Hybridization affects life-history traits and host specificity in Diorhabda spp

Authors: Bitume, Ellyn; BEAN, DAN - Colorado Department Of Agriculture; STAHLKE, AMANDA - University Of Idaho; HUFBAUER, RUTH - University Of Idaho

Submitted to: Biological Control
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/22/2017
Publication Date: N/A
Citation: N/A

Interpretive Summary: Invasive weeds are a substantial problem for ecosystems and the environment, as well as for landowners and landscape managers. Tamarix, also known as saltcedar, is one of the top three invaders of shoreline habitats along arroys, creeks, rivers, lakes and reservoirs in dry desert and rangeland regions of the western U.S., occupying over 1 million acres in these habitats. Tamarix consumes water, spreads fires, and degrades the shoreline habitats for plants and animals, including threatened and endangered species, that depend on these habitats in desert regions. When mechanical and chemical options are no longer viable, or the problem with the invasive weed such as Tamarix is too large to control, we rely on applied biocontrol: the practice of searching out natural enemies of the invasive weed in its native habitat and bringing the enemy, in this case an insect, to the invaded range to effectively decrease the biomass of the invasive weed. Part of a successful biocontrol program is understanding the mechanisms behind how an insect, released to control an invasive plant, will effectively establish and survive in its new habitat while minimizing the impacts to non-target plants, such as plants that are native and naturally occurring in the environment in the U.S., and beneficial crops in the U.S., both native and introduced. One suggestion is to increase the variation in populations of introduced insects so that they can better respond to novel environmental pressures. This task can be achieved by searching out insects from different geographical regions of the parts of the world where it is native, and mixing these populations before releasing them in the areas needed for biocontrol, where the weed is non-native and invasive. In the case of Diorhabda spp, a beetle released to control invasive Tamarix spp, different "ecotypes", or populations from different regions, were released in Texas and other states. These ecotypes were later deemed separate species, and indeed the beetles are now hybridizing. We tested the effects of hybridization on development time, egg production, adult mass, and host choice. We found that in these species, hybridization had either neutral or positive effects on these important life history traits. Furthermore, we show that host preference of the hybrids can vary depending on their particular cross. In this case, hybridization in a released biocontrol agent might increase the effectiveness of the beetles to do damage to Tamarix, but it could also increase the chances of non-target impacts to athel, a closely-related, non-native but sometimes beneficial shade tree in desert communities.

Technical Abstract: Hybridization is an influential evolutionary process that has been viewed alternatively as an evolutionary dead-end or as an important creative evolutionary force. In colonizing species, such as introduced biological control agents, hybridization can negate the effects of bottlenecks and genetic drift through increasing genetic variation. Such changes could be beneficial to a biological control program by increasing the chances of establishment success. However, hybridization can also lead to the emergence of transgressive phenotypes that could alter host specificity; an important consideration when assessing potential non-target impacts of planned agents. In a series of lab experiments, we investigated the effects of hybridization between three species of Diorhabda released to control invasive Tamarix (saltcedar) on life history traits through two generations, and through the third generation for one cross. Depending on the cross, hybridization had either a positive or neutral impact on development time, adult weight, and fecundity. We evaluated preference for the target (saltcedar) relative to a non-target host Tamarix aphylla (athel), and found host specificity patterns varied in two of the three hybrids, demonstrating the possibility for hybridization to alter host preference. Importantly, the overall effects of hybridization were inconsistent by cross, leading to unpredictability in the outcome of using hybrids in biological control.