Effects of a simple plant morphological mutation on the arthropod community and the impacts of predators on a principal insect herbivore

Abstract

Plant features that enhance predator effectiveness can be considered extrinsic-resistance factors because they result in reduced insect herbivory. In this paper we test the hypothesis that reduced epicuticular wax (EW) in Pisum sativum L. is an extrinsic-resistance factor contributing to field resistance to Acyrthosiphon pisum (Harris). We monitored pea aphid populations in the field on reduced EW and normal EW near isolines of peas for two seasons and confirmed that aphid populations are lower on reduced EW peas than on normal EW peas. We also monitored predators within the canopies of the two pea lines to discover community level patterns in response to differences in EW. We found that while predator numbers were similar between the two lines, there were more syrphids on the normal EW peas, and a trend towards more coccinellids on reduced EW peas. We tested the impact of predators on pea aphids on the two EW lines by monitoring their population levels in cages that excluded predators, and in cages that allowed predators to enter. We found that pea aphid populations were similar on the two EW lines when predators were excluded. When predators were allowed access to the plants, pea aphid populations were reduced more on reduced EW peas than on normal EW peas. We also examined the intrinsic resistance to aphids in reduced EW peas with laboratory dual-choice tests comparing aphid response to reduced EW and normal EW peas, and found that walking, apterous aphids displayed no preference for one pea line over the other. Bioassays to measure growth and fecundity of the pea aphid on the two EW types in the greenhouse and in the field showed that intrinsic rate of increase, and other life table parameters, were not different for aphids on the two lines. Together these results support the hypothesis that reduced EW in peas is a predator-dependent extrinsic resistance factor. Genetically reducing EW bloom in peas and other waxy crop plants might improve the effectiveness of arthropod natural enemies of insect pests. More generally, the results show that a subtle change in plant morphology can substantially influence the impact of predators on insect herbivore populations. The benefit of extrinsic resistance to herbivory conferred by reduced EW may balance any benefits of a prominent EW bloom, thereby sustaining EW polymorphisms in some natural plant populations.

Delineating the effects of a plant trait on interactions among associated insects

“…In North America, a complex of predators, parasitoids, and pathogens attacks the pea aphid (Fluke 1929). Several of these are more effective at capturing pea aphids on reduced wax peas (Eigenbrode et al 1998a, unpublished data;White and Eigenbrode 2000b;Rutledge et al 2003). The lady beetle Hippodamia convergens is the most abundant predator of aphids on the foliage of peas (White and Eigenbrode 2000a;Rutledge et al 2003) and more generally in the region of our study (Elberson 1992;Youssef 2000).…”

“…Plant wax had no direct effect on aphid density in the greenhouse, but fewer aphids were found on reduced wax plants in the 2002 field experiment. A previous experiment on reduced and normal wax pea found a trend toward poorer aphid survival on reduced wax pea in field clip cages (Rutledge et al 2003). Rutledge et al (2003) suggested that reduced wax peas suffer higher drought stress than normal peas, which could explain why reduced wax peas grew less rapidly than normal peas in 2002 (Fig.…”

“…In field trials conducted over several years, reduced wax peas have consistently hosted lower densities of pea aphids than normal peas (White and Eigenbrode 2000b;Rutledge et al 2003; Table 1). Natural enemies of the pea aphid are one reason why aphid densities on the two pea lines differ.…”

Intraguild predation and successful invasion by introduced ladybird beetles

“…Self-sustaining populations were first located near port cities rather than near release points (Day et al 1994;Wheeler and Stoops 1996). Coccinella septempunctata reached eastern Washington and northern Idaho in the early 1990s (Elberson 1992), and by 1998 C. septempunctata had almost completely replaced the native Coccinella species, with C. septempunctata representing 20-34% of the coccinellid community and C. transversoguttata less than 1% (Youssef 2000;White and Eigenbrode 2000;Rutledge et al 2003). However, the native H. convergens appeared to be more tolerant of C. septempunctata, and still represented 65-80% of the coccinellid community following invasion by C. septempunctata (Youssef 2000;White and Eigenbrode 2000).…”

Coccinellids in Biological Control

“…Similarly, Eigenbrode et al (1998) showed that adult Hip. convergens foraged more effectively on pea cultivars with reduced epicuticular wax and Rutledge et al (2003) found that such cultivars hosted lower pea aphid populations in fi eld plots than isolines that lacked this trait. Comparing these isolines in cage and laboratory tests without predators revealed no differences between them in plant acceptance by aphids or their subsequent performance.…”