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Phytochemical responses to herbicide exposure and effects on herbivorous insects

10. Relationships between herbicide treatment of host plants and the performance of herbivorous insects

The present chapter tests whether the negative effects on leaf chewing herbivores found in the literature are a general phenomenon or it is characteristic for the specific plant-herbivore pair and the compound tested. Therefore, the present study evaluates three insect/plant interaction systems for their susceptibility to sulfonylurea herbicides. The three test systems are representative of species which could be exposed to sulfonylurea herbicides due to spray drift into other crops or into semi-natural habitats. The organism pairs used are Pieris brassicae/Brassica napus (hereafter Pieris), G. polygoni/F. convolvulus (hereafter Gastrophysa), and Sitobion avenae/Triticum aestivium (hereafter Sitobion), respectively. The test design differs between systems due to their different feeding guilds.

10.1 Methods and Materials

10.1.1 Pieris-Brassica test system

Brassica napus is an annual plant (either spring or winter annual). This species occurs both as crop, weed, and wild species with scattered distribution in disturbed habitats. Pieris brassicae is a pierid butterfly. The larvae feed mainly on plants of the Brassica family (Feltwell, 1982) and prefer foliage for consumption in all life stages. The egg clutches are normally so large that the host plant is totally defoliated and the larvae are forced to migrate to other host plants in order to complete larval development (Davies and Gilbert, 1985). This species pair may experience herbicide effects both in natural habitats and in crops exposed to spray drift from adjacent fields. Both acute and chronic tests were conducted with the insects.

Brassica napus in the vegetative stage (i.e. possessing 4 true leaves) was sprayed with herbicide. Four days after spraying, 20 newly hatched larvae of Pieris brassicae were placed on each of the plants. After four days, the weight gain and survival of the larvae were assessed. Each dosage consisted of three independently prepared replicates of three samples each (spray dosages and used pesticides are presented in Table 1). At the day of placing larvae on the plants, a randomised sample of 40 larvae was collected in order to measure the average size of the experimental animals. The fresh and dry weights of these animals were measured as eight samples of five individuals each. The experiments were conducted in a controlled-environment-chamber administered at 20°C, 70% RH, and 16 h photoperiod. Living specimens were counted daily, and on day 8 after spraying, the weight of the larvae was measured. This time span was chosen because a pilot experiment had shown that plants at the higher dosages loose their leaves shortly after this point in time. Survival was estimated as the fraction of larvae remaining on the plant on day 4 after placing the larvae on the plant in relation to the numbers present after one day. This was done because a single experiment (chlorsulfuron) lost a good deal of specimens over the first 24 hours even in the control treatment. Subsequently, this was examined in a small experiment, which showed that within a short time span of 1-2 hours after hatching the larvae are vulnerable to handling. The procedure of applying larvae to the plants was hereafter altered, and larval disappearance was avoided in the remaining experiments.

In a long-term (chronic) experiment, larvae were allowed to complete larval development. Brassica napus plants were sprayed in the vegetative stage (i.e. possessing 4 true leaves) with the sulfonylurea-herbicide metsulfuron-methyl at different dosages, i.e. 0, 0.05, 0.1 and 0.2 times the recommended field rate. Four days after spraying, 20 newly hatched larvae of Pieris brassicae were placed on the sprayed plants. The experiments was conducted in a controlled-environment-chamber administered at 20°C, an RH of 70%, and a 16 h photo period. At the time when the larvae started to move on the plant these were moved to cages in a greenhouse cell and remained here for the rest of their development.

The survival of the larvae was registered throughout development, and developmental time from egg hatch to adult stage was measured. Hereafter, biomass of the adults was determined.

Simultaneously with the acute insect bioassay, host plants without larvae were treated with the herbicides to assess effects on the plants. The set-up was similar with respect to soil type, temperature, etc., but the plants were treated with a broader range of dosages, in order to a establish dose - response relation for the plants. The plants were treated when they possessed four leaves. Each treatment dosage was sprayed in three independently prepared replicates with three plants per replicate. At the time of spraying, 20 plants of similar size as the experimental plants were weighed, i.e. measures of root and shoot biomass (both dry and fresh weight). On the eighth day after spraying, the experimental plants were harvested, both shoot and root biomass (fresh and dry weight) was measured.

