Do maternal allocations towards offspring quality and quantity ameliorate the effects of predators on offspring survival?

Reproductive allocation is often balanced between the quantity and quality of offspring. Ecological stresses, like exposure to predators, can cause organisms to shift their allocations along this continuum. While the consequences of such plastic shifts for offspring performance are often untested, they are critical to understanding the potential long-term benefits of manipulating predation risk as an agricultural pest management technique. Predation risk induces reductions in egg production and increases in nutritional condition due to maternal provisioning in Colorado potato beetles (Leptinotarsa decemlineata, CPB). Here we tested whether reductions in density or increases in offspring condition, which may increase per-capita larval survival, can compensate for the reduction in total egg production, especially when offspring are exposed to predators. In two field trials, we manipulated the density and condition of larval CPB and measured survival through development to adulthood in field cages with and without predaceous stink bugs (Podisus maculiventris). As expected, cages with the higher initial larval densities had more larvae and adults surviving in the treatments without predators –about 30-50% survival across densities. When predators were present this relationship did not hold because of density-dependent predation. Larval condition interacted with density and impacted larval survival in both trials albeit in different ways. In Trial 1, unprovisioned beetles had higher survival at the higher densities, in Trial 2 provisioned beetles had higher survival across densities.

Synthesis and Applications: Overall, our test of the effects of predation risk via manipulations of larval density and condition revealed few net compensatory benefits to the prey of reduced density and higher condition. Benefits to the prey of shifts in allocation from the quantity to quality of offspring may depend on factors that influence the strength of density dependence, including predation intensity. Our results suggest a new strategy of taking advantage of the reductions in prey density due to the non-consumptive effects of predators as a pest management approach to protect plants.

Methods
Using the herbivorous Colorado potato beetle and predaceous spined soldier bug system, we tested whether parental reproductive allocation shifts could compensate for reductions in the total number of eggs laid. In this system, mothers exposed to predators shift their reproductive investment to make fewer offspring with higher levels of provisioning. We tested 1) the individual and combined effects of larval density and condition on the number of offspring surviving to the last larval stage and emerging as adults, in the presence and absence of predators, 2) whether larval survival is density-dependent in the presence and absence of predators, and 3) whether density-dependent survival depended on larval condition. Lastly, 4) we measured larval feeding, growth, and development midway through the larval period to assess levels of competition and effects on plant damage.

We placed larval beetles from all combinations of density and condition treatments in field mesocosms with and without predators. We then measured the number of beetles surviving daily throughout larval development and adult emergence and assessed beetle growth and plant damage early in the experiment. These experiments allowed us to determine the consequences of reproductive allocation for the local beetle population (total number surviving) and survival during larval development when the offspring were reared with or without predators.