Spatial distribution of cabbage root maggot (Delia radicum) and clubroot (Plasmodiophora brassicae) in winter oilseed rape crops in the Czech Republic

The amount and spatial distribution of plants afflicted with cabbage root maggot (Delia radicum; CRM) and clubroot (Plasmodiophora brassicae) in winter oilseed rape crops were assessed in the Olomouc region (Northern Moravia, Czech Republic) over the course of 2012–2014. A total of 16 commercial rape fields were included in the assessments. Plants with tumours showed a significantly lower (P < 0.001) level of infestation induced by CRM (24% of plants infested) compared to plants without tumours (37% of plants infested). According to a generalised linear mixed model, plants with thicker hypocotyls are predisposed to significantly higher levels (P < 0.001) of root surface damage induced by CRM. The correlation analysis indicates rather weak or intermediate levels of correlation between the two variables (hypocotyls thickness × root surface damage induced by CRM). Both CRM and clubroot symptomatic plants showed a significant tendency for aggregation in rape crops, but not in all cases. Distributions of CRM and clubroot symptomatic plants were either significantly spatially dissociated or not associated in crops. Ovipositing D. radicum females showed some tendency to avoid zones with higher number of plants infected by P. brassicae. Distributions of CRM and hypocotyl thickness levels were significantly spatially associated in crops in several cases.

Erroneous records of Aleochara bipustulata from North America: an assessment of the evidence

Aleochara bipustulata (L., 1761) (Coleoptera: Staphylinidae) is a Palearctic species and a natural enemy of the cabbage root maggot, Delia radicum (L., 1758) (Diptera: Anthomyiidae). It has been identified as a candidate for introduction to Canada for classical biological control of D. radicum. Recent taxonomic studies assert that A. bipustulata is absent from the Nearctic; however, there are numerous publications reporting the presence of the species in North America. We examined voucher material relating to these publications and additional museum specimens labeled as A. bipustulata. In addition, we reared Aleochara spp. from D. radicum puparia collected in the Canadian prairie provinces. Specimens that, based on external anatomy, could be A. bipustulata were definitively identified using characters of the genitalia. All of the 141 museum specimens labeled A. bipustulata were found to be Aleochara verna Say, 1836. A total of 811 individuals of Aleochara spp. were reared from D. radicum puparia; of these, 690 were Aleochara bilineata Gyllenhal, 1810, 121 were A. verna, and none were A. bipustulata. We have found no evidence that A. bipustulata occurs in North America.

INTERPOPULATION DIFFERENCES IN THE COLDHARDINESS OF DELIA RADICUM (DIPTERA: ANTHOMYIIDAE)

The cabbage root maggot, Delia radicum (L.), was introduced to North America in the mid-1800s, likely from northwestern Europe, and probably reached Quebec and British Columbia before 1885 and Manitoba by 1922. The mean temperature of crystallization (Tc) for overwintering pupae was −22.8 ± 1.2 °C for the St-Jean-sur-Richelieu, Quebec, population and −23.8 ± 0.7 °C for the Vancouver, British Columbia, population. The mean Tc for these two populations and for Winnipeg, Manitoba (−24.4 °C), Ascot, England (−22.8 °C), Tallinn, Estonia (−25.2 °C), and St. Petersburg, Russia (ca. −20 °C), did not show any relation to mean January temperatures. These locations represent both temperate oceanic and temperate continental climates and a range of mean January temperatures from +4.6 to −17.7 °C. Survival of puparia from St-Jean-sur-Richelieu exposed to nonfreezing temperatures decreased as temperature decreased and exposure time lengthened. The parameters for the regression equations describing this relationship were similar to those describing the Winnipeg population, and both were more coldhardy than the Ascot population. The upper limit of the cold injury zone (ULCIZ) for the St-Jean-sur-Richelieu population was −12.7 °C and the lower limit of this zone (LLCIZ) was −27.6 °C. Coldhardy populations from temperate continental climates (St-Jean-sur-Richelieu, Winnipeg) showed a rate of decrease in survival with increased cold stress (lower temperature, longer exposure) within the cold injury zone which was much slower than in the less coldhardy population (Ascot). Thus, some individuals from the coldhardy populations would be physiologically capable of surviving exposure to temperatures below Tc, whereas in the Ascot population nonfreezing injury would kill all the overwintering puparia at a temperature (−19.6 °C) well above Tc (−22.8 °C). The observed survival of puparia from Vancouver, following various nonfreezing exposures, resembled more closely the calculated survival for these exposures when the equations describing the Ascot population were used than when the equations for Winnipeg or St-Jean-sur-Richelieu were used. The Ascot and Vancouver populations, both from temperate oceanic climates, are less coldhardy than the populations from St-Jean-sur-Richelieu and Winnipeg (temperate continental climates). The founder populations of D. radicum in North America, which probably originated in the temperate oceanic climates of northwestern Europe, have adapted to the colder temperate continental climates by increasing their ability to survive longer exposures to all temperatures within the cold injury zone and not by lowering Tc. Therefore, selection for coldhardiness in D. radicum must have operated on structures, processes, or physiological-biochemical mechanisms that help the organism to avoid or repair nonfreezing cold injury but not on those that determine Tc.

