Effects of different herbicides on host plant quality for phytophagous insects
Martin Geisthardt, Diploma thesis
Study Description
Phytophagous insects fulfill an important function in the food chain, since they act as a link between primary producers and organisms at higher trophic levels. In the agricultural landscape, phytophagous insects are threatened due to destruction of their habitats and the intensive use of pesticides. The latter can directly affect the insects (insecticides) or remove their food resources (herbicides).
Another possible threat has attracted less attention so far. Plants which are exposed to subletal herbicide dosages (for example due to pesticide inputs via spray drift in field margins) might show a stress reaction and change their value as host plant for phytophagous insects.
For instance, herbicides can change the content of secondary metabolites in plants. Many secondary plant metabolites are known to be defense compounds during plant stress, some are especially produced under feeding pressure and act as feeding deterrents or toxins. The few previous studies concerning this topic revealed different possible effects: Sap-sucking insects seemed to benefit from herbicide treatment of their host plants, whereas reactions of other insect groups like folivores are less well investigated. However, folivorous insects seemed to be either not affected or showed detrimental effects like increased mortality. Since the tested herbicides did not show a direct insect toxicity, the detrimental effects might be caused by a stress reaction of the plants, induced or enhanced by the herbicide.
Due to the different observed effects of herbicides on host plant quality and to get more information about the underlying mechanisms, it is important to test further combinations of herbicides, phytophagous insects, and their host plants.
Therefore, young Mamestra brassicae caterpillars (5 days old) were reared on different host plants (Plantago lanceolata, P. major, Ranunculus acris) which were treated, beforehand, with sublethal (and field margin relevant) dosages of two herbicides, Atlantis WG (Mesosulfuron-methyl, Iodosulfuron-methyl-sodium) and Roundup LB Plus (glyphosate). Weight of the caterpillars and their development time to adults were assessed for each plant-herbicide combination.
Caterpillars feeding on R. acris treated with the herbicide Atlantis WG showed significantly lower weights in comparison to caterpillars feeding on untreated control plants. This indicates a reduced host plant quality possibly caused by defence components produced in the plants following the herbicide application.
