Aversive responses of Queensland fruit flies towards larval-infested fruits are modified by fruit quality and prior experience.

For frugivorous fruit flies, the decision whether to accept or reject a host fruit for oviposition is influenced by a variety of fruit quality factors. Additionally, ovipositing flies may be influenced by the presence of eggs or larvae already within the host fruit. Species of the genus Bactrocera have been shown to avoid ovipositing into larval-infested fruits. However, the observed oviposition aversion in Bactrocera is variable, with some studies showing that deterrence to infested fruits may not always occur, but what may influence such variation is unknown. Using the polyphagous fruit fly Bactrocera tryoni (Froggatt), we tested if the quality of host fruit for offspring survival was a factor in influencing a female fly's decision whether to oviposit or not into larval-infested fruits. In both small cages and field cages, ovipositing B. tryoni did not discriminate between infested and non-infested high-quality fruits. However, when given a choice between poor-quality infested and non-infested fruits, significantly more flies selected and oviposited in non-infested fruits. For example, B. tryoni did not discriminate between infested and non-infested guava (a fruit in which there is high offspring survival), but more flies selected and oviposited on non-infested than on infested green apples (a fruit in which there is poor offspring survival). Small cage experiments also showed that prior oviposition experience on a larval-infested host negated the previously observed aversive response for that particular infested host fruit. The results are discussed in the light of a long recognised, but often ignored fact that herbivore host choice is about the sum of both the positive and negative cues received from the host.

Response of Male Queensland Fruit Fly (Diptera: Tephritidae) to Host Fruit Odors.

The surveillance and management of Dacini fruit fly pests are commonly split by fly gender: male trapping focuses on the dacine 'male-lures', whereas female trapping focuses on lures based on host-fruit volatiles. Although the males of several Dacini species have been reported to be attracted to host fruit volatiles, the option of using host-fruit traps for males has, to date, been ignored. Males of the cue-lure responsive fruit fly Bactrocera tryoni (Froggatt) have been recorded as responding to host-fruit volatile blends, but it is not known how frequently this happens, if it is age-dependent, or the strength of the response relative to cue-lure throughout the year. Here, we conducted an olfactometer experiment to test the lifetime (weeks 1-15) response of B. tryoni males to the odor of tomato, a known host of this fly, and compare catches of wild males to tomato-based traps and cue-lure traps in the field. Bactrocera tryoni males started to respond to tomato odor as they sexually matured (2 to 3 wk olds) and thereafter showed consistent olfactory response until advanced age (15 wk). In the field, wild males were captured by tomato-based traps throughout the year at a level not significantly different from cue-lure traps. The reason for the consistent B. tryoni male response to host fruit odor at this stage is not known, but it certainly occurs at a level greater than can be continued to be ignored for both basic and applied research.

The "sequential cues hypothesis": a conceptual model to explain host location and ranking by polyphagous herbivores.

Successfully locating a host plant is crucial for an insect herbivore to feed and/or oviposit. However, locating a host within a complex environment that may contain an array of different plant species is a difficult task. This is particularly the case for polyphagous herbivores, which must locate a host within environments that may simultaneously contain multiple suitable and unsuitable hosts. Here we review the mechanisms of host selection used by polyphagous herbivores, as well as exploring how prior experience may modify a generalist's response to host cues. We show that recent research demonstrates that polyphagous herbivores have the capacity to detect both common cues from multiple host species, as well as specific cues from individual host species. This creates a paradox in that generalists invariably rank hosts when given a choice, a finding at odds with the "neural limitations" hypothesis that says generalist insect herbivores should not have the neural capacity to identify cues specific to every possible host. To explain this paradox we propose a model, akin to parasitoid host location, that postulates that generalist herbivores use different cues sequentially in host location. We propose that initially common host cues, associated with all potential hosts, are used to place the herbivore within the host habitat and that, in the absence of any other host cues, these cues are sufficient in themselves to lead to host location. As such they are true "generalist" cues. However, once within the host habitat, we propose that the presence of a smaller group of cues may lead to further host searching and the location of preferred hosts: these are "specialist" cues. This model explains the current conflict in the literature where generalists can respond to both common and specific host-plant cues, while also exhibiting specialist and generalist host use behavior under different conditions.

