Temperature-dependent Functional Response of Cryptolaemus montrouzieri (Col.: Coccinellidae) to the Citrus Mealybug, Planococcus citri (Hem.: Pseudococcidae)

The citrus mealy bug,  Planococcus citri (Risso) is one of the major pests of citrus and many other orchards crops, as well as ornamental plants in subtropical and tropical regions of the world, including Iran. Due to the harmful effects of chemical pesticides on biological control agents and non-target organisms, along with developing resistance to various pesticides by the pests, there has been an increasing interest in integrating biological control methods in the pest management programs. The predatory ladybird, Cryptolaemus montrouzieri Mulsant is one of the most widely used biological control agents. It is an important natural enemy of many species of mealybugs such as P. citri. Functional response of a predator that refers to the number of prey consumed per predator as a function of a prey density is the important characteristic of predator–prey interactions. Temperature can affects the consumption behaviour of the predators. The aim of the present study is to evaluate the effects of temperature on the functional response of C. montrouzieri to different densities of P. citri. Improving our understanding about predator-prey interactions between C. montrouzieri and P. citri may be helpful to optimize biological control of the mealy bug.

The colony of P. citri was collected from bitter orange in Mehmansara region, Ghaemshahr, Mazandaran province, and was reared on pumpkin, Cucurbita maxima. The laboratory colony of C. montrouzieri was obtained from Baharan Dasht Sahel insectarium in Sari, Mazandaran province, and was reared on P. citri. The functional response experiments were performed with fourth instar larvae and adult females of C. montrouzieri to the adult female of P. citri. The experiments were conducted at the temperatures of 18, 23, 27, 32 and 37±1 °C, 60±10% RH and L16:D8 h. Prey in densities of 3, 5, 7, 10, 15 and 30 was offered to the predators. The predators used for the experiments were < 48 h old and were starved for 24 h before starting the tests. A single bitter orange leaf disc (3 cm2) was centred upside down in each ventilated Petri dish (10 cm diameter). Ten replicates were conducted for each prey density. After 24 h, the predators were removed from the experimental arena and the number of consumed prey was recorded and they were not replaced during the functional response tests. The type of the functional response was determined by logistic regression analysis (SAS/STAT, CATMOD-procedure) of the proportion of prey killed in relation to prey offered. To determine the type of functional response, the sign of the linear coefficient was estimated by the logistic regression. The negative sign is evident to a functional response type II, whereas a positive sign reveals a type III functional response. Parameters of attack rate and handling time were estimated using the PROC NLIN procedure of SAS and compared through the indicator variable method. 

The logistic regression for fourth instar larvae and adult females of C. montrouzieri to P. citri at all temperatures showed a negative linear parameter and the proportion of prey eaten by the predators declined with increasing prey density. This suggested that fourth instar larvae and females of C. montrouzieri exhibited a type II functional response to change in P. citri density at all tested temperatures. The highest attack rate and lowest handling time of fourth instar larvae were estimated at 37 and 27 °C, respectively. Whereas the highest attack rate and lowest handling time of adult females were estimated at 37 and 32 °C, respectively. At the same temperature, the attack rate estimated for the adult female was higher than that of the larval instar of the predator, showing higher predation potential of adult female in compared to the fourth larval instar of the predator. Theoretical maximum attack rates (T/Th) for fourth larval instar of the predator at above-mentioned temperatures were respectively estimated 8.20, 28.81, 43.80, 19.82 and 14.44 and for adult female predator were 9.70, 9.17, 9.10, 18.47 and 17.92, respectively.          

Temperature affected significantly functional response parameters of fourth instar larvae and adult females of C. montrouzieri to P. citri densities. Although the last instar larvae and adult females of C. montrouzieri showed high predation potential at all temperatures, adult females of the predator was more voracious compared to the last instar larvae of the predator especially at higher temperatures.