Field persistence of the entomopathogenic nematode Heterorhabditis bacteriophora in different crops

Abstract

To investigate nematode establishment and persistence, dauer juveniles (DJs) of Heterorhabditis bacteriophora were applied at 50 cm -2 in different crops in June and July with conventional spraying equipment and 420 l water ha -1 . Application hardly had any effects on survival and infectivity. The number of DJs reaching the soil was assessed and the establishment and persistence recorded by baiting soil samples with larvae of the wax moth Galleria mellonella. The better the plant canopy was developed the fewer DJs reached the soil during application. Whereas in pasture 77% and in potatoes 78% of the applied nematodes reached the soil, in wheat and peas little less than 50%, in oil-seed rape only 5% and in lupine 6% were recorded. Between 50 and 60% of the soil samples contained H. bacteriophora a month after application with the exception of wheat (>90%) and potatoes (<5%) indicating that the number of nematodes reaching the soil during application had no influence on their establishment in the soil. Probably DJs can survive in the plant canopy and reach the soil later after application. The percentage of nematode-positive soil samples dropped considerably after tillage. In potatoes no nematodes were recovered after two months, which probably was also due to the intensive movement of the soil. Although nematodes are susceptible to freezing, temperatures below 08C during the winter did not extinguish the H. bacteriophora population. In field crops EPN usually persisted not much longer than one year. The longest persistence of H. bacteriophora was detected 23 months after release in beans followed in rotation by wheat with red clover as cover crop. In this field larvae of the pea weevil Sitona lineatus (Coleoptera: Curculionidae) were detected in soil samples and found infected with the released nematode population. In the laboratory the field soils were tested for persistence of H. bacteriophora at 88C and a half-life of 24.8 days was recorded in the absence of host insects and plants. Thus long-term persistence in the field was a result of recycling in host insects, which could not be detected in other crops than beans and clover. As H. bacteriophora seems to be restricted in its host potential, this species disappears after release once the host population is not available anymore.

Prospects for Using Entomopathogenic Nematodes to Control the Vine Mealybug, Planococcus ficus, in South African Vineyards

“…On the other hand, the large decline in the percentage of mortalities in the Block B vineyard between four and 12 weeks post-application may be due to the fact that, between these two periods, tillage had occurred between the grapevine rows. Work done by Entomopathogenic Nematodes to Control the Vine Mealybug Susurluk and Ehlers (2008) found that ploughing caused a decline in H. bacteriophora persistence.…”

“…As a result, a laboratory persistence trial for both H. zealandica and S. yirgalemense was conducted in the hope that comparable results could be produced for current and future studies. When investigating the persistence of nematodes in the field, results could possibly be influenced by the availability of insect hosts (Susurluk & Ehlers, 2008). Consequently, in-field persistence studies will not be able to distinguish between the persistence of the released population of EPNs and the continuation of offspring from the released population, otherwise known as recycling (Curran, 1993;Susurluk & Ehlers, 2008).…”

Behaviour and Population Dynamics of Entomopathogenic Nematodes Following Application

“…The percentage of Heterorhabditis bacteriophora Poinar (Rhabditida: Heterorhabditidae) IJs reaching soil level immediately after spraying ranged from 5 to 6 % in dense canopy crops (oilseed rape and lupine) to 77-78 % in pasture and potatoes (Susurluk & Ehlers, 2008). As this was measured by placing Petri dishes on the soil it probably overestimates the numbers actually reaching the soil in a pasture with dense thatch, which can form a significant barrier to EPN dispersal (Zimmerman & Cranshaw, 1991).…”

Selective breeding of entomopathogenic nematodes for enhanced attraction to a root signal did not reduce their establishment or persistence after field release

“…and help in developing the experimental set-ups. The fieldwork was possible thanks to the hospitality of the Plant Health Service in Hodmezovasarhely in Hungary, factors such as soil type, 23 humidity, 24 temperature 25 or pH. 24 But the main factor that is thought to determine long-term persistence in the field is the presence of available host insects.…”

“…24 But the main factor that is thought to determine long-term persistence in the field is the presence of available host insects. 25 In field trials in Hungary, three EPN species, H. bacteriophora, H. megidis and Steinernema feltiae, were released to test their control potential against WCR. They all persisted at least as long WCR were present in soil, during the same year.…”