Bringing Ecology Back: How Can the Chemistry of Indirect Plant Defenses Against Herbivory Be Manipulated to Improve Pest Management?

Research on insect-plant interactions has highlighted the intricacies of constitutive and induced plant defenses. Of particular interest has been the relationship of natural enemies (especially parasitic hymenoptera) to herbivore induced changes to plants, especially their responses to herbivore induced plant volatiles (HIPVs). In recent decades this has been a fertile area for research, with elegant experiments showing that HIPVs are important in attracting natural enemies to plants. We critically appraise the application of work on HIPVs in plant-insect-natural enemy interactions. The promise of applications to improve pest management has not been forthcoming. We attribute this to a failure to include the multifaceted aspects of natural enemy-prey interactions - attraction, location, subjugation and experience. Attraction in an olfactometer by naïve parasitoids has not been translated to methodologically sound field-based estimates of higher parasitism rates. We highlight what needs to be done to better understand the information that HIPVs convey, how this is utilized by parasitoids and how a greater understanding of these interactions might lead to the development of new strategies so that this knowledge can be effectively deployed for improved pest management.

Effects of single and dual species herbivory on the behavioral responses of three thrips species to cotton seedlings.

This study investigated the olfactory responses of 3 thrips species [Frankliniella schultzei Trybom, F. occidentalis Pergrande and Thrips tabaci Lindeman (Thysanoptera: Thripidae)] to cotton seedlings [Gossypium hirsutum L. (Malvales: Malvaceae)] simultaneously damaged by different combinations of herbivores. Cotton seedlings were damaged by foliar feeding Tetranychus urticae Koch (Trombidiforms: Tetranychidae), Helicoverpa armigera Hübner (Lepidoptera: Noctuidae), Aphis gossypii Glover (Hemiptera: Aphididae) or root feeding Tenebrio molitor L. (Coleoptera: Tenebrionidae). Thrips responses to plants simultaneously damaged by 2 species of herbivore were additive and equivalent to the sum of the responses of thrips to plants damaged by single herbivore species feeding alone. For example, F. occidentalis was attracted to T. urticae damaged plants but more attracted to undamaged plants than to plants damaged by H. armigera. Plants simultaneously damaged by low densities of T. urticae and H. armigera repelled F. occidentalis but as T. urticae density increased relative to H. armigera density, F. occidentalis attraction to coinfested plants increased proportionally. Thrips tabaci did not discriminate between undamaged plants and plants damaged by H. armigera but were attracted to plants damaged by T. urticae alone or simultaneously damaged by T. urticae and H. armigera. Olfactometer assays showed that simultaneous feeding by 2 herbivores on a plant can affect predator-prey interactions. Attraction of F. occidentalis to plants damaged by its T. urticae prey was reduced when the plant was simultaneously damaged by H. armigera, T. molitor, or A. gossypii and F. schultzei was more attracted to plants simultaneously damaged by T. urticae and H. armigera than to plants damaged by T. urticae alone. We conclude that plant responses to feeding by 1 species of herbivore are affected by responses to feeding by other herbivores. These plant-mediated interactions between herbivore complexes affect the behavioral responses of thrips which vary between species and are highly context dependent.

Effect of the postfeeding interval on olfactory responses of thrips to herbivore-induced cotton plants.

We investigated the responses of 3 thrips species, Frankliniella schultzei Trybom, F. occidentalis Pergrande, and Thrips tabaci Lindeman (Thysanoptera: Thripidae) to herbivore-damaged and undamaged cotton seedlings (Gossypium hirsutum L. [Malvales: Malvaceae]) at a range of time intervals following damage by adult Tetranychus urticae (Koch), adult T. ludeni (Zacher) (Acari: Tetranychidae) or Helicoverpa armigera (Hübner) (Lepidoptera: Noctuidae) larvae in olfactometer assays. The intensity/frequency of the response of thrips to herbivore-induced plants decreased with time and ultimately disappeared in all cases; however, the rate at which the response declined was related to the herbivore species that inflicted the damage. All 3 species of thrips were attracted to plants damaged by T. urticae for longer than they were to plants damaged by T. ludeni. The duration for which damaged plants remained attractive was also affected by the degree of damage inflicted on cotton seedlings. For example, F. schultzei was attracted to plants damaged by a higher density of two-spotted spider mites (100/plant) for much longer than to plants damaged by a lower density of these mites (50/plant). The results reinforce previous studies that demonstrate that arrangement of variables influences the responses of thrips to their herbivore-induced cotton host plants. Results also show that these responses are variable in time following herbivore damage to cotton plants, which further demonstrates how difficult it is to generalize about the functional significance of these interactions.

