Solomon Balagawi

Guild-specific patterns of species richness and host specialization in plant-herbivore food webs from a tropical forest

1. The extent to which plant-herbivore feeding interactions are specialized is key to understand the processes maintaining the diversity of both tropical forest plants and their insect herbivores. However, studies documenting the full complexity of tropical plant-herbivore food webs are lacking. 2. We describe a complex, species-rich plant-herbivore food web for lowland rain forest in Papua New Guinea, resolving 6818 feeding links between 224 plant species and 1490 herbivore species drawn from 11 distinct feeding guilds. By standardizing sampling intensity and the phylogenetic diversity of focal plants, we are able to make the first rigorous and unbiased comparisons of specificity patterns across feeding guilds. 3. Specificity was highly variable among guilds, spanning almost the full range of theoretically possible values from extreme trophic generalization to monophagy. 4. We identify guilds of herbivores that are most likely to influence the composition of tropical forest vegetation through density-dependent herbivory or apparent competition. 5. We calculate that 251 herbivore species (48 of them unique) are associated with each rain forest tree species in our study site so that the ∼200 tree species coexisting in the lowland rain forest community are involved in ∼50,000 trophic interactions with ∼9600 herbivore species of insects. This is the first estimate of total herbivore and interaction number in a rain forest plant-herbivore food web. 6. A comprehensive classification of insect herbivores into 24 guilds is proposed, providing a framework for comparative analyses across ecosystems and geographical regions.

Evolution of lure response in tephritid fruit flies: phytochemicals as drivers of sexual selection

The males of many Bactrocera species (Diptera: Tephritidae) respond strongly and positively to a smallnumber of plant-derived chemicals (¼ male lures). Males that have imbibed the lures commonly havea mating advantage over unfed males, but no female benefits have been demonstrated for femalesmating with lure-fed males. It has been hypothesized that the strong lure response is a case of runawayselection, where males receive direct benefits and females receive indirect benefits via ‘sexy sons’,ora case of sensory bias where females have a lower threshold response to lures. To test these hypotheseswe studied the effects of lure feeding on male mating, remating and longevity; while for females that hadmated with lure-fed males we recorded mating refractoriness, fecundity, egg viability and longevity. Weused Bactrocera tryoni as our test animal and as lures the naturally occurring zingerone and chemicallyrelated, but synthetic chemical cuelure. Feeding on lures provided direct male benefits in greater matingsuccess and increased multiple mating. For the first time, we recorded direct female effects: increasedfecundity and reduced remating receptivity. Egg viability did not differ in females mated with lure-fed orunfed males. The life span of males and females exposed to lures was reduced. These results reveal direct,current-generation fitness benefits for both males and females, although the male benefits appeargreater. We discuss that while lure response is indeed likely to be a sexual selection trait, there is no needto invoke runaway selection to explain its evolution. 2013 The Association for the Study of Animal Behaviour. Published by Elsevier Ltd. All rights reserved.

Host specialization and species richness of fruit flies (Diptera: Tephritidae) in a New Guinea rain forest

Frugivorous dacine fruit flies were studied in a lowland tropical rain forest in Papua New Guinea to determine their host specificity, abundance, and the number of species attacking various plant species. Plant species hosted 0-3 fruit fly species at median (1-3 quartile) densities of 1 (0-17) fruit flies per 100 fruits. Fruit flies were mostly specialized to a single plant family (83% species) and within each family to a single genus (88% species), while most of the species (66%) were able to feed on >1 congeneric plant species. Only 30 from the 53 studied plant species were colonized by fruit flies. The plant-fruit fly food web, including these 30 plant species and the total of 29 fruit fly species feeding on them, was divided into 14 compartments, each including 1-8 plant species hosting mutually disjunct assemblages of fruit flies. This structure minimizes indirect interactions among plant species via shared herbivores. The local species pool was estimated at 152 ± 32 (± SE) fruit fly species. Forty per cent of all taxonomically described species known from Papua New Guinea were reared or trapped in our study area. Such a high proportion indicates low beta-diversity of fruit flies. Steiner traps were highly efficient in sampling the lure-responsive fruit fly species as they re-collected 84% of all species trapped in the same area 5 y before. Fruit fly monitoring by these traps is a cheap, simple and efficient method for the study of spatial and temporal changes in rain-forest communities.

Simultaneous tests of the preference-performance and phylogenetic conservatism hypotheses: is either theory useful?