ROOT MAGGOT DAMAGE TO CANOLA AND THE DISTRIBUTION OF DELIA SPP. (DIPTERA: ANTHOMYIIDAE) IN MANITOBA

The cabbage root maggot, Delia radicum (L.), was the only Delia species capable of attacking undamaged roots of canola [oilseed rape, Brassica napus (L.)] that was found in commercial fields in Manitoba. Adults of the overwintering generation infest canola. Their progeny, adults of the first generation, left the maturing plants after completing a single generation. Root maggot damage occurred in fields in all agricultural regions of the province. The mean percentage of the roots with damage was higher in the more northerly agricultural regions (15.8%) than in the south (8.0%). Most of the damage was light and the damage did not visibly affect the growth or vigour of the plants. Foot rot fungi were not associated with the feeding damage. In cage experiments, the seed yield of canola did not decrease with increasing proportions of plants damaged by D. radicum. The percentage of plants with root maggot damage (DAM) was best described in a polynomial by the mean daily air temperature (TEMP) and the total precipitation (RAIN) for June and July:log(DAM + 1)=–12.3 + 0.66 TEMP + 0.09 RAIN – 0.004(TEMP × RAIN).The level of damage caused by the larvae of D. radicum to canola in commercial fields during the years 1985 through 1988 did not appear to have caused yield losses. However, the level of damage increased with increasing precipitation and temperature during June and July so a series of favourable years could lead to increased damage and possibly to significant yield losses.

Rearing Root Maggots, Chiefly Hylemya brassicae (Bouché) (Diptera: Anthomyiidae) for Bioassay

Details of a method of mass rearing eggs and larvae of the cabbage root maggot, Hylemya brassicae (Bouché) for bioassay and of continuous propagation of all stages of the pest are described. The technique was equally suitable for rearing the onion maggot H. antiqua (Meig.), the seed corn maggot H. calicruraliturata (Rond.) and the carrot rust fly, Psila rosa (L.). Under the conditions described, 2000-2500 eggs or first-instar larvae of H. brassicae were obtained from 50-60 female flies in 24 hours, or 1000-1200 mature larvae or pupae were reared from eggs produced by 25 female flies in approximately 20 days.

Chemical Control of the Cabbage Root Maggot Integrated with Natural Controls

Application of 5 Ibs. toxicant Ileptachlor or aldrin per acre placed in a 4- to 5-inch hand about 1½ inches below the surface of the soil in a ridged seeding drill have controlled infestations of the cabbage root maggot, Hylemya brassicae (Bouche) in cabbage and rutabagas for eight consecutive years on one farm in Prince Edward Island without detectable indications of resktance. The insecticide is so placed in the soil as to be most concentrated against the young root maggot larvae and least concentrated against the following predators and parasites: Coenosia tigrina (Fall.) and Scatophaga stercoraria. (L.), which attack and destroy H. brassicae flies in flight, on the soil surface, or on plant foliage; various species of Carabid beetles which destroy H. brassicae eggs at or near the soil surface, Trybliographa rapae (L.), the larvae of which parasitize and destroy H. brassicae larvae in the plant roots; and Aleochara bilineata (Gyll.), with adults destroying H. brassicae eggs or young larvae near the soil surface, and larvae in the plant roots, and the larvae parasitising H. brassicae puparia and destroying the pupae. Records of field observations, supplemented with data obtained on green-house determinations of the potential reproduction of both predators and host, indicate that any one of these predators could theoretically eliminate the pest population from an area in two to three generations. They do not eliminate or even give apparent economic control of the pest because a) the aerial attackers do not find and destroy adults of the pest before many eggs are deposited in the soil; b) many eggs are hidden in the soil by wind and rain and thus protected from discovery by predators; and c) larval and puparial parasites attack after the pest has injured the crop.