P.C.J. van Rijn

Plant-provided food for carnivorous insects : a protective mutualism and its applications

Plants provide insects with a range of specific foods, such as nectar, pollen, and food bodies. In exchange, they may obtain various services from arthropods. The role of food rewards in the plant–pollinator mutualism has been broadly covered. This book addresses another category of food-mediated interactions, focussing on how plants employ foods to recruit arthropod ‘‘bodyguards’’ as a protection against herbivores.

Comparative life history studies of Frankliniella occidentalis and Thrips tabaci (Thysanoptera: Thripidae) on cucumber

Shortly after its invasion into Europe, the western flower thrips, Frankliniella occidentalis (Pergande), became a more severe pest of greenhouse crops than the onion thrips, Thrips tabaci Lindeman. To test whether this differential pest status was due to a larger capacity of population increase, a comparative life history study was carried out on cucumber ( Cucumis sativus cv. Corona). Experiments at 25°C showed that the egg-to-egg period of F. occidentalis was shorter, but its peak ovipositional rate was lower and its offspring sex ratio more male biased. These differences resulted in a slightly lower intrinsic rate of population increase (r m ) for F. occidentalis than for T. tabaci (0.166 vs. 0.176/day). It was shown experimentally that between 15 and 28°C, developmental rate of F. occidentalis is linearly related to temperature, with a theoretical threshold temperature similar to the value reported for T. tabaci (10.9 vs. 11.5°C). It is argued that the r m -value of F. occidentalis will not be higher than that of T. tabaci for any temperature within this range. Alternative explanations for the difference in pest status between the two thrips species are discussed.

Preselection of predatory mites to improve year-round biological control of western flower thrips in greenhouse crops

In spring and summer, two groups of natural enemies are successfully used for biological control of western flower thrips, Frankliniella occidentalis (Pergande) in greenhouses: phytoseiid mites (Amblyseius cucumeris (Oudemans) and, to a lesser extent, A. barkeri (Hughes)) and anthocorid bugs (Orius spp.). During winter, however, these predators often fail to control the pest. One likely cause for failure is the predators tendency to enter diapause under short day conditions. In addition, eggs of predatory mites are generally susceptible to low humidity conditions, which often arise in greenhouses when outside temperatures drop below zero, or at bright, hot days in summer. In search for a thrips predator that is not hampered by these conditions, five subtropical phytoseiid species were selected which were known to feed on thrips: A. hibisci (Chant), A. degenerans Berlese, A. limonicus s.s. Garman and McGregor, A. scutalis (Athias‐Henriot) and A. tularensis (Congdon). These species were compared to A. cucumeris and A. barkeri, with respect to the following features: (1) predation and oviposition rate with young F. occidentalis larvae as prey, (2) oviposition rate on a diet of sweet pepper pollen, (3) drought tolerance spectrum of eggs, and (4) incidence of reproductive diapause under short day conditions. The results showed that A. limonicus exhibited the highest predation and oviposition rates on a diet of thrips larvae. Moreover, A. limonicus females showed total absence of diapause under the conditions tested. A major disadvantage of this species was, however, that its eggs were most sensitive to low air humidity conditions. Least sensitive to low air humidity were eggs of A. degenerans and A. hibisci. Females of A. degenerans and A. hibisci also showed total absence of diapause, and intermediate rates of predation and oviposition, on both thrips larvae and pollen. In conclusion, we argue that A. degenerans and A. hibisci are the most promising candidates for biological control of F. occidentalis under conditions of low humidity and short day length. The success of these candidates remains to be shown in greenhouse experiments.

The contribution of extrafloral nectar to survival and reproduction of the predatory mite Iphiseius degenerans on Ricinus communis

The phytoseiid mite Iphiseius degenerans (Berlese) is an effective predator of western flower thrips, Frankliniella occidentalis (Pergande), in Dutch greenhouses. In the Mediterranean area, castor bean, Ricinus communis L., is known as a year-round host plant for this predatory mite. On flowering castor bean plants in greenhouses, I. degenerans can be found in densities of more than 100 per leaf. For this reason, the plant is being used as a ‘banker’ plant to augment biological control. It has been shown that pollen produced by the large apical flowers sustains reproduction and development for these mites. The objective of this study was to measure the contribution of the extrafloral nectar of this plant to the reproductive success of this predatory mite. A study conducted at 25°C in presence of free water showed that (1) I. degenerans is unable to develop beyond the protonymphal stage when fed only nectar and leaf tissue, (2) its ovipositional rate is higher when pollen is supplemented with nectar, (3) its reproduction ceases within a few days when fed on nectar only, but the predator can survive for several weeks and resume oviposition when fed pollen again and (4) the feeding of young females for one or two weeks with nectar only extends their longevity by approximately the same period and only slightly diminishes their lifetime reproductive potential (R0), as compared to mites continuously fed pollen. It can be concluded that extrafloral nectar can provide an important contribution to population growth and maintenance of I. degenerans on R. communis, particularly in pre- and post-blooming periods. Assuming these predators are beneficial to the plant in clearing them of herbivorous mites and thrips, this relationship may be regarded as an example of plant–predator mutualism. The combination of pollen and extrafloral nectar makes castor bean an ideal rearing and banker plant for I. degenerans.

Kin recognition by the predatory mite Iphiseius degenerans: discrimination among own, conspecific, and heterospecific eggs

1. Kin recognition is important in many social insects, but has also been found in several nonsocial insects such as parasitoids, where it plays an important role in oviposition behaviour. In nonparasitic arthropods, however, the fitness of ovipositing females also depends on the oviposition behaviour of related and unrelated females, especially when eggs are oviposited in clusters by several females.