Response to Short-Term Cold Storage for Eggs of Agasicles hygrophila (Coleoptera: Chrysomelidae), a Biological Control Agent of Alligator Weed Alternanthera philoxeroides (Caryophyllales: Amaranthaceae)

Cold Acclimation and Supercooling Capacity of Agasicles hygrophila Adults

Agasicles hygrophila Selman and Vogt is used in the biological control of the invasive weed Alternanthera philoxeroides (Mart.) Griseb. However, with the northward establishment of A. philoxeroides in China, the weak adaptivity of A. hygrophila to cold weather has resulted in the ineffective control of A. philoxeroides in northern China. Cold acclimation can significantly enhance insect cold tolerance, enabling them to cope with more frequent climate fluctuations. To improve the biological control efficacy of A. hygrophila in cold climates, we compared the effects of rapid cold hardening and acclimation on A. hygrophila under laboratory conditions. On initially transferring adults from 26 to -10 °C for 2 h, mortality reached 80%. However, when pre-exposed to 0 °C for 2 h and then transferred to -10 °C for 2 h, adult mortality was reduced to 36.67%. These findings indicate that cold acclimation can enhance the cold tolerance of A. hygrophila under laboratory conditions. However, the beneficial cold acclimation effects waned after more than 15 min of recovery at 26 °C. Exposure to 15 °C for 24 h or gradual cooling from 0 to -10 °C at 1 °C·min^-1 also induced cold acclimation, indicating that long-term cold and fluctuating cold acclimation are also potentially effective strategies for enhancing low-temperature tolerance.

Induction of Heat Shock Protein Genes is the Hallmark of Egg Heat Tolerance in Agasicles hygrophila (Coleoptera: Chrysomelidae)

Insects are ecotothermic organisms. Their development, survival, reproduction as well as distribution and abundance are affected by temperature. Heat shock protein (HSP) gene expression is closely associated with temperature variation and influences the adaptation of organisms to adverse environments. The beetle Agasicles hygrophila has successfully been used for biological control of the invasive plant alligator weed (Alternanthera philoxeroides). As A. hygrophila populations are substantially inhibited by high temperatures in the summer, increasing global temperatures may limit the efficacy of this control agent. We previously established that A. hygrophila eggs have low tolerance to heat and this factored into the decreased numbers of A. hygrophila beetles at temperatures of 37.5°C and above. Here, we identified 26 HSP genes in A. hygrophila and examined the relationship between the transcript levels of these genes and heat tolerance. The temperature at which the expression of these 21 HSP genes peaked (Tpeak) was 37.5°C, which is in line with the limit of the high temperatures that A. hygrophila eggs tolerate. Therefore, we speculate that the Tpeak of HSP gene expression in eggs indicates the upper limit of temperatures that A. hygrophila eggs tolerate. This study identifies HSP genes as potential robust biomarkers and emphasizes that determining species' heat tolerance in their natural habitats remains an important consideration for biocontrol. HSP gene expression data provide information about a species' heat tolerance and may be used to predict its geographical distribution.

Molecular Cloning and Expression Profiles of Thermosensitive TRP Genes in Agasicles hygrophila

Simple Summary

The increase of hot days with temperatures over 37 °C in southern China due to global warming has led to summer collapse of the alligator weed flea beetle, an introduced biological agent for the invasive alligator weed. To promote understanding of the beetle’s adaption/tolerance to hot temperatures, we obtained TRPA1, Painless, and Pyrexia, three thermosensitive transient receptor potential channel genes from the beetle, and analyzed their expression patterns across different developmental stages and hot temperatures. Their constitutive expressions were dramatically different from each other and stage-specific. As temperature increased, their expressions in eggs elevated to their peak levels at 30 or 37.5 °C, and then fell back to their preferred temperature levels at temperatures > their peak temperatures. These results imply that (1) they may have different and stage-specific roles in perceiving high temperatures/chemicals and mediating the corresponding responses; and (2) their expressions may be decoupled from their activation. These findings lay a foundation for further understanding of the summer collapse of the beetle.

Abstract

Global warming has gradually reduced the control efficacy of Agasicles hygrophila against the invasive weed Alternanthera philoxeroides. To better understand the summer collapse of A. hygrophila populations, we cloned the cDNA sequences of the high temperature-sensing TRPA1, Painless, and Pyrexia from A. hygrophila, and analyzed their temporal expressions and the impacts of high temperatures on their expression in eggs, the most vulnerable stage of A. hygrophila to hot temperatures. All the three genes obtained had the signature domains of TRPA channels and were constitutively expressed in eggs, larvae (L1, L2, L3), pupae, and adults, but AhPainless had the highest expression, followed by AhPyrexia, and AhTRPA1. The lowest and highest expression stages were adult and pupae for AhTRPA1, egg and L3 for AhPainless, and pupae/adult and L2 for AhPyrexia. The expressions of AhTRPA1, AhPainless, and AhPyrexia remained low at the preferred temperature range of 25–28 °C, elevated to their peak levels at 37.5, 30, and 30 °C, respectively, then fell to their 25–28 °C levels (AhTRPA1, AhPainless) or a lower level (AhPyrexia) at one or more temperatures >30 or 37.5 °C. These results suggest that their temperature-sensing roles and importance may be different, stage-specific, and their expression may be decoupled from their activation.