How predictable are the behavioral responses of insects to herbivore induced changes in plants? Responses of two congeneric thrips to induced cotton plants.

Changes in plants following insect attack are referred to as induced responses. These responses are widely viewed as a form of defence against further insect attack. In the current study we explore whether it is possible to make generalizations about induced plant responses given the unpredictability and variability observed in insect-plant interactions. Experiments were conducted to test for consistency in the responses of two congeneric thrips, Frankliniella schultzei Trybom and Frankliniella occidentalis Pergrande (Thysanoptera: Thripidae) to cotton seedlings (Gossypium hirsutum Linneaus (Malvales: Malvaceae)) damaged by various insect herbivores. In dual-choice experiments that compared intact and damaged cotton seedlings, F. schultzei was attracted to seedlings damaged by Helicoverpa armigera (Hübner) (Lepidoptera: Noctuidae), Tetranychus urticae (Koch) (Trombidiforms: Tetranychidae), Tenebrio molitor Linnaeus (Coleoptera: Tenebrionidae), F. schultzei and F. occidentalis but not to mechanically damaged seedlings. In similar tests, F. occidentalis was attracted to undamaged cotton seedlings when simultaneously exposed to seedlings damaged by H. armigera, T. molitor or F. occidentalis. However, when exposed to F. schultzei or T. urticae damaged plants, F. occidentalis was more attracted towards damaged plants. A quantitative relationship was also apparent, F. schultzei showed increased attraction to damaged seedlings as the density of T. urticae or F. schultzei increased. In contrast, although F. occidentalis demonstrated increased attraction to plants damaged by higher densities of T. urticae, there was a negative relationship between attraction and the density of damaging conspecifics. Both species showed greater attraction to T. urticae damaged seedlings than to seedlings damaged by conspecifics. Results demonstrate that the responses of both species of thrips were context dependent, making generalizations difficult to formulate.

Estimating the economic cost of one of the world's major insect pests, Plutella xylostella (Lepidoptera: Plutellidae): just how long is a piece of string?

Since 1993, the annual worldwide cost of diamondback moth, Plutella xylostella (L.) (Lepidoptera: Plutellidae), control has been routinely quoted to be US$1 billion. This estimate requires updating and incorporation of yield losses to reflect current total costs of the pest to the world economy. We present an analysis that estimates what the present costs are likely to be based on a set of necessary, but reasoned, assumptions. We use an existing climate driven model for diamondback moth distribution and abundance, the Food and Agriculture Organization country Brassica crop production data and various management scenarios to bracket the cost estimates. The "length of the string" is somewhere between US$1.3 billion and US$2.3 billion based on management costs. However, if residual pest damage is included then the cost estimates will be even higher; a conservative estimate of 5% diamondback moth-induced yield loss to all crops adds another US$2.7 billion to the total costs associated with the pest. A conservative estimate of total costs associated with diamondback moth management is thus US$4 billion-US$5 billion. The lower bound represents rational decision making by pest managers based on diamondback moth abundance driven by climate only. The upper estimate is due to the more normal practice of weekly insecticide application to vegetable crops and the assumption that canola (Brassica napus L.) is treated with insecticide at least once during the crop cycle. Readers can decide for themselves what the real cost is likely to be because we provide country data for further interpretation. Our analysis suggests that greater efforts at implementation of even basic integrated pest management would reduce insecticide inputs considerably, reducing negative environmental impacts and saving many hundreds of millions of dollars annually.