantly higher in cocoons collected from undergrowth vegetation than from the forest canopy. Other pupal parasitoids contributed relatively little to FTC mortality.

Parasitoids of the African stem borer, Busseola fusca (Lepidoptera: Noctuidae), in South Africa

Eighteen parasitoids were recorded from the African stem borer, Busseola fusca (Fuller), on maize and grain sorghum, in Delmas and Cedara, South Africa. In Delmas, larval parasitism on both crops fluctuated below 20% with occasional peaks of 40–60%. In Cedara, larval parasitoids were active throughout the season with peaks of 75% and 60% parasitism during January and March-April, respectively on the ratoon crop, and 20% in May on the crop. Pupal parasitism peaked in Cedara at 100% during February-March, and at 80% during November, when parasitoids attacked pupae that formed after B. fusca larvae had emerged from diapause and pupated. In Delmas activity by pupal parasitoids was negligible. The egg parasitoids, Telenomus busseolae Gahan (Hymenoptera: Scelionidae) and Trichogrammatoidea lutea Girault (Hymenoptera: Trichogrammatidae), were rare. The larval parasitoid, Cotesia sesamiae (Cameron) (Hymenoptera: Braconidae), was active throughout the season and was by far the most abundant, emerging from about 90% of parasitized larvae. Its cocoons were often attacked by Aphanogmus fijiensis (Ferrière) (Hymenoptera: Ceraphronidae). Second in abundance among the larval parasitoids was Bracon sesamiae Cameron (Braconidae) whose cocoons were attacked in turn by Eurytoma braconidis Ferrière (Hymenoptera: Eurytomidae). Eurytoma braconidis was reared also from cocoons of Euvipio sp. and Aleiodes sp. (both Braconidae). All other larval parasitoids were rare. Procerochasmias nigromaculatus (Cameron) (Hymenoptera: Ichneumonidae) was the most abundant pupal parasitoid. Trichogrammatoidea lutea, Glyptapanteles maculitarsis (Cameron) (Braconidae) and Odontepyris transvaalensis (De Buysson) (Hymenoptera: Bethylidae) have not been recorded before from B. fusca. Some considerations and proposals for introductions of parasitoids into South Africa against B. fusca are discussed.