The preference-performance and phylogenetic conservatism hypotheses have been postulated to explain the mechanisms driving host-use patterns of phytophagous insects. The preference-performance hypothesis predicts that insects will use plants that provide higher offspring fitness, while the phylogenetic conservatism hypothesis predicts that insects will use phylogenetically closely related plants over more distantly related plants. Although some studies have supported these two hypotheses, others have not. Simultaneous tests of the two hypotheses on more than one species are lacking, and this limits comparative interpretation of previous studies. We undertook a comparative investigation to determine whether preference-performance and/or the phylogenetic conservatism hypothesis can explain host-use patterns of two phytophagous insects, the fruit flies Bactrocera cucumis and B. tryoni. Within a nested, plant phylogenetic framework, oviposition preference and offspring performance of the two fruit fly species were tested on fruits of plant species from across different plant families, from within a family and across cultivars within a species. The results show that both the preference-performance and the phylogenetic conservatism hypotheses can, depending on the host plant taxonomic level, explain host usage patterns in B. cucumis, while neither theory explained the host patterns seen in B. tryoni. In the light of increasing recognition of the complexity of host plant–herbivore relationships, and of ongoing studies which as often as not fail to find support for these theories as those that do, we discuss the limited value of either theory as a basis for future research.

Multiple mating by females of two Bactrocera species (Diptera: Tephritidae: Dacinae)

Multiple mating was investigated in two tephritid species when females were under minimal male pressure because they were each confined with a single male in cages 20 x 20 x 20 cm and observed daily until they died. Laboratory-reared females of Bactrocera cucumis (French) lived up to 274 days and refractory periods averaged 59-63 days. However, the distribution of matings among B. cucumis females was not significantly different to that expected by chance. Wild females of Bactrocera cacuminata (Hering) reared from field-collected fruits of Solanum mauritianum Scopoli lived for up to 134 days and mated up to three times with refractory periods between matings averaging 27-39 days. The distribution of the number of matings among females of B. cacuminata was non-random because of the high proportion of non-maters (50%); but, when only females mating more than once were considered, there was no significant departure from random expectation.

Nutritional status and the foraging behaviour of Bactrocera tryoni with particular reference to protein bait spray

Poisoned protein baits comprise a recognized method for controlling tephritid fruit flies in the form of a ‘lure‐and‐kill’ technique. However, little is known about how a flys internal protein and carbohydrate levels (i.e. nutritional status) might influence the efficacy of this control. In the present study, the relationships between the internal levels of protein (as measured by total body nitrogen) and carbohydrate (as measured by total body carbon) of the fruit fly Bactrocera tryoni (Froggatt) (Diptera: Tephritidae) are investigated, as well as its foraging behaviours in response to protein, fruit and cue‐lure (a male‐specific attractant) baits. Small cage behavioural experiments are conducted using flies from cultures of different nutritional status and wild flies sampled from the field during the fruiting cycle of a guava crop. For female flies, increasing total body nitrogen is correlated with decreased protein foraging and increased oviposition activity; increasing total body carbon levels generate the same behavioural changes except that the oviposition response is not significant. For males, there are no significant correlations between changes in total body nitrogen and total body carbon and protein or cue‐lure foraging. For wild flies from the guava orchard, almost all of them are sexually mature when entering the crop and, over the entire season, total body nitrogen and total body carbon levels are such that protein hunger is unlikely for most flies. The results infer strongly that the requirements of wild, sexually mature flies for protein are minimal and that flies can readily gain sufficient nutrients from wild sources for their physiological needs. The results offer a mechanistic explanation for the poor response of male and mature female fruit flies to protein bait spray.

Resting sites, edge effects and dispersion of a polyphagous Bactrocera fruit fly within crops of different architecture

The spot or strip application of poisoned protein bait is a lure‐and‐kill technique used for the management of fruit flies. Knowledge of where flies occur in the crop environment is an important part of maximizing the efficacy of this tool. Bactrocera tryoni is a polyphagous pest of horticulture for which very little is known about its distribution within crops. With particular reference to edge effects, we monitored the abundance of B. tryoni in two crops of different architecture; strawberry and apple. In strawberries, we found more flies on the crop edge early in the fruiting season, which lessened gradually and eventually disappeared as the season progressed. In apple orchards, no such edge effect was observed and flies were found equally throughout the orchard. We postulated these differences may be due to differences in crop height (high vs. short) and/or crop canopy architecture (opened and branched in apple, dense and closed in strawberry). In a field cage trial, we tested these predictions using artificial plants of different height and canopy condition. Height and canopy structure type had no significant effects on fly oviposition and protein feeding, but the ‘apple’ type canopy significantly influenced resting. We thus postulate that there was an edge effect in strawberry because the crop was not providing resting sites and flies were doing so in vegetation around the field margins. The finding that B. tryoni shows different resting site preferences based on plant architecture offers the potential for strategic manipulation of the fly through specific border or inter‐row plantings.