10.1.2 Gastrophysa-Fallopia test system

F. convolvulus is a strictly annual weed species. It often climbs adjacent crop plants. Under greenhouse conditions, the plant reproduces mainly by self-fertilisation. F. convolvulus has no leaf loss during development, but continues to grow until senescence, when all leaves die almost synchronously and the seeds ripen. Details of the life cycle of F. convolvulus are presented by (Hume et al., 1983). Plants for experiments and insect food were grown in pots in a greenhouse. Seeds were harvested and stored cold to break dormancy before sowing. G. polygoni is a chrysomelid beetle utilising mainly two food plants, viz. F. convolvulus and Polygonum aviculare L. It eats the foliage of the plants in all three larval instars and as adults. The larvae pupate in the soil. The beetles were kept in culture on F. convolvulus plants at 20°C and 16 h photoperiod.

A controlled environment experiment was conducted to measure the suitability of herbicide-sprayed F. convolvulus plants as hosts for G. polygoni. Newly hatched larvae of G. polygoni were placed on leaves of plants subjected to different herbicide dosages. After spraying, the plants were caged singly in polystyrene cylinders and placed in a controlled-environment-chamber (photoperiod of 16 h, constant temperature of 20°C, relative humidity of 60%, and a photo flux density of 350 mE m^-2 sec^-1). One day after herbicide treatment, 20 newly hatched G. polygoni larvae were placed on the leaves of the plants. Each treatment was replicated four times. The larvae were placed on the lower side of leaves in the middle section of the plant according to the behaviour of ovipositing females (Kjær et al., 1998). Subsequently, the number of larvae and adults emerging after pupation were recorded every second or third day. When the adult beetles emerged, they were removed and weighed.

Simultaneously with the insect bioassay host plants without insects were treated with the herbicides to assess effects on the plants. The set-up was similar to the insect bioassay. At the time of spraying, shoot dry weight of seven plants of similar size as the experimental plants were measured. On day 16 after spraying, the experimental plants were harvested and shoot biomass was measured.

10.1.3 Sitobion-Triticum test system

Sitobion avenae is an aphid pest of cereal crops and sucks from the phloem of the host plants. This aphid prefers to feed in the upper part of the host plant, due to a better food quality (El-Sayed in Klingauf (1987)) ). Unlike broad-leaved weeds, cereal crops are tolerant to sulfonylurea herbicides. Therefore, no plant bioassay was performed. The difference between sensitive and tolerant species is based on their ability to metabolise the pesticide . It is possible that this metabolism evokes a reallocation of resources from storage organs to the leaves to the potential benefit of the aphids. It is therefore important to test if the use of herbicide may improve the performance of the aphids.

The effects of the sulfonylurea herbicide metsulfuron on the performance of the aphid Sitobion avenae were assessed in a feeding experiment in a controlled-environment-chamber (20°C, 16 h photoperiod and 20°C). Seeds of Triticum aestivium (var. Lambros) germinated in compost soil at 20°C. The day following herbicide treatment, one 0-24 h old aphid nymph was applied to each plant. Prior to application the nymphs were weighed. Plants and aphids were enclosed in glass cylinders (diameter of 5 cm) which were closed in the top with a 0.1 mm mesh. Hereafter, the aphids were observed two times a day to register developmental time until adult stage (D± 0.5 day). When the aphids had reached adult stage, they were weighed and placed on the plant again. Hereafter, the fecundity of the aphids were assessed in a period equal to two times the developmental time of the specific individual, i.e. D. Approximately halfway in this period all nymphs produced were killed so that the original adult could be recognised and nymph production from the F1-generation was avoided. Replicates in which the adult had disappeared were excluded from the measures of fecundity.

10.1.4 Spraying procedure

Plants were sprayed with a pot sprayer designed for automatic and controlled spraying of larger plants (Kristensen pot sprayer, Ringsted, Denmark; l ´ w ´ h = 120 ´ 100 ´ 170 cm). Table 10.1 gives the tested herbicides and the actual dosages. The trial with chlorsulfuron was conducted with the surfactant Citowett (BASF) added to the spray solutions (0.5% v/v). Control plants were treated with water. The sprayer was equipped with two Hardi flat fan nozzles type 411014 separated by 53 cm and used at a working pressure of 2 bar. The sprayer was calibrated to deliver a spray volume of 200 l ha^-1.

Table 10.1
Presentation of the dosages and herbicides used on the Pieris/Brassica system.