L. E. Perkins

Where to from here? The mechanisms enabling the movement of first instar caterpillars on whole plants using Helicoverpa armigera (Hübner)

Helicoverpa armigera (Hübner) (Lepidoptera: Noctuidae) is an economically-important, polyphagous herbivore in Old World countries. The distribution of larvae within various host plants has been described, but few studies have tried to determine the behavioural mechanisms by which the given distributions arose. Our aim was to determine the mechanisms which enable larval movement on pea plants, starting with first instars. Observations and bioassays determined larval movement in response to light and angled surfaces, as well as the effect of feeding and plant volatiles on these responses. The majority (68–72%) of 1st instars were positively phototactic towards blue, green and white light and 42% towards UV light. In the dark, larvae showed negative geotaxis. The angle of their substrate also had a kinetic effect on larvae; the steeper the angle from horizontal the more larvae moved under all conditions. Phenylacetaldehyde (a flower volatile) suppressed larval movement except at 90°. (Z)-3-Hexenyl acetate (a green leaf volatile) reversed the direction of movement at the flattest angle. Feeding lessened the probability of moving. We suggest that phototaxis and geotaxis are behaviours common to larval lepidopterans (caterpillars), and that these basic behaviours are modulated by environmental, larval, and plant factors to give observed distributions. Using a multinomial model approach, we created a flow chart to qualitatively and quantitatively represent the decision-making process of first instar H. armigera in response to the factors influencing movement.

Generalist insects behave in a jasmonate-dependent manner on their host plants, leaving induced areas quickly and staying longer on distant parts

Plants are sessile, so have evolved sensitive ways to detect attacking herbivores and sophisticated strategies to effectively defend themselves. Insect herbivory induces synthesis of the phytohormone jasmonic acid which activates downstream metabolic pathways for various chemical defences such as toxins and digestion inhibitors. Insects are also sophisticated animals, and many have coevolved physiological adaptations that negate this induced plant defence. Insect behaviour has rarely been studied in the context of induced plant defence, although behavioural adaptation to induced plant chemistry may allow insects to bypass the hosts defence system. By visualizing jasmonate-responsive gene expression within whole plants, we uncovered spatial and temporal limits to the systemic spread of plant chemical defence following herbivory. By carefully tracking insect movement, we found induced changes in plant chemistry were detected by generalist Helicoverpa armigera insects which then modified their behaviour in response, moving away from induced parts and staying longer on uninduced parts of the same plant. This study reveals that there are plant-wide signals rapidly generated following herbivory that allow insects to detect the heterogeneity of plant chemical defences. Some insects use these signals to move around the plant, avoiding localized sites of induction and staying ahead of induced toxic metabolites.

Towards a resistance management strategy for Helicoverpa armigera in Bt-cotton in northwestern China: an assessment of potential refuge crops

Transgenic Bt-cotton now dominates the cotton-growing belt in northwestern China where there are few natural plant refuges to act as sources of moths susceptible to Bt toxin. As an initial step towards developing an insect resistance management (IRM) plan for the pest moth, Helicoverpa armigera, on Bt-cotton in this part of China, we assessed the potential of six crops grown in two configurations to act as refuge hosts for susceptible moth production in both broad-acre and small-holder farms. Egg and larval abundance indicated that H. armigera had a preference for chickpea, pigeon pea, and corn, over cotton, sorghum, and benne (sesame). There were no significant differences in egg or larval abundance between plot and strip configurations of these crop hosts. We found that sorghum was not attractive to H. armigera, contrary to the findings in other cotton production areas. Moreover, chickpea, pigeon pea, and corn were determined to be potential refuge hosts based on egg and larval abundance over the growing season. From the adult moth population dynamics in broad-acre and small-holder farms, the efficiency of alternative refuge crops, and local agricultural practices, we recommend that spring corn be grown as a strip crop as part of an IRM strategy to improve the probability of Bt-susceptible moth production and mating with resistant moths in broad-acre farms. In small-holder farms where current agricultural practices are entrenched, wheat and summer corn should be evaluated as refuge crops in the future.

Effects of plant micro-environment on movement of Helicoverpa armigera (Hübner) larvae and the relationship to a hierarchy of stimuli

Locomotory behaviour of 1st instar Helicoverpa armigera is influenced by a complex of micro-attributes, the leaf “environment”, comprising odours, wax chemistry, trichomes and grip texture. Larval movement speeds on leaves of different types varied more than eight fold. On garden pea, Pisum sativum, there is a hierarchy of stimuli perceived by larvae resulting in differing behavioural responses. Light and angle are paramount in responses to micro-environment. These influence responses to local stimuli. Experiments in darkness produce different responses from those under laboratory light. In darkness, on horizontal surfaces as found for most leaves, preference for leaf surface is driven by micro-environment associated with leaf waxes. Larvae prefer the abaxial surface. In light, on horizontal surfaces, larvae seek enclosed spaces and foray under leaf surfaces. They wander more openly in the dark. Such information is important in building a model of larval behaviour and predicting behaviour on differing plant architectures.

Oviposition site selection and survival of susceptible and resistant larvae of Helicoverpa armigera (Lepidoptera: Noctuidae) on Bt and non-Bt cotton

In Australia Bt cotton has been planted since 1996, and has greatly improved the control of its key target Helicoverpa armigera (Hübner). There is no strong evidence that genetically modified cotton has been selected for significant physiological resistance to Bt toxin in field populations. There are many possible explanations for the lack of apparent selection that range from high compliance with the resistance management strategy for this technology to a lack of behavioral preference in key traits such as oviposition that could favor survival. To date most experiments that test oviposition of H. armigera on Bt cotton vs. conventional cotton have been done with susceptible moths. We determine the oviposition preference of a field isolated Bt resistant line of H. armigera and a susceptible counterpart when given a choice of non-Bt cotton and Bt-cotton with the same genetic background, and test whether there is any relationship between oviposition site selection (different plant structures) and the survival of the first instar larvae. Within cotton plants, our experiments consistently showed that both resistant and susceptible moths did not choose plants or plant parts that were less toxic in terms of Bt toxin on which to lay eggs. There was one exception in that susceptible moths were more likely to lay eggs on squares of Bt cotton plants than squares of non-Bt cotton. As expected, the mortality of susceptible H. armigera neonates was significantly higher on structures of Bt cotton plants than on those structures of conventional cotton, and survival was greater on flowers than on other structures of Bt cotton. This confirms opportunities for selection for resistance, and demonstrates no advantage in this respect to carrying resistance genes that might overcome the Bt toxins.

The movement and distribution of Helicoverpa armigera (Hübner) larvae on pea plants is affected by egg placement and flowering.

The distribution and movement of 1st instar Helicoverpa armigera (Hübner) (Lepidoptera: Noctuidae) larvae on whole garden pea (Pisum sativum L.) plants were determined in glasshouse trials. This economically-important herbivore attacks a wide variety of agricultural, horticultural and indigenous plants. To investigate the mechanisms underlying larval intra-plant movement, we used early-flowering and wild-type plant genotypes and placed eggs at different vertical heights within the plants, one egg per plant. Leaf water and nitrogen content and cuticle hardness were measured at the different plant heights. Of 92 individual larvae, 41% did not move from the node of eclosion, 49% moved upwards and 10% moved downwards with the distance moved being between zero and ten plant nodes. Larvae from eggs placed on the lower third of the plant left the natal leaf more often and moved further than larvae from eggs placed in the middle or upper thirds. The low nutritive value of leaves was the most likely explanation for more movement away from lower plant regions. Although larvae on flowering plants did not move further up or down than larvae on non-flowering plants, they more often departed the leaflet (within a leaf) where they eclosed. The final distribution of larvae was affected by plant genotype, with larvae on flowering plants found less often on leaflets and more often on stipules, tendrils and reproductive structures. Understanding intra-plant movement by herbivorous insects under natural conditions is important because such movement determines the value of economic loss to host crops. Knowing the behaviour underlying the spatial distribution of herbivores on plants will assist us to interpret field data and should lead to better informed pest management decisions.

The urticating setae of Ochrogaster lunifer, an Australian processionary caterpillar of veterinary importance

The bag‐shelter moth, Ochrogaster lunifer Herrich‐Schaffer (Lepidoptera: Notodontidae), is associated with a condition called equine amnionitis and fetal loss (EAFL) on horse farms in Australia. Setal fragments from O. lunifer larvae have been identified in the placentas of experimentally aborted fetuses and their dams, and in clinical abortions. The gregarious larvae build silken nests in which large numbers cohabit over spring, summer and autumn. The final instars disperse to pupation sites in the ground where they overwinter. Field‐collected O. lunifer larvae, their nests and nearby soil were examined using light and electron microscopy to identify setae likely to cause EAFL and to determine where and how many were present. Microtrichia, barbed hairs and true setae were found on the exoskeletons of the larvae. True setae matching the majority of setal fragments described from equine tissue were found on third to eighth instar larvae or exuviae. The number of true setae increased with the age of the larva; eighth instars carried around 2.0–2.5 million true setae. The exuvia of the pre‐pupal instar was incorporated into the pupal chamber. The major sources of setae are likely to be nests, dispersing pre‐pupal larvae and their exuviae, and pupal chambers.

Is the effect of priming plants, and a functional JAR1, negligible on the foraging behaviour and development of a generalist lepidopteran, Helicoverpa armigera?

Theory and recent literature suggest strong effects of induced plant defences in some plant herbivore systems. Few have studied behavioural effects on intact plants. Differences in foraging behaviour as well as weight gain were determined for first instar Helicoverpa armigera (Hübner) (Lepidoptera: Noctuidae) on Arabidopsis thaliana (L.) Heynh. (Brassicaceae) mutant and wild type plants, non‐primed, or primed by herbivore feeding or methyl jasmonate. The differences in feeding were primarily in the length of feeding time as opposed to the area fed on, feeding location, or frequency. More larvae dispersed from plants after priming by mite feeding than dispersed after caterpillar feeding. Other behavioural activities such as resting were not significantly affected. Early instars gained less weight feeding on ein2 (ethylene insensitive) mutant, but there was no difference in weight gain between larvae feeding on induced and non‐induced plants of the same type. We concluded that there are fitness consequences for neonates of the generalist H. armigera after feeding on induced plant tissues in some cases, and that distinct changes in behaviour are recognisable both at the fine scale and at grosser levels (dispersal). However, these changes are more subtle than might be expected.

Feeding behaviour and survival of Bacillus thuringiensis-resistant and Bacillus thuringiensis-susceptible larvae of Helicoverpa armigera (Lepidoptera: Noctuidae) exposed to a diet with Bacillus thuringiensis toxin

Although higher than expected numbers of surviving larvae of Helicoverpa spp. are reported in some Bollgard II cotton fields from time to time, there is no convincing evidence that field resistance has developed. A behavioural component, specifically the avoidance of the toxin, was considered to contribute to larvae surviving on Bacillus thuringiensis (Bt) toxin‐expressing plants. Experiments were conducted with Bt‐susceptible and Bt‐resistant larvae of Helicoverpa armigera to investigate (1) how larval responses to food deprivation might lead to differences in survival and therefore how far neonates could move to find a suitable feeding site and (2) whether larvae that are physiologically susceptible vs. resistant behave differently when feeding on diet with Bt and without Bt toxins. Bt‐susceptible neonates of H. armigera, which were significantly heavier, could survive without food for longer and recover better than Bt‐resistant neonates. Although H. armigera neonates did not shift their behaviour with respect to Bt toxin on artificial diet before their first feeding event, the tendency of Bt‐susceptible larvae to remain on non‐Bt diet and move off Bt diet may allow them to survive in a Bt environment. This behaviour resulted in a higher percentage of survival in situations where a diet choice was offered.

Subterranean termite (Blattodea: Termitoidae) pests in metropolitan Brisbane, Australia, 1997–2006: patterns and implications

The distribution and abundance of subterranean termites in Brisbane, Australia, has not previously been investigated, despite the sub‐tropical climate being favourable for these xylophagous insects which often cause damage to the built environment. Data on the incidence of termite infestations, including taxonomic identity, locality and site of infestation within premises were collected by a large commercial pest management company in metropolitan Brisbane during a ten‐year period (1997 – 2006). General Linear Models based on a negative binomial distribution were used to examine the influence of the climatic variables of rainfall and Southern Oscillation Index (SOI), and the demographic variables of premise density and socioeconomic status, on the number of termite infestations found. Two genera of termites dominated: Schedorhinotermes, which was the most abundant both indoors and outdoors, and Coptotermes. That Schedorhinotermes spp. were the major xylophagous subterranean termites recorded is unique in Australian capital cities where Coptotermes spp. usually dominate. The patterns of abundance and distribution of both genera are discussed with respect to their differing biology. SOI, premise density per ha and household income all significantly explained the variation in termite abundance reported across metropolitan Brisbane over the 10 year study. Our results shows the potential for using SOI as a predictor of termite activity. Further study is needed to more accurately correlate the key drivers of termite activity with the incidence of termite infestations in premises.