Virulence of the insect-pathogenic fungi Metarhizium spp. to Mormon crickets, Anabrus simplex (Orthoptera: Tettigoniidae)

The Mormon cricket (MC), Anabrus simplex Haldeman, 1852 (Orthoptera: Tettigoniidae), has a long and negative history with agriculture in Utah and other western states of the USA. Most A. simplex populations migrate in large groups, and their feeding can cause significant damage to forage plants and cultivated crops. Chemical pesticides are often applied, but some settings (e.g. habitats of threatened and endangered species) call for non-chemical control measures. Studies in Africa, South America, and Australia have assessed certain isolates of Metarhizium acridum as very promising pathogens for Orthoptera: Acrididae (locust) biocontrol. In the current study, two isolates of Metarhizium robertsii, one isolate of Metarhizium brunneum, one isolate of Metarhizium guizhouense, and three isolates of M. acridum were tested for infectivity to MC nymphs and adults of either sex. Based on the speed of mortality, M. robertsii (ARSEF 23 and ARSEF 2575) and M. brunneum (ARSEF 7711) were the most virulent to instars 2 to 5 MC nymphs. M. guizhouense (ARSEF 7847) from Arizona was intermediate and the M. acridum isolates (ARSEF 324, 3341, and 3609) were the slowest killers. ARSEF 2575 was also the most virulent to instar 6 and 7 nymphs and adults of MC. All of the isolates at the conidial concentration of 1 × 107 conidia ml−1 induced approximately 100% mortality by 6 days post application of fungal conidia. In conclusion, isolates ARSEF 23, ARSEF 2575, and ARSEF 7711 acted most rapidly to kill MC under laboratory conditions. The M. acridum isolates, however, have much higher tolerance to heat and UV-B radiation, which may be critical to their successful use in field application.

Elevational Changes in Mormon Cricket Life Histories: Minimum Temperature for Nymphal Growth Declines With Elevation

As the mean temperature and the duration of the growing season decline with elevation, growth of immature insects should initiate at a lower temperature, but it should also be faster to complete development prior to season’s end. Although flightless, Mormon crickets migrate in large aggregations across broad spatial and elevational distances that might limit adaptations to local environments. In addition, selection to be active at cooler temperatures might limit selection to maximize growth rate. I measured growth rate in controlled environments for nymphs from three populations that vary in altitude (87–2,688 m) but are similar in latitude (43.2–45.7°N). Growth rate increased significantly with mean rearing temperature between 22 and 30°C. The intercept of the regression of growth rate on temperature increased with elevation, whereas the slope did not change significantly. For any given rearing temperature, growth rate increased with elevation, which suggests that selection to initiate growth at cooler temperatures did not compromise growth rate. Body mass did not differ between the two lower elevations, whereas the highest elevation population had smaller hatchlings and adults. Critical thermal minimum (base temperature) declined with elevation (0.7°C per 1,000 m), and the degree days were 509 across all elevations. For pest management, a base temperature from midelevation of 15.3°C (60°F) and growing degree days of 509 (equivalent to 916 Fahrenheit-based degree days) are reasonable estimates for applications from sea level to 2,700 m.

Structure, culture, and predicted function of the gut microbiome of the Mormon cricketAnabrus simplex(Orthoptera: Tettigoniidae)

SUMMARYThe gut microbiome of insects plays an important role in their ecology and evolution, participating in nutrient acquisition, immunity, and behavior. Microbial community structure within the gut is heavily influenced by differences among gut regions in morphology and physiology, which determine the niches available for microbes to colonize.We present a high-resolution analysis of the structure of the gut microbiome in the Mormon cricketAnabrus simplex,an insect known for its periodic outbreaks in the western United States and nutrition-dependent mating system. The Mormon cricket microbiome was dominated by eleven taxa from the Lactobacillaceae, Enterobacteriaceae, and Streptococcaeae. While most of these were represented in all gut regions, there were marked differences in their relative abundance, with lactic-acid bacteria (Lactobacillaceae) more common in the foregut and midgut and enteric (Enterobacteriaceae) bacteria more common in the hindgut.Differences in community structure were driven by variation in the relative prevalence of three groups: aLactobacillusin the foregut,Pediococcuslactic-acid bacteria in the midgut, andPantoea agglomerans, an enteric bacterium, in the hindgut. These taxa have been shown to have beneficial effects on their hosts in insects and other animals by improving nutrition, increasing resistance to pathogens, and modulating social behavior.Using PICRUSt to predict gene content from our 16S rRNA sequences, we found enzymes that participate in carbohydrate metabolism and pathogen defense in other orthopterans. These were predominately represented in the hindgut and midgut, the most important sites for nutrition and pathogen defense.Phylogenetic analysis of 16S rRNA sequences from cultured isolates indicated low levels of divergence from sequences derived from plants and other insects, suggesting that these bacteria are likely to be exchanged between Mormon crickets and the environment.Our study shows strong spatial variation in microbiome community structure, which influences predicted gene content and thus the potential of the microbiome to influence host function.

Mating promotes lactic-acid gut bacteria in a gift-giving insect

Mating is a ubiquitous social interaction with the potential to influence the microbiome by facilitating transmission, modifying host physiology, and in species where males donate nuptial gifts to females, altering diet. We manipulated mating and nuptial gift consumption in two insects that differ in nuptial gift size, the Mormon cricket Anabrus simplex and the decorated cricket Gryllodes sigillatus, with the expectation that larger gifts are more likely to affect the gut microbiome. Surprisingly, mating, but not nuptial gift consumption, affected bacterial community structure, and only in Mormon crickets. The change in structure was due to a precipitous drop in the abundance of lactic-acid bacteria in unmated females, a taxon known for their beneficial effects on nutrition and immunity. Mating did not affect phenoloxidase or lysozyme-like antibacterial activity in either species, suggesting that any physiological response to mating on host-microbe interactions is decoupled from the systemic immunity. Protein supplementation also did not affect the gut microbiome in decorated crickets, suggesting that insensitivity of gut microbes to dietary protein could contribute to the lack of an effect of nuptial gift consumption. Our study provides experimental evidence that sexual interactions can affect the microbiome and suggests mating can promote beneficial gut bacteria.

Immune Response of Mormon Crickets That Survived Infection byBeauveria bassiana

Beauveria bassiana(Fungi: Ascomycota) is an entomopathogenic fungus that serves as a biological control agent of Mormon cricketsAnabrus simplexHaldeman (Orthoptera: Tettigoniidae) and other grasshopper pests. To measure the dose-dependent response of Mormon crickets to fungal attack, we appliedB. bassianastrain GHA topically to adults using doses of 5.13 × to 1.75 × conidia in sunflower oil, with oil only as a control. After three weeks, we assessed the survivors' hemolymph for fungal cells, active phenoloxidase (PO), and lysozyme. Mortality increased and body mass of survivors decreased with conidial dose. survivors' PO activity was elevated to the same level independent of dose. Those with fungal cells visible in their hemolymph did not differ in PO activity from those with clear hemolymph. We conclude that circulating PO may be an important enzymatic defense againstBeauveriainfection and that it is associated with attempted clearing ofBeauveriablastospores and hyphae from Mormon cricket hemolymph.