Concepts in Insect Host-Plant Selection Behavior and Their Application to Host Specificity Testing
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Proceedings: Host Specificity Testing of Exotic Arthropod Biological Control Agents: The Biological Basis for Improvement in Safety Concepts in Insect Host-Plant Selection Behavior and Their Application to Host Specificity Testing Tim A. Heard CSIRO Entomology Long Pocket Laboratories 120 Meiers Rd, Indooroopilly 4068 Brisbane, Australia Abstract Testing the host specificity of potential agents is an important part of biocontrol method- ology. An understanding of the behavioral processes involved in selection of a host plant can be used to improve the accuracy of host specificity testing by biocontrol practitioners and others interested in predicting field host use. These behavioral processes include the sequential nature of host selection behavior, the effects of experience, and time-dependent changes of host acceptance or rejection. Each of these three aspects of behavioral science is reviewed and its potential effect on the outcome of host testing is examined. The means by which practitioners can incorporate these concepts into the design, implementation and interpretation of host specificity tests are discussed. Practical matters affected by these issues include: (a) choice of arena size and design (e.g., small cages versus wind tunnels versus open field tests), (b) duration of tests, (c) use of behavioral observations to examine the process instead of the end result, and (d)interpretation of the results of choice vs no- choice tests, sequential versus parallel tests, and open field versus cage tests. Because of the diversity of behavioral factors and the inconsistent ways in which they can produce false results in host specificity tests, guidelines cannot be generalized. Hence, all biocontrol practitioners are encouraged to become familiar with the relevant concepts and apply them appropriately. Introduction Serious concerns about the non-target effects of behavioral concepts gives us an opportunity to improve biocontrol agents are increasingly being expressed by the design, conduct, and interpretation of host ecologists, the wider scientific community and specificity testing. Insect behavior is a large and fast biocontrol practitioners themselves (Thomas and Willis, moving area of research. The practice of host specificity 1998; Anonymous, 1999). Many of these warnings call testing has benefited much from such basic studies but for biocontrol practitioners to better understand the we can continue to fine tune testing methodology by effects of releases of biocontrol agents. The best single applying the latest information and concepts (Marohasy, way of predicting both direct and indirect non-target 1998; Withers et al., 1999; Withers et al., 2000). effects is to understand the host specificity of agents (see Biocontrol practitioners have argued that their testing is Secord and Kareiva, 1996). Host specificity testing sufficiently rigorous because they have made few provides the primary information for making decisions mistakes. Several recent examples show, however, that on whether to release an agent. the outcome of host specificity tests can be influenced Given the importance of host specificity testing, can we by behavioral phenomena that express themselves improve it? In this paper, I argue that the application of differently in tests of different designs. The mirid Concepts in Insect-Host Plant Selection Behvaior 1 Proceedings: Host Specificity Testing of Exotic Arthropod Biological Control Agents: The Biological Basis for Improvement in Safety Eucerocoris suspectus Distant completed development on Host Plant Selection several plant species including guava (Psidium guajava L.) in open field tests when all feeding sites on the target in Phytophagous Insects weed, the paperbark tree, Melaleuca quinquenervia Sequential Behavioral Responses (Cavier) Blake were destroyed. Extensive cage choice in Host Plant Selection tests and field surveys did not reveal any attack on guava (Purcell et al., 2000). This insect will not be released There is a long held and widely accepted view that against paperbark trees in Florida but could have been if insects use a sequence of behavioral responses in host the former tests were not done. Later I will show how selection. This was first recognized in parasitoids and behavioral factors were responsible for this serious later in phytophagous insects (e.g., Kennedy, 1965). disparity in the results of different types of tests. The sequence of steps in host selection includes habitat location, host location, host acceptance, and host use. A second example is of greater concern because the Insects use a number of sensory cues in host selection insect has already been released. Bruchidius villosus including visual, olfactory, gustatory, and tactile stimuli Fabricius, a seed bruchid, was recently released in New as well as humidity and light intensity (Bernays and Zealand and Australia against broom (Cytisus scoparius Chapman, 1994). These cues stimulate receptors, [L.]) but is attacking tagasaste (Chamaecytisus palmensis generating sensory input and finally behavioral [L. Fil.] Link), a non-target plant. This attack on responses. A large number of sensory receptors of tagasaste does not represent a host range expansion but a different modalities receive stimulation at each step in failure of host specificity testing to predict field host the host selection process. This information must be range (Fowler et al., 2000). Testing relied on choice tests processed and integrated by the central nervous system, alone and under the conditions of this test, tagasaste was interpreted as a positive or negative signal and a decision not attacked. Many examples are known of the made as to whether to make a certain behavioral expression of a broader host range in no-choice tests response. Courtney and Kibota (1990) critically review compared to choice tests (Hill et al., 1995; Marohasy, host plant selection for oviposition while Mayhew 1998). Behavioral factors generate these results. When (1997) reviews adaptive patterns of host plant selection. the design and interpretation of trials fail to recognize and understand these factors, problems can and do arise. Different species express high specificity at different Other studies show differences between host ranges stages in the host selection process. High specificity early measured in the field compared to the laboratory in the host selection process has been demonstrated in (Balciunas et al., 1996) but this is not always the case nature with Drosophila magnaquinaria Wheeler, which (Cordo et al., 1995). shows very high specificity to its habitat: wet, low-lying areas. Low levels of host specificity are expressed at later In this paper, I first review the proximate behavioral stages – pre-alighting attraction to cues from many plant factors that influence host acceptance and choice. I species occurs and larval survivorship on many divide the host selection behavior into: (1) sequential substrates is very high. However, high host specificity in behavioral responses in host plant selection (and use), the field occurs because skunk cabbage is the only (2) effects of time dependent factors, and (3) effects of suitable substrate in its preferred habitat (Kibota and experience. I treat the effects of time dependent factors Courtney, 1991). After habitat selection, distance cues very briefly as they are covered by Withers et al., 2000. are used in host location. For example, adult apple To illustrate the above points, I give an example of a maggot flies, Rhagoletis pomonella (Walsh), show positive hypothetical insect and follow its life and behavior. I responses to host odor within a few meters of the source then discuss how the behavioral factors can influence (Aluja and Prokopy, 1992). Post-alighting cues are the the results of host tests and what we can do to improve most important stage in host selection for some insects, our testing using this information. including the bean aphid, Aphis fabae Scopoli. This insect alights with equal frequency on host and non- host plants proving the lack of a role for pre-alighting cues. After contact with non-hosts, the aphids leave non-hosts but remain on hosts. Antennation of the leaf surface allows contact cues to be assessed (Kennedy et al., 1959). 2 Concepts in Insect-Host Plant Behavior Proceedings: Host Specificity Testing of Exotic Arthropod Biological Control Agents: The Biological Basis for Improvement in Safety It will be shown later that the testing methods that are prevents all feeding on the second encounter. Another best to determine the host specificity of agents will example is the positive response to a previously neutral depend on the behavioral stage in the agent’s host phagostimulant following contact with that selection sequence in which it expresses the greatest phagostimulant. Priming is a related concept that occurs specificity. where experience with an innate stimulus makes the insect more responsive to other stimuli such as other Experience and Learning foraging cues (Turlings et al., 1993). The effects of experience – learning, memory and Central excitation and central inhibition. Contact with forgetting – are important behavioral components in the a highly ranked host will increase the responsiveness and host selection process. Learning is the modification of readiness of an insect to oviposit or feed. Central behavior due to the effect of prior experience. Learning excitation is a similar effect to sensitization but is shorter can happen very quickly. The effects of experience